• dispersal;
  • house sparrow;
  • life expectancy;
  • lifetime reproductive success;
  • mating success


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1. Dispersal affects many important ecological and evolutionary processes. Still, little is known about the fitness of dispersing individuals.

2. Here, we use data from a long-term study of a house sparrow Passer domesticus metapopulation to compare lifetime reproductive success (LRS) of resident and immigrant individuals, all with known origin.

3. Lifetime production of recruits by immigrant males was much lower than for resident males, because of shorter life span and lower annual mating success. In contrast, lifetime production of recruits did not differ significantly between immigrant and resident females.

4. Over their lifetime, dispersers contributed fewer recruits to the local population than residents. This shows that immigrant house sparrows have different, sex specific, demographic effects on the population dynamics than residents.


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Dispersal of individuals and their genes influence many important ecological and evolutionary processes (Clobert et al. 2001; Bowler & Benton 2005; Ronce 2007). Due to loss and fragmentation of critical habitat types at an increasing rate, the number of populations with metapopulation structure, i.e. local populations interconnected by dispersing individuals, is increasing (Hanski & Gaggiotti 2004). The main adaptive explanations of dispersal include leaving low-quality habitat, avoidance of inbreeding and kin competition (Johnson & Gaines 1990; Clobert et al. 2001; Bowler & Benton 2005; Szulkin & Sheldon 2008). For instance, immigrant individuals may obtain higher fitness than residents if residents are inbred, and offspring of immigrants have survival advantages from heterosis (Ingvarsson & Whitlock 2000; Ebert et al. 2002; Saccheri & Brakefield 2002). Furthermore, locally adapted parasites may reduce local host fitness more than fitness of immigrant hosts (Altermatt, Hottinger & Ebert 2007).

Emigration from the natal habitat may also involve several costs. The movement itself may have substantial costs in terms of energy and predation. During and after settlement, lack of familiarity with the new habitat may incur costs through the ability to find high-quality nest sites and food (Greenwood 1980; Stamps 1987, 1995), loss of social status and effects of prior residency when joining a new group of individuals (Greenwood 1980; Lambin, Aars & Piertney 2001; Kokko, Lopez-Sepulcre & Morrell 2006). These costs may affect the two sexes differently. For instance, competition for resources, which may be linked to the mating system, may entail different costs and benefits of dispersal for males and females (Greenwood 1980). In most birds, males defend resources necessary for obtaining a mate, such as territories or nest sites. Therefore, males may benefit more from familiarity with the breeding area and prior residency, and costs of dispersal may be higher than in females. On the other hand, females, being the choosy sex in such a mating system, may benefit more from dispersal through increased access to mates or resources (Greenwood 1980).

The evolution of dispersal and demographic and genetic consequences of immigration will, among other things, depend on fitness differences between immigrant and resident individuals (Lemel et al. 1997; Whitlock & McCauley 1999; Whitlock 2001). For example, in order for an immigrant to affect gene flow, it must successfully mate and breed, and at least some of its decendants must survive to adulthood. Although many studies have compared some components of fitness between dispersing and non-dispersing individuals, long-term estimates of survival and reproduction over larger geographical areas are much fewer (Belichon, Clobert & Massot 1996; Clobert et al. 2001; Doligez & Pärt 2008). Some studies have found that residents and immigrants differ in fitness-related traits (e.g. insects: Roff 1977; Saccheri & Brakefield 2002; reptiles: Massot et al. 1994; mammals: Krohne & Burgin 1987; reviewed in Belichon et al. 1996; Clobert et al. 2001; Marr, Keller & Arcese 2002; Clobert, Ims & Rousset 2004; Doligez & Pärt 2008). Furthermore, in birds, most studies involve comparisons between residents and immigrants with respect to a few demographic traits, often measured only during their first year of breeding (e.g. great tit Parus major, Greenwood, Harvey & Perrins 1979; sparrow hawk Accipiter nisus, Newton & Marquiss 1983; marsh tit Parus palustris, Nilsson 1989; collared flycatchers Ficedula albicollis, Pärt 1991, 1994). Although these studies provide important information about short-term correlates of dispersal, data on lifetime reproductive success (LRS) are important for fully assessing the fitness of immigrants and to understand the consequences of immigration in the receiver population. The relationship between dispersal behaviour and LRS, i.e. the number of recruits to the breeding population that an individual produces over its lifetime (Clutton-Brock 1988), has been analysed in six bird species: great reed warbler Acrocephalus arundinaceus (Bensch et al. 1998; Hansson, Bensch & Hasselquist 2004), savannah sparrow Passerculus sandwichensis (Wheelwright & Mauck 1998), black kite Milvus migrans (Forero, Donazar & Hiraldo 2002), red-cockaded woodpecker Picoides borealis (Pasinelli, Schiegg & Walters 2004), great tit (Verhulst & van Eck 1996) and long-tailed tit Aegithalos caudatus (Maccoll & Hatchwell 2004). However, extra-pair paternity is common in a wide range of species (Westneat & Stewart 2003), and if, for example, females prefer resident over immigrant males as extra-pair mates, then genetic parentage is necessary to reduce bias when comparing fitness of resident and immigrant males. Such a comparison of genetic parentage of residents and immigrants has previously only been determined in one species, the great reed warbler, where immigrants were shown to have lower LRS than residents (Bensch et al. 1998; Hansson et al. 2004).

Here, we use data from a long-term study on an insular metapopulation of resident house sparrows Passer domesticus to compare LRS of residents and immigrants. The study system is based on extensive capture–mark–recapture methods and determination of genetic reproductive success. Most birds on the main study islands included here are individually marked (>90% of adults) and the study area is large in relation to the dispersal range. This allows unambiguous identification of dispersal events between islands and a high probability of observing recruiting individuals within the study area, which in turn reduces the bias in fitness caused by non-random dispersal behaviour out of the study area (Doligez & Pärt 2008). Thus, it is possible to follow both resident and dispersing individuals from hatching to recruitment, and until their death (Pradel 1996). Together this allows us to obtain relatively unbiased sex-specific estimates of LRS both for residents and immigrants.

The house sparrow is a small, passerine bird and is one of the most widely distributed bird species on earth (Anderson 2006). In the study area, polygyny and extra-pair matings are common: the proportion of within-pair fledglings ranges from 0% to 80% and the occurrence of broods sired by at least to fathers is over 30% (L.K. Larsen, H. Pärn, H. Jensen, T.H. Ringsby & B.-E. Sæther, unpublished data). The house sparrow is a highly sedentary species, and has limited natal dispersal (c. 10%) and negligible breeding dispersal (<2‰) in the study population (Altwegg, Ringsby & Sæther 2000; Tufto et al. 2005). A previous study on dispersal of house sparrows in the study area (Altwegg et al. 2000) showed that dispersers of both sexes had higher adult survival rates than residents. In the study area, inbreeding level is relatively high and has negative fitness consequences (Jensen et al. 2007). Accordingly, dispersal may be a way to escape such costs of inbreeding. In house sparrows, males compete over nest sites and females. Therefore, philopatry and prior local experience is expected to be of greater advantage to males than females (Greenwood 1980; Pärt 1994; Lambin et al. 2001).

The main objective of this study was to compare lifetime fitness of resident and immigrant individuals. By analysing several fitness components in both sexes, we also aim to further understand sex-specific effects of dispersal on individual variation in fitness.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

General field procedures

The study was conducted from 1993 to 2006 in a house sparrow metapopulation in northern Norway (66°N, 13°E; Fig. 1). Individuals were observed and captured annually during the study period, on 18 islands covering an area of more than 1600 km2 (Sæther et al. 1999b; Ringsby et al. 2002). Fitness of resident and immigrant individuals were compared on five of these islands, where the sparrows breed in small colonies in close association with human habitation and dairy farms. On the five main study islands, the capture rate is higher and the proportion of accessible nests is larger than on the other study islands. Accordingly, on the main study islands, reproductive success can be determined for a larger proportion of individuals, and the proportion of individually colour marked sparrows on these islands is larger (>90% vs. >60% on surrounding study area). However, data from the other islands and from the mainland was used to identify immigrants to the five main study islands and to identify any dispersing recruits produced by adults on these five islands. The mean population size on each of the five main study islands during the years 1993–2002 ranged from 18 to 93 adult individuals. The mean population size on all the 18 islands in the total metapopulation was approximately 450 adult individuals during the same time period. The breeding season lasts from early May to mid August (Ringsby, Sæther & Solberg 1998; Ringsby et al. 2002). Fieldwork was performed each year during this period and for approximately one month from the end of September to the beginning of November. The birds reach sexual maturity in their second calendar year, and can lay up to three clutches per season, each containing on average five eggs (Husby et al. 2006). The house sparrows build their nests in cavities found in buildings. Thus, although behavioural differences may well force immigrant males to use low-quality nest sites, nest site selection by immigrants in entirely different areas where nests are much more difficult to find (e.g. in the forest) or access is therefore unlikely. Each island was searched for any active nests at least once a week during the summer. Nests were usually visited 2–3 times during the incubation period. Hatching date was either determined directly by observation, or estimated by a subjective estimation of the age of the nestlings on the first visit after hatching. The nestlings fledge at an age of about 14 days, and when the nestlings were between 8 and 12 days old, 25 μL of blood was collected from their brachial vein and they were banded with a numbered metal ring and unique combinations of plastic colour leg rings (see Jensen et al. 2003, 2004). A large proportion of the adult birds and unmarked fledged juveniles on the study islands was captured using mist nets during the summer and autumn field periods. At first capture, previously unmarked individuals were blood sampled and marked (see above).


Figure 1.  Map showing the house sparrow metapopulation study system at the coast of Norway (66°N, 13°E). The five main study islands are shown in black, and the other 13 study islands and the mainland are shown in grey.

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Due to the extensive capture and ringing protocol of fledglings, fledged juveniles and adults, more than 90% of all adult birds on the five main study islands were individually marked during the study period (see Ringsby et al. 1999). Recaptures and observations of marked adult birds were used to determine whether fledglings and juveniles survived until recruitment, whether adult birds survived to the next breeding season, and whether inter-island dispersal had occurred. The island on which unmarked juveniles were captured during summer and autumn was assumed to be their natal island. Thus, in this study, resident individuals were those that were marked as nestlings or juveniles on any of the five main study island and that recruited on their natal island. Immigrant individuals were those that were marked as nestlings or juveniles on any island in the study system and that performed inter-island dispersal and recruited on one of the five main study islands. Among the individuals classified as residents and that were marked the first time as juveniles in the autumn, less than 0·7% of the males and 2% of the females may be wrongly classified as residents, due to dispersal that takes place during the autumn (T.H. Ringsby, H. Pärn, H. Jensen & B.-E. Sæther, unpublished data). To avoid inclusion of individuals with ambiguous dispersal history, we excluded individuals captured as unmarked adults. We estimated two components of fitness in both sexes, namely fecundity and survival (Jensen et al. 2004). In males, we also estimated mating success (Jensen et al. 2008). Fecundity was estimated as the number of eggs (for females), number of fledglings and number of recruits an individual produced, determined by genetic parentage analyses (see below). Male mating success within a breeding season was based on genetically determined paternity of individual fledglings in clutches of different genetically determined mothers. Moreover, males that were genetic fathers of fledglings in clutches where no genetic mother was identified were given one extra mate as long as the clutch was not in a nest where they had fathered a fledgling earlier in the breeding season. Accordingly, all clutches in the same nest were assumed to have the same mother, except in the few cases where the genetic analyses suggested that different females had produced subsequent clutches in the same nest. Measures of reproductive success based on genetic parentage were determined for the cohorts 1993–2002 (see below). The life span of a bird was defined as the number of years from hatching to the last year (up to and including 2008) it was either observed or recaptured, as this year was assumed to be the last it was alive (see below).

Parentage analyses

Parentage was determined by genetic analyses of DNA extracted from blood by standard methods. Up to nine highly polymorphic microsatellite loci were used in the genotyping procedure (see Jensen et al. 2003, 2004). To determine the parentage, the software cervus 2.0 (Marshall et al. 1998) was used. The total exclusionary power in the parenthood analyses was >0·9994 for both first and second parent. The resulting assigned maternity or paternity was correct in at least 90% of cases. The genetic paternity or maternity could not be determined with 90% confidence for all fledglings using the above procedures. This could be because an unmarked adult bird present in the population was the parent, or that the genetic analyses could only determine with less than 90% certainty which of the candidate parents that was the genetic parent. The genetic mother was unidentified for 16% of the fledglings and the genetic father was unidentified for 30%, which bias fitness downwards. If residents are more genetically similar to each other than to immigrants, correct assignment of parentage may be more difficult for residents. Thus, if anything, this type of bias may be more pronounced for resident individuals. In addition, some nestlings died before blood sampling, and a few nests were not discovered or were inaccessible. However, we assume that these individuals were a random sample of the individuals included in the analyses (see above) and we expect no bias in fitness between immigrants and residents for these reasons.

Data selection

For individuals belonging to the 1993–2002 cohorts, data on the last year a bird was alive exists until 2008. However, LRS for individuals that were alive after 2002, which was the last year where genetic parentage was determined in our study, was censored. Therefore, we decided to exclude individuals from cohort 2000 and onwards from analysis of reproductive success. However, we believe that the excluded individuals were a random sample of the adult birds. Among the cohorts included in the analyses (1993–1999), 11 residents (but no immigrants) were still alive after 2002. Thus, for these individual the LRS may be underestimated. In analyses of onset of breeding only the first clutch in the current breeding season, and only clutches with parents in their first breeding year were selected.

Statistical analyses

Reproductive success and life span were analysed using the software r version 2.8.1 (R Development Core Team, 2008). Reproductive success was analysed by fitting linear mixed models, using the lme4 package (Bates, Maechler & Dai 2008). Life span was analysed with a Cox proportional hazards model (coxph function in survival package; Terry Therneau and original R port by Thomas Lumley 2008). In all models of reproductive success and life span, the fixed predictor variables were sex and dispersal category (resident or immigrant). To test if the effect of dispersal category differed between males and females, we also included the interaction term sex × dispersal category. In analyses of reproduction, the random factors were adult island and year nested in adult island (first year of reproduction) or cohort year nested in adult island (lifetime reproduction). Year and cohort year were nested in island to account for island-specific effects of these two factors. To test if the effect of dispersal category change with age (familiarity), we fitted separate models for the two sexes and included age (categorized as second calendar year, i.e. first year on the new island for immigrants, or after second calendar year), dispersal category and the interaction thereof as fixed factors, and adult island, year and individual as random factors. To estimate survival and recapture probabilities, Cormack–Jolly–Seber capture–mark–recapture methods (Lebreton et al. 1992) implemented in program mark (White & Burnham 1999) were used. As an interface to program mark, the package RMark (Laake 2008) was used. The model consisted of survival (Φ) and resight/recapture ( p) probability parameters that could vary by sex and dispersal category and the interaction thereof. Due to insufficient data and to avoid problems with overfitting, the fitted MARK model was time independent. Goodness-of-fit tests of the model revealed overdispersion (variance inflation factor ĉ = 2·03, < 0·01, based on the bootstrapped GOF approach; White & Burnham 1999). Standard errors and confidence interval for annual survival rate and detection rate were adjusted accordingly. Coefficients for the interaction term sex × dispersal category were always calculated relative to an intercept representing resident males. Coefficients for dispersal category within each sex were always calculated relative to an intercept representing residents of the same sex.


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In the cohorts 1993–1999, there was no difference between the proportion of male (30 of 300) and female (29 of 264) recruits that were immigrants to the five study islands (χ2 = 0·06, d.f. = 1, P = 0·808).

Sex-specific differences in lifetime reproduction

The relationship between dispersal category and lifetime production of fledglings and recruits differed significantly between the sexes, as indicated by the positive interaction between sex and dispersal category on number of fledglings (β = 0·97 ± 0·22, = 4·37, < 0·001) and recruits (β = 2·07 ± 0·77, = 2·70, P = 0·007). In males, immigrants performed less well than residents in terms of lifetime number of mates, number of fledglings and number of recruits (Table 1, Fig. 2). This could partly be due to the shorter life span of immigrant males (Table 1, Fig. 3a). However, after controlling for life span, immigrant males still had fewer mates, produced fewer fledglings and recruits than resident males (Table 1), suggesting a lower annual fitness in immigrant males. In females, dispersal category was not a significant predictor of lifetime number of eggs, fledglings and recruits (Table 1) or life span (Table 1, Fig. 3b). Thus, there was a sex-specific effect of dispersal category on lifetime reproduction, where immigrants had lower fitness than residents in males, but not in females.

Table 1.   Lifetime fitness-related traits of male and female house sparrows, categorized as residents and immigrants
 ResidentImmigrantβ ± SEzP
Mean ± SEMean ± SE
  1. Life span was analysed with a Cox proportional hazard model. Other traits were analysed with mixed models with dispersal category (resident or immigrant), sex and the interaction thereof as fixed factors, and adult island and cohort year nested in island as random factors. Parameter estimates for dispersal category within each sex was calculated relative to an intercept representing residents of the same sex. The results from the models are presented as parameter estimates β ± SE, z and P-values. Sample sizes: resident males, n = 270; immigrant males, n = 30; resident females, n = 235; immigrant females, n = 29.

 Life span2·21 ± 0·09 1·73 ± 0·170·39 ± 0·192·000·045
 No. mates1·47 ± 0·140·33 ± 0·10−1·43 ± 0·33−4·38<0·001
 No. mates controlled for life span  −1·14 ± 0·33−3·49<0·001
 No. fledglings2·87 ± 0·291·00 ± 0·32−1·09 ± 0·19−5·79<0·001
 No. fledglings controlled for life span  −0·80 ± 0·19−4·20<0·001
 No. recruits0·59 ± 0·070·07 ± 0·05−2·19 ± 0·72−3·060·002
 No. recruits controlled for life span  −1·87 ± 0·72−2·600·009
 Life span2·11 ± 0·09 2·03 ± 0·220·06 ± 0·200·330·740
 No. eggs5·73 ± 0·644·86 ± 1·03−0·11 ± 0·10−1·130·258
 No. eggs controlled for life span  −0·03 ± 0·10−0·330·739
 No. fledglings3·28 ± 0·372·97 ± 0·73−0·12 ± 0·12−1·020·306
 No. fledglings controlled for life span  0·00 ± 0·120·010·990
 No. recruits0·64 ± 0·090·55 ± 0·20−0·12 ± 0·27−0·450·654
 No. recruits controlled for life span  0·02 ± 0·280·080·936

Figure 2.  Lifetime number of (a) mates, (b) fledglings and (c) recruits in resident (R) and immigrant (I) male house sparrows. Bars show means and error bars indicate ±1 standard error.

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Figure 3.  Survival curves for the 1993–1999 cohorts of male (a) and female (b) house sparrows categorized as residents (solid line) or immigrants (hatched line). The Kaplan–Meier method for censored data was used to compute the curves.

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It was then tested if the relationship between annual fitness and dispersal category differed between the first year in the new habitat, when the lack of familiarity is largest, and subsequent years. Individuals were categorized as being either 1 year old (i.e. second-year individuals, SY) or more than 1 year old (after-second-year individuals, ASY). However, the relationship between annual fitness and dispersal category did not differ significantly between SY and ASY males, as indicated by non-significant interactions between age and dispersal category on annual number of mates (β = 0·01 ± 0·70, = 0·01, = 0·991), annual number of fledglings (β = 0·43 ± 0·49, = 0·87, = 0·375), and annual number of recruits (β = 0·26 ± 1·66, = 0·16, = 0·874). This suggests that, in males, the effect of dispersal category on annual fitness did not change with increasing familiarity with the island of residency.

In females, the coefficient for dispersal category was significantly larger in ASY females than in SY females (interaction age × dispersal category; eggs: β = 0·88 ± 0·27, = 3·25, = 0·001; fledglings: β = 0·88 ± 0·33, = 2·70, =0·007). Among SY females, the annual number of eggs (β = 0·03 ± 0·48, = 0·07, = 0·945) and fledglings (β = −0·13 ± 0·41, = −0·33, = 0·741) did not differ significantly between residents and immigrants. On the other hand, among ASY females, immigrants tended to produce more eggs (β = 0·91 ± 0·50, = 1·83, = 0·067) and fledglings (β = 0·75 ± 0·43, = 1·75, = 0·080) than residents. Thus, in females, the importance of dispersal category as a predictor of annual fitness increases with increasing age and familiarity with the island. However, the positive interaction between age and dispersal category can be interpreted in an alternative way, namely that the effect of age on number of eggs and fledglings differs between immigrant and residents. Among resident females, there was no significant effect of age on annual number of eggs (β = −0·14 ± 0·09, = −1·46, = 0·127) or fledglings (β = −0·15 ± 0·10, =2·79, = 0·005) and fledglings (β = 0·73 ± 0·31, = 2·34, = 0·019) than SY females. Thus, in females, the effect of increasing age and familiarity on annual reproduction was stronger among immigrants. For annual number of recruits, there was no significant interaction between age and dispersal category: (β = 1·05 ± 0·72, = 1·46,  0·143).

Fitness-related traits during first breeding year

During the first breeding year, immigrant males had significantly fewer mates (β = −1·10 ± 0·46, = −2·40, = 0·016) and fledglings (β = −0·90 ± 0·28, = −3·22, = 0·001) and tended to have fewer recruits (β = −1·96 ± 1·03, z = −1·90, = 0·057) than resident males. Onset of breeding (β = −7·81 ± 13·13, = −0·60, P = 0·554) and survival from second to third calendar year (β = −0·34 ± 0·39, = −0·86, = 0·389) did not depend on dispersal category in males. Among females, residents and immigrants did not differ in any of the fitness-related traits during the first breeding year (onset of breeding: β = −2·25 ± 8·67, = −0·259, = 0·796; number of eggs: β= −0·17 ± 0·14, z = −1·26, = 0·207; number of fledglings: β = −0·13 ± 0·17, = −0·75, = 0·453; number of recruits: β = −0·44 ± 0·44, = −1·00, = 0·319; survival from second to third calendar year: β = −0·09 ± 0·40, = 0·23, = 0·812). The relationship between dispersal category and the number of fledglings (i.e. the effect of being an immigrant) was significantly less negative in females than in males (interaction sex × dispersal category: β = 0·77 ± 0·33, z = 2·35, P = 0·019). However, the relationship between dispersal category and number of recruits in the first year of reproduction (interaction sex × dispersal category: β = 1·51 ± 1·12, z = 1·35, P = 0·176) or survival from second to third calendar year (interaction sex × dispersal category: β = 0·24 ± 0·56, z = 0·44, P = 0·663) did not differ between the sexes.

Survival rate and life span

The annual survival rate did not differ significantly between residents and immigrants, neither in males (Φ = −0·15 ± 0·29, CI = −0·72–0·43), nor in females (Φ = −0·02 ± 0·29, CI = −0·59–0·55; interaction sex × dispersal: Φ= 0·12 ± 0·41, CI = −0·69–0·93). The annual recapture/resight rate did not differ significantly between residents and immigrants, neither in males ( = −0·57 ± 0·51, CI = −1·57–0·43), nor in females ( = 0·07 ± 0·36, CI = −0·65–0·78; interaction sex × dispersal: = 0·64 ± 0·73, CI = −2·07–0·79). In the Cox proportional hazards model, there was a higher hazard (indicating poorer survival) in immigrant males than in resident males (β = 0·39 ± 0·19, = 2·00, = 0·045). In females, hazard did not differ between immigrants and residents (β = 0·06 ± 0·20, = 0·33, = 0·740). The importance of dispersal category as a predictor of hazard did not differ significantly between the sexes (interaction sex × dispersal category: β = −0·32 ± 0·28, = −1·17, P = 0·240).


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Lifetime production of recruits in male house sparrows was lower for immigrant than for resident males (Table 1, Fig. 2c). This was due to a combined effect of immigrant males having shorter life span (Table 1, Fig. 3a), and a lower mating success (Table 1, Fig. 2a). In females, lifetime production of recruits did not differ between residents and immigrants (Table 1). There was a positive effect of age and familiarity on number of eggs and number of fledglings in females, but only among immigrant females.

Annual survival rates were not significantly related to dispersal category in either sex, although they had the expected sign and magnitude given the results from the analysis of their cumulative effect on life span. In males, life span was shorter in immigrant males than in resident males (Fig. 3a). If immigrants were more prone to perform breeding dispersal out of the study area, the shorter life span found in the study area could be an artefact. However, only two cases of breeding dispersal was recorded during the study period and were both performed by individuals that had not performed natal dispersal (Altwegg et al. 2000). In contrast to males, females of the two dispersal categories did not differ in life span. Sex-specific costs on life span of unfamiliarity with the area in which the disperser settle may explain this pattern. Although immigrants of both sexes are unfamiliar to their new habitat and may suffer from less efficient foraging or higher predation rates compared to philopatric individuals (Stamps 1995; Stamps & Krishnan 1999), males may in addition need to spend more time and energy in search and competition for nest sites and mates, and may also be more frequently challenged by residents (Greenwood 1980; Arcese 1989; van der Jeugd 2001), which may be costly in terms of survival.

The similar survival rates among residents and immigrants are not consistent with a previous study on house sparrows in the same area, which found that inter-island dispersers of both sexes had higher adult survival than residents (Altwegg et al. 2000). One reason for this discrepancy could be that Altwegg et al. (2000) included an additional set of islands, with no farms and colony living sparrows, where sparrows breed in scattered nest boxes. Interestingly, it seems that the dispersal patterns are different in the two set of islands (Ringsby et al., in review), which in turn may affect the fitness consequences of dispersal. A major methodological difference is that the models fitted in the two studies are not identical. First, we allowed recapture probability to depend on dispersal category in our models, whereas Altwegg et al. (2000) did not. Furthermore, our dataset did not allow us to fit a fully time-dependent model with interactions, which was done by Altwegg et al. (2000), which may have affected parameter estimation.

To date, this is the first study that has found that life span is shorter in immigrants. Similar to Altwegg et al. (2000), survival probability increased with increased dispersal distance in western gulls (Spear, Pyle & Nur 1998), and immigrant males had higher survival than residents in song sparrows (Marr et al. 2002). However, other studies on birds have not found any relationship between dispersal category and life span or survival in males (great reed warbler, Bensch et al. 1998; Hansson et al. 2004; great tit, Clobert et al. 1988; willow tit, Orell et al. 1999; reviewed in Doligez & Pärt 2008). In females, we found no relationship between dispersal category and life span, which seems to be the norm in the species considered to date (great reed warbler, Bensch et al. 1998; Hansson et al. 2004; great tit, Clobert et al. 1988; song sparrow, Marr et al. 2002; willow tit, Orell et al. 1999).

Immigrant males had fewer mates during their lifetime than resident males. The only bird species for which lifetime number of mates previously has been determined for immigrants is the great reed warbler (Bensch et al. 1998; Hansson et al. 2004). Also in this species the lifetime number of females was lower in immigrant males. As in this study, the difference remained when controlling for life span (Bensch et al. 1998). Similarly, the speed and probability of obtaining a mate the first breeding year was higher in philopatric than in immigrant collared flycatchers (Pärt 1994), and in song sparrows fewer immigrant males bred each year (Marr et al. 2002). Thus, it may be a general pattern that immigrant males have a lower mating success than resident males.

There are several potential reasons for immigrant males having lower mating success. First, there is a broad support across taxa that most animals dominate opponents in aggressive interactions within a familiar area (resident advantage; reviewed in Kokko et al. 2006). Greenwood (1980) suggested that males with prior local experience would have greater chance of obtaining a high quality nest, a resource necessary for obtaining a female, which in turn gives fitness advantages to resident males. Second, the cost of searching and assessing alternative nest sites is higher in an unfamiliar area (Stamps 1995; Stamps & Krishnan 1999). Third, arriving to a new area may also entail costs due to agonistic interactions with resident individuals, for example if residents are more hostile against immigrant males. This has been shown in the colonial barnacle geese, where first year male immigrants suffered more attacks by territory owners than residents (van der Jeugd 2001). This may also occur in house sparrows: they live in loose colonies with frequent interactions among birds and there may be dominance hierarchies in flocks (Solberg & Ringsby 1977; Anderson 2006). McGillivray (1980) observed that house sparrows nesting in close proximity vigorously chased strange birds, whereas neighbours were tolerated. Together these observations suggest that house sparrows are capable of individual recognition and that residents may react aggressively against immigrants (see also Beletsky & Orians 1989).

Competition for resources at the natal site may force inferior individuals to disperse (Ims & Hjermann 2001; Murren et al. 2001). For example, in song sparrows, subordinate individuals emigrated more often in both sexes (Arcese 1989). Thus, the phenotype per se of immigrants may make them poor competitors in their new area. In house sparrows, the importance of the male throat badge in competition over mates, nest sites and other resources (Solberg & Ringsby 1997; Jensen et al. 2004, 2008; Nakagawa et al. 2007) renders it a plausible candidate trait that could differ between dispersal categories. However, in our study area badge size did not differ between immigrants and residents (Altwegg et al. 2000; H. Pärn, H. Jensen, T.H. Ringsby & B.-E. Sæther, unpublished data) and is probably not an explanation for the difference in number of mates. Neither did other morphological traits differ between residents and immigrants (Altwegg et al. 2000; Pärn et al., unpublished data). Thus, lower mating success of male immigrants does not seem to be due to inferior competitive ability over mates due to morphological differences.

We found that LRS of immigrant males was much lower than the LRS of residents (Table 1, Fig. 2c). To our knowledge, this is the second bird species (the first being the Great reed warbler; Bensch et al. 1998; Hansson et al. 2004) in which fitness estimated as lifetime production of recruits determined by genetic parentage has been compared between residents and immigrants. In the migratory great reed warbler, LRS was about 1·5 times lower in immigrant males than in residents (estimated from Fig. 1 in Bensch et al. 1998). However, the differences in fitness between philopatric and immigrants found in the Great reed warbler may be biased due to parent–offspring resemblance in dispersal behaviour (Doligez & Pärt 2008; see also below). In great tit, LRS did not differ between resident and immigrant males, whereas LRS was higher in resident females (Verhulst & van Eck 1996). In the cooperative breeding red-cockaded woodpeckers there was a negative relationship between dispersal distance and lifetime production of recruits in females, but not in males (Pasinelli et al. 2004). However, it should be noted that the study focused on dispersers only, and excluded resident individuals from the analyses. In long-tailed tit females, only immigrants produced recruits, whereas among males LRS were not related to dispersal (Maccoll & Hatchwell 2004). Using initial banding age as a measure of philopatry Wheelwright & Mauck (1998) found that more philopatric male savannah sparrows had higher LRS. In black kites, LRS decreased with dispersal distance in males (Forero et al. 2002). Thus, in the species studied to date, there is either a negative relationship or a lack of relationship between LRS and dispersal behaviour in males. However, there are some methodological issues that need to be considered. First, except for the study on great reed warblers, genetic parentage was not used in assignment of parentage. If, in species where extra-pair paternity occur, immigrant males have a higher risk of being cuckolded by their mate and reproductive success is not based on genetic information, then reproductive success of immigrants may be overestimated. For example, although the effect of dispersal category on extra-pair paternity in the great reed warbler was not tested (Hansson et al. 2004), song repertoire size was related to success in obtaining to extra-pair fertilizations (Hasselquist, Bensch & von Schantz 1996), and repetoire size was in turn larger in philopatric males than in immigrant males (Hansson et al. 2004). Thus, the magnitude of the effect of dispersal on reproductive success must be treated with some caution when parentage is based on social information only (e.g. Nilsson 1989; McCleery & Clobert 1990; Verhulst & van Eck 1996; Wheelwright & Mauck 1998; Marr et al. 2002; Marr 2006). Second, it could be argued that if the tendency to disperse is a heritable trait, then immigrants will produce offspring that are more likely to disperse themselves and possibly disperse out of the study area (Doligez & Pärt 2008). If so, the lower production of recruits by immigrant males within the study area would not only be due to a lower number of mates and shorter life span, but may be partly an artefact from offspring that escape our sampling. The underestimation of fitness in immigrants will depend on the study area size in relation to the distribution of dispersal distances and the level of parent–offspring resemblance (Doligez & Pärt 2008). In the studies above that found a differences in LRS between philopatric and dispersing individuals (in great reed warbler, savannah sparrow, black kite, red-cockaded woodpecker and great tit), biased fitness estimates due to non-random dispersal behaviour cannot be ruled out (Doligez & Pärt 2008). In this study, pairs consisting of at least one immigrant parent were not more likely to produce recruits that dispersed and were observed within the study area (0 of 31 offspring) than pairs consisting of two resident individuals (23 of 273 offspring). It should be noted that also parent–offspring resemblance, which we use here to infer if bias in observed fitness is likely, may itself be biased for the same reason – because long-distance dispersers out of the study area are censored. However, if there was a true difference in dispersal distances between offspring from residents and offspring from immigrants, then one would expect to see a difference also in observed (non-censored) dispersal behaviour, which is not the case in this study (Tufto et al. 2005). Together, the lack of parent–offspring resemblance, the large study area in relation to dispersal distance, and no difference in detection rate between residents and immigrants, makes bias in local estimates of recruitment and life span, due to dispersal category unlikely. Nevertheless, to test the robustness of the difference in LRS between residents and immigrants in this study, we accounted for an underestimation of 20% (the largest bias for passerines presented in table 2 in Doligez & Pärt 2008) of LRS for immigrant males compared to resident males, by using an offset argument in the model. Also when correcting for a bias, that has to be considered unrealistically high in our study system, the fitness of immigrant males was still significantly lower than for resident males.

It has been suggested that a reduction in one fitness component in immigrants could be compensated through an increase in another component (e.g. fecundity vs. survival; see e.g. Lemel et al. 1997; Belichon et al. 1996). In the current study, we have measured several components related to fitness (onset of breeding, mating success, number of eggs, number of fledglings, number of recruits, survival rate, life span) and in neither sex we were able to detect any compensation. In blue tits, female immigrants produced fewer but heavier fledglings, which were suggested to be more prone to disperse and have a lower probability of recruit in the study area (Julliard, Perret & Blondel 1996). However, this mechanism is not a very likely explanation for the lower number of recruits produced by immigrants and observed within our study area. First, a previous study in the population found no relationship between the probability of natal dispersal and body condition or body mass (Altwegg et al. 2000). Second, in this study, offspring morphology is not related to parental dispersal category (results not shown).

If lack of familiarity is the major cause of the reduced LRS in immigrant males, one could expect the effect to be strongest the first breeding season in the new area and then decrease with increased experience (i.e. age). However, the negative effect of being an immigrant male did not differ between first and subsequent breeding seasons in our study. Although not statistically tested, Bensch et al. (1998) described a similar pattern, where the annual production of fledglings remained consistently lower in immigrant males irrespective of age. The lack of change in the effect of dispersal category despite increased experience may have two non-mutually exclusive explanations. First, lack of familiarity in the first breeding season may have consequences that persist through subsequent breeding seasons. In house sparrows, there is a tendency for individuals to utilize the same nest repeatedly (Anderson 2006). Thus, if immigrant males are more likely to obtain a low-quality nest the first year, a low overall turnover of nest sites may constrain the options to improve nest quality. Second, there may be consistent phenotypic differences between immigrants and residents in traits affecting reproductive success. Although residents and immigrants are morphologically similar, other phenotypic traits not measured in this study (e.g. immune function; Snoeijs et al. 2004) may differ between residents and immigrants, and correlate with fitness.

In females, lifetime production of eggs, fledglings and recruits did not depend on dispersal category, which agrees with most other studies on birds that have quantified LRS (recruits: Bensch et al. 1998; Hansson et al. 2004; Forero et al. 2002; Wheelwright & Mauck 1998; fledglings: Marr et al. 2002). These results are expected in a male resource defence system, such as in the house sparrow, where females benefit less from philopatry than males do (Greenwood 1980). In a study on introduced house sparrows, the probability of dispersal was related to wing length in females (Skjelseth et al. 2007). However, similar to Altwegg et al. (2000) we could not detect any differences in morphology between immigrant and resident females (Pärn et al., unpublished data). Interestingly, we found a positive interaction between age and dispersal category in females: the annual production of eggs and fledglings increased with age, but only for immigrants. This suggest that the effect of increasing age and familiarity on annual reproduction was stronger among immigrants. Similarly, in collared flycatcher, reproduction of long- and short-dispersing females differed only in their first breeding year (Pärt 1991). Dispersing females in these species may be constrained in their search and choice of mate and nest site, due to unfamiliarity and possibly due to late arrival to the new area. In blue tits on the other hand, the increase in clutch size from age 1 to age 2 was similar in residents and immigrants (Julliard et al. 1996). In males, there was no significant interaction between age and dispersal category on fitness. This suggests that in house sparrow the negative effect of unfamiliarity on fitness seem to persist through life.

It should be noted that this study has focused on fitness consequences of between-island dispersal only. Shorter distance, within-island dispersal was not considered. However, the selective forces acting on dispersal may depend on dispersal distance. Thus, causes and consequences of dispersal on different spatial scales deserves further studies (Ronce et al. 2001).

Many models of the evolution of dispersal take a cost of dispersal into account, often modelled as a lower probability to survive the dispersal phase. However, these models assume that immigrants and residents have similar fitness once settled (see Cohen & Motro 1989; Johnson & Gaines 1990; Lemel et al. 1997; and references therein). Similarly, an underlying assumption of many models on gene flow and its evolutionary consequences is that immigrants and residents have similar fitness (see e.g. Whitlock & McCauley 1999; Whitlock 2001; Wang 2004).

This study has provided evidence, using lifetime production of recruits as estimate of fitness, that the relative fitness contribution of immigrants to future population growth is sex-specific and may be much lower for immigrant than for resident males. Thus, care must be taken when assuming no demographic differences among dispersers and residents when modelling the dispersal process. In fact, we believe that such demographic differences together with density dependent effects (Sæther, Engen & Lande 1999a) may be crucial for our understanding of evolution of dispersal and for the effects of immigration on gene flow, and dynamics and persistence of natural populations (Hanski 2001; Whitlock 2001).


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We want to thank: Ben Sheldon, Blandine Doligez and three anonymous reviewers for comments; our dedicated fieldworkers; H. Ellegren, S. C. Griffith, L. K. Larsen and S. Skjelseth for help with laboratory analyses; I. Herfindal for making the map, and the inhabitants in our study area for their hospitality. This study was funded by the Norwegian University of Science and Technology (NTNU), the Norwegian Research Council [Storforsk, Strategic University Program (SUP) in Conservation Biology], the Norwegian Directorate for Nature Management, and the EU-commission (METABIRD).


  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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