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Keywords:

  • breeding density;
  • brood desertion;
  • brood movement;
  • food availability;
  • parental care

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    One of the fundamental insights of behavioural ecology is that resources influence breeding systems. For instance, when food resources are plenty, one parent is able to care for the young on its own, so that the other parent can desert and became polygamous. We investigated this hypothesis in the context of classical polyandry when females may have several mates within a single breeding season, and parental duties are carried out largely by the male.
  • 2
    We studied a precocial wader, the Kentish plover Charadrius alexandrinus, that exhibits variable brood care such that the chicks may be raised by both parents, only by the female or, more often, only by the male. The timing of female desertion varies: some females desert their brood at hatching of the eggs and lay a clutch for a new mate, whereas other females stay with their brood until the chicks fledge. Kentish plovers are excellent organisms with which to study breeding system evolution, as some of their close relatives exhibit classical polyandry (Eurasian dotterel Eudromias morinellus, mountain plover Charadrius montanus), whereas others are polygynous (northern lapwing Vanellus vanellus).
  • 3
    Kentish plovers raised their broods in two habitats in our study site in southern Turkey: saltmarsh and lakeshore. Food intake was higher on the lakeshore than in the saltmarsh as judged from feeding behaviour of chicks and adults. As the season proceeded and the saltmarsh dried out, the broods moved toward the lakeshore.
  • 4
    As the density of plovers increased on lakeshore, the parents spent more time defending their young, and female parents stayed with their brood longer on the lakeshore.
  • 5
    We conclude that the influence of food abundance on breeding systems is more complex than currently anticipated. Abundant food resources appear to have profound implications on spatial distribution of broods, and the social interactions between broods constrain female desertion and polyandry.

Introduction

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

Ecological constraints are often invoked in explaining the diversity of breeding systems (Reynolds 1996). Spatial distribution of males and females, food resources and the risk of predation have been shown to influence mating systems and parental care in vertebrates, especially fishes and birds (Davies 1991; Ligon 1999; Conway & Martin 2000; Bennett & Owens 2002; Reynolds, Goodwin & Freckleton 2002). One of the main factors proposed to promote the evolution of uniparental care and polygamy is high food availability (Davies 1991; Andersson 2005): when resources are plentiful a single parent can care for the offspring so that the other parent may desert and remate.

We investigated this hypothesis in the context of classical polyandry (reviewed by Andersson 2005). In species with classical polyandry males carry out most (or all) parental duties, whereas females may pair up with several mates within a single breeding season. Food abundance is thought to have a key role in the evolution of classical polyandry, by allowing females to lay successive clutches within a short time period for different mates (Erckmann 1983; Andersson 2005). For example, Graul (1976) argued that females only lay multiple clutches when food is abundant in mountain plover Charadrius montanus.

Waders or shorebirds (sandpipers, plovers and allies; Charadriides infraorder, excluding Laroidea, Monroe & Sibley 1993) exhibit an unusual range of breeding systems among birds (Erckmann 1983; Oring 1986; Székely & Reynolds 1995) and they harbour some of the best known cases of classical polyandry (Oring & Lank 1986; Holt, Whitfield & Gordon 2002; Emlen & Wrege 2004). A recent phylogenetic analysis identified chick development as a major constraint on their breeding system evolution (Thomas & Székely 2005): species with more demanding chicks have strict biparental care and monogamy, whereas the evolutionary transition toward less demanding young has opened the route for uniparental care and polygamy.

Offspring development, however, is unlikely to explain all variation in breeding systems (see References above), as breeding systems are often different between closely related species with similar developmental mode. For example, female Eurasian dotterel Eudromias morinellus, a small wader, usually deserts the male after clutch completion (Kålås & Byrkjedal 1984), whereas both sexes incubate in the Kentish plover Charadrius alexandrinus (Linnaeus) (Fraga & Amat 1996; Kosztolányi & Székely 2002).

Here we investigate the influence of an ecological variable, food abundance, on parental behaviour in the Kentish plover (body mass approximately 42 g). Kentish plovers nest on the ground; their chicks are precocial, and leave the nest within a day after hatching. Both parents attend the chicks at hatching, although as the chicks get older, either the male or the female may desert the brood (Warriner et al. 1986; Székely & Lessells 1993; Amat, Fraga & Arroyo 1999). Thus some broods are raised by both parents until the chicks fledge, whereas in others either only the male or only the female cares for the chicks. Kentish plovers are predominantly monogamous for a given brood (only 4·6% of broods contain extra-pair young; Blomqvist et al. 2002), although the deserting parent often remates and renests (27–37% of deserting females; Warriner et al. 1986; Székely & Williams 1995).

We investigated Kentish plovers in southern Turkey where they raise their young in two habitats: on the lakeshore and in the saltmarsh. We had two objectives: (1) to determine the distribution of broods between the two brood-rearing habitats over the breeding season, and (2) to evaluate the consequences of brood movement on parental care. First, we show that the broods move between these habitats, and broods hatched late in the season are concentrated on the lakeshore. Second, we test a hypothesis for why broods move toward the lakeshore: the saltmarsh provides less food than the shore, particularly late in the breeding season when it dries out. Third, if competition for food is more intense on lakeshore, particularly late in the season, then this should influence the parental behaviour and the timing of brood desertion. Specifically, we predicted that females, which desert the brood more often than males, will stay with their chicks for longer on the shore than in the saltmarsh to assist the male defending the chicks from neighbouring parents.

Materials and methods

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

study area and field methods

Fieldwork was carried out at Lake Tuzla (36°43′ N, 35°03′ E), southern Turkey (Székely & Cuthill 1999; Székely, Cuthill & Kis 1999) in 4 years (1996: 13 April−16 July; 1997: 15 April−30 June; 1998: 8 April−10 July; 1999: 14 April−8 July). The Kentish plovers reared their chicks on the north shore of Lake Tuzla (‘shore’ henceforward), and in alkaline grassland of Salicornia europaea and Antrochnemum fruticosum (Uzun et al. 1995) situated in a 50–800 m strip on the north side of the lake (‘saltmarsh’ henceforward). Shore also included mudflats along the lake edge and edges of shallow temporary pools. Saltmarsh also included cultivated fields with short vegetation that bordered the study area on the north. The distance between the two habitats was variable, the shortest distance was about 50 m while the longest was about 1 km. The distribution of these two habitats is not related to field sites mentioned in previous studies of the same population (see Székely & Cuthill 1999, 2000; Székely et al. 1999).

Both parents were caught and ringed with a metal ring and a unique combination of colour rings. Parents were caught by funnel traps either during late incubation or just after hatching of the clutch. Chicks were ringed either in the nest scrape, or if they had already left the nest we ringed them at the first encounter. The age of the latter broods was estimated using the length of their right tarsus (see Székely & Cuthill 1999). We studied 144 broods altogether, and we visited each brood every other day until the chicks died or reached the age of 25 days. Kentish plover chicks fledge at about 29 days of age (Cramp & Simmons 1983). During these visits the number and sex of attending parent(s), and the type of brood-rearing habitat were recorded. Behavioural records were taken for 1–2 h every fourth–sixth day by scanning the behaviour of parents and of chicks every 30 s (see Székely & Cuthill 1999). Brood-rearing habitat was also recorded during behavioural observations. In 1998 we estimated the density of plovers by recording the number of Kentish plovers (including adults and chicks, except the focal brood) in a 25 m radius around the focal brood every 5 min. Ambient temperature was measured to the nearest 0·1 °C at ground level at the end of behavioural observations.

statistical analyses

We considered each brood as the unit of analysis. If more than one record was available for a brood, we took the mean of these records. Varying samples sizes in the analyses are due to missing values. Dates were calculated as the number of days since 1 March each year. Hatching dates and dates of behavioural observations did not differ between years (one-way anovas, hatching date: F3,140 = 0·746, P = 0·527; date of behavioural observations: F3,140 = 0·405, P = 0·749), thus we did not consider year-effects in the analyses. Nevertheless, we tested the robustness of our conclusions by including year in all models of temperature, plover density and behaviour (results not shown); in all cases our conclusions remained unaltered.

Brood-rearing habitat was calculated in two comparable ways. First, in the analysis of movement of broods and duration of biparental care, we treated it as a continuous variable, i.e. brood-rearing habitat was the proportion of time each family spent on the shore during brood visits. Second, habitat was treated as a dichotomous variable in the analyses of behavioural samples. For those broods that spent some time in both habitats, the habitat where they spent more time was selected, and we included only those behavioural records in the analyses that were collected in the corresponding habitat.

Kentish plovers are insectivorous (Cramp & Simmons 1983), feeding with a characteristic run-and-peck style by taking food items from the ground, vegetation or water. They may also catch prey, such as flies, from the air. We use feeding efficiency (no. of pecks × 102 × (no. of pecks + no. of runs)−1) as a measure of food abundance, where the number of pecks and number of runs were calculated from the behavioural records (see above). This measure appears to be a better indicator of foraging success than percentage of time pecking, because it reflects the effort the plover puts into gaining a prey item. Furthermore, observations of foraging rates may provide a better estimate of resource abundance than standard arthropod sampling methods, as the arthropod samples do not necessarily reflect prey availability (Palmer, Lane & Bromley 2001). Plover density was calculated as number of plovers × ha−1. Brooding by a parent was defined as per cent of time the parent brooded at least one chick, whereas brooding of a chick was defined as per cent of time when the chick was brooded by a parent. Fighting and feeding were defined as per cent of time a bird spent on fighting and feeding, respectively.

We analysed plover density and behavioural data by General Linear Models (GLMs). For the analyses of plover density and feeding efficiency of parents we included habitat (factor), observation date (covariate), and their interaction in the models. Feeding behaviour, fighting, brooding and chick feeding efficiency may also be influenced by the age of the chicks, thus brood age (covariate) was also included in GLMs of these variables in addition to habitat, observation date and the habitat × observation date interaction. Behavioural variables (brooding, fighting, feeding and feeding efficiency) were arcsine-square root transformed (Sokal & Rohlf 1995). The variance of plover density was not constant between the habitats; therefore, we used a gamma error distribution in the analyses of plover density (Crawley 2002). Brooding, fighting and feeding were also analysed by manovas; we quote the F- and P-values associated with Wilks’λ statistic, and the probability from univariate F-tests (Norušis 1994).

Over half (55·5%) of the broods (n = 110 broods) were biparental, the female deserted in 40·9% of broods and the male deserted in 3·6% of broods. Desertion by males was thus rare, so we investigated the duration of parental care only for females. Duration of biparental care (response variable, in days) was analysed by a single GLM whereby both brood-rearing habitat (proportion of time on the shore, covariate) and observation date (covariate) were included in the model. All broods in which either the female deserted the brood, or both parents stayed with their chicks for at least 25 days were included in the GLM. The latter analysis, however, may be biased because it did not include those broods that died while the female still attended the brood. Thus we also analysed duration of biparental care by Cox regression (Norušis 1994), in which the response variable was the duration of biparental care, the terminal event was desertion by the female, and both habitat and observation date were covariates. Censored cases included (1) broods that were cared for by both parents over 25 days, and (2) broods that died while both parents attended. Note that in Cox regression a negative sign of the parameter estimate indicates a decrease in the probability of the event, i.e. desertion by female.

We carried out several experimental manipulations during the study (e.g. Székely & Cuthill 1999, 2000), and these manipulations may potentially influence our results presented here. However, these manipulations did not influence the behaviour of parents (manovas, 1996–99, males: Wilks’λ = 0·671–0·918, P = 0·373–0·980; females: Wilks’λ = 0·705–0·751, P = 0·094–0·595), the behaviour of their chicks (Wilks’λ = 0·832–0·959, P = 0·427–0·972), and the use of brood-rearing habitat (one-way anovas, 1996–99, P = 0·201–0·924) in the current dataset, and thus manipulations are not analysed further.

We also tested whether the location of brood (‘sites’ in the terminology of Székely & Cuthill 1999, 2000; Székely et al. 1999; unrelated to the saltmarsh/shore dichotomy) may influence the movement of broods and the behaviour of parents and their chicks. However, by including sites in statistical analyses, our conclusions were not changed (results not shown).

Ambient temperature may be different between the two type of habitats, and may influence the movement of families especially when the chicks are young and do not have well developed thermoregulation (Visser & Ricklefs 1993). Nevertheless, ambient temperature is unlikely to explain the movement of broods, because neither the slopes nor the intercepts of temperature over the breeding season were different between the habitats (GLM on log10(x + 1) transformed data, habitat: F1,140 = 0·234, P = 0·629; observation date: F1,140 = 44·748, P < 0·001; habitat × observation date: F1,140 = 0·109, P = 0·742).

Statistical analyses were carried out using R 1·7·1 for Windows (Ihaka & Gentleman 1996) and SPSS for Windows 8·0 (Norušis 1994). In all GLMs we used Type III sum of squares. Two-tailed probabilities and mean ± SE are given.

Results

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

movement of broods

One hundred and sixteen broods of 144 were observed in both habitats; 22 broods stayed only in the saltmarsh, whereas six stayed only on the shore. The first observed move of 95 of 116 broods was toward the shore (binomial test with P = 0·5 hypothesized probability, P < 0·001). Broods spent more time on the shore as the season proceeded (slope of the least-squares regression line: B = 0·005 ± 0·002, n = 116, P = 0·001).

The movement of broods toward the lake influenced the densities of plovers that was estimated by the number of plovers around the focal family: there were more plovers on the shore (7·8 ± 1·63 plovers × ha−1, n = 22) than in the saltmarsh (2·8 ± 0·33 plovers × ha−1, n = 23, GLM with gamma errors including only the habitat effect, F1,43 = 17·028, P < 0·001). The density of plovers increased over the breeding season on the shore, whereas the density was unrelated to observation date in the saltmarsh (Fig. 1). A significant interaction term between habitat and observation date indicated that the difference between habitats was particularly strong later in the breeding season (GLM with gamma errors, F1,41 = 7·124; P = 0·011).

image

Figure 1. Seasonal variation in density of Kentish plovers (no. of individuals × ha−1) around focal broods on the shore (solid circles and solid line) and in the saltmarsh (open circles and broken line, slopes estimated by GLMs with gamma errors separately for the two habitats, shore: B = 0·51 ± 0·204, n = 22, P = 0·021, saltmarsh: B = 0·023 ± 0·025, n = 23, P = 0·372).

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factors influencing the movement of broods

Prey availability, as indicated by feeding efficiency of parents and their chicks, showed a consistent pattern (Fig. 2). First, feeding efficiency was higher on the shore (males: 32·5 ± 1·79, n = 72; females: 25·4 ± 1·85, n = 68; chicks: 27·1 ± 1·10, n = 76) than in the saltmarsh (males: 14·0 ± 1·58, n = 58, t128 = 7·614, P < 0·001; females: 9·7 ± 1·19, n = 55, t121 = 6·844, P < 0·001; chicks: 12·2 ± 0·94, n = 68, t142 = 9·553, P < 0·001). Second, feeding efficiency decreased marginally significantly in the saltmarsh over the season, whereas it tended to increase on the shore (Fig. 2). The latter effect was also indicated by the significant (or marginally so) interactions between habitat and observation date (GLMs, males: F1,126 = 5·421, P = 0·021; females: F1,119= 3·417, P = 0·067; chicks: F1,139 = 2·764, P = 0·099).

image

Figure 2. Feeding efficiency of male (A), female (B) and chick (C) Kentish plovers on the shore (solid circles and solid line), and in the saltmarsh (open circles and broken line). Slopes of least-squares regression line separately for the two habitats in males (shore: B = 0·002 ± 0·002, n = 72, P = 0·189, saltmarsh: B = −0·003 ± 0·002, n = 58, P = 0·054), females (shore: B = 0·002 ± 0·002, n = 68, P = 0·422, saltmarsh: B = −0·003 ± 0·002, n = 55, P = 0·057) and chicks (shore: B = 0·002 ± 0·001, n = 76, P = 0·160, saltmarsh: B = −0·002 ± 0·001, n = 68, P = 0·065). Feeding efficiency was arcsine-square root transformed.

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behaviour of parents and their chicks

Parents spent more time fighting on the shore than in the saltmarsh (Table 1), and the difference between habitats tended to increase in females as the breeding season progressed (Fig. 3, Table 2). To separate the effects of density from observation date, we used GLMs in which fighting behaviour was the response, and both plover density and observation date were explanatory variables. These models showed that fighting behaviour remained correlated with plover density (parameter estimates for slope, males: B = 0·006 ± 0·003, n = 44, P = 0·082; females: B = 0·011 ± 0·004, n = 40, P = 0·004), whereas observation date was nonsignificant (males: B = 0·001 ± 0·002, P = 0·716; females: B = −0·0001 ± 0·002, P = 0·962).

Table 1.  Behaviour of Kentish plover parents and their chicks on the shore and in the saltmarsh (mean ± SE percentage of time)
 ShoreSaltmarshtP
  • *

    Number of broods on the shore and in the saltmarsh, respectively.

Males (72, 58)*
Fighting11·9 ± 1·23 6·2 ± 0·654·003< 0·001
Feeding12·8 ± 1·00 5·0 ± 0·716·650< 0·001
Brooding 7·5 ± 1·2821·2 ± 2·505·618< 0·001
Females (68, 55)*
Fighting10·3 ± 0·98 5·9 ± 0·813·691< 0·001
Feeding 7·5 ± 0·71 2·5 ± 0·406·574< 0·001
Brooding12·6 ± 1·6423·2 ± 2·603·439  0·001
Chicks (76, 68)*
Feeding15·4 ± 0·79 6·2 ± 0·579·320< 0·001
Brooding13·7 ± 1·6533·0 ± 2·706·028< 0·001
image

Figure 3. Behaviour of male and female Kentish plovers on the shore (solid circles and solid line) and in the saltmarsh (open circles and broken line): fighting (A and B), feeding (C and D) and brooding (E and F). Slopes of least-squares regression line separately for the two habitats in males (shore n = 72, saltmarsh n = 58; fighting, shore: B = −0·002 ± 0·002, P = 0·117; saltmarsh: B = −0·001 ± 0·001, P = 0·561; feeding, shore: B = 0·001 ± 0·001, P = 0·418; saltmarsh: B = −0·002 ± 0·001, P = 0·063; brooding, shore: B = 0·001 ± 0·002, P = 0·726; saltmarsh: B = −0·004 ± 0·002, P = 0·095), and females (shore n = 68, saltmarsh n = 55; fighting, shore: B = 0·003 ± 0·001, P = 0·049; saltmarsh: B = −0·001 ± 0·001, P = 0·377; feeding, shore: B = 0·001 ± 0·001, P = 0·214; saltmarsh: B = −0·002 ± 0·001, P = 0·004; brooding, shore: B = −0·004 ± 0·002, P = 0·057; saltmarsh: B = −0·002 ± 0·003, P = 0·422). Variables were arcsine-square root transformed.

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Table 2.  Behaviour of Kentish plover parents and their chicks in relation to brood-rearing habitat, date of observation and brood age (variable d.f., error d.f.)
 Habitat F (P)Observation date F (P)Brood age F (P)Observation date × habitat F (P)
Males
Multivariate analysis (3, 123)3·014 (0·033)1·575 (0·199) 5·587 (0·001)2·741 (0·046)
Univariate analyses (1, 125)
 Fighting (r2 = 0·141)2·815 (0·096)3·016 (0·085) 0·858 (0·356)1·283 (0·260)
 Feeding (r2 = 0·290)0·653 (0·420)0·519 (0·473) 1·758 (0·187)2·760 (0·099)
 Brooding (r2 = 0·309)6·521 (0·012)0·462 (0·498)16·036 (< 0·001)4·328 (0·040)
Females
Multivariate analysis (3, 116)1·759 (0·159)1·178 (0·321) 5·329 (0·002)3·506 (0·018)
Univariate analyses (1, 118)
 Fighting (r2 = 0·140)2·155 (0·145)0·711 (0·401) 0·431 (0·513)3·518 (0·063)
 Feeding (r2 = 0·314)3·288 (0·072)0·513 (0·475) 0·037 (0·847)7·509 (0·007)
 Brooding (r2 = 0·224)0·100 (0·753)2·884 (0·092)16·206 (< 0·001)0·039 (0·843)
Chicks
Multivariate analysis (2, 138)1·868 (0·158)1·555 (0·215)13·662 (< 0·001)2·959 (0·055)
Univariate analyses (1, 139)
 Feeding (r2 = 0·482)0·651 (0·421)0·001 (0·977)20·112 (< 0·001)3·431 (0·066)
 Brooding (r2 = 0·306)2·038 (0·156)2·861 (0·093)15·817 (< 0·001)0·881 (0·349)

Males, females and their chicks spent more time on feeding on the shore than in the saltmarsh (Table 1), and the difference between habitats tended to increase over the breeding season (Figs 3 and 4, Table 2). In contrast, brooding time was higher in the saltmarsh than on the shore (Figs 3 and 4, Table 1).

image

Figure 4. Behaviour of Kentish plover chicks on the shore (solid circles and solid line) and in the saltmarsh (open circles and broken line): feeding (A), brooding (B). Slopes of least-squares regression line separately for the two habitats (shore n = 76, saltmarsh n = 68; feeding, shore: B = 0·002 ± 0·001, P = 0·042; saltmarsh: B = −0·001 ± 0·001, P = 0·128; brooding, shore: B = −0·003 ± 0·002, P = 0·211; saltmarsh: B = −0·003 ± 0·002, P = 0·124). Variables were arcsine-square root transformed.

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These results were consistent with multivariate tests that showed significant, or marginally significant, interactions between observation date and habitat in males (F3,123= 2·741, P = 0·046), females (F3,116 = 3·506, P = 0·018) and chicks (F2,138 = 2·959, P = 0·055, for full models see Table 2).

brood desertion by females

Duration of biparental care was influenced by both the time spent on the shore and observation date (Fig. 5; GLM with both covariates entered). First, the duration of biparental care increased with the time spent on the shore (F1,78 = 11·192, P = 0·001). Second, the duration of biparental care increased over the breeding season (F1,78 = 21·798, P < 0·001).

image

Figure 5. The length of biparental care in relation to the time spent on shore (A), and observation date (B, ▵, broods deserted by female; +, broods attended by both parents until the age of 25 days). Residual biparental care was calculated by regressing the duration of biparental care against either the observation date (A) or time spent on the shore (B), and taking the residuals from the respective least-squares equations. The slopes of least-squares regression line are (A) B = 6·97 ± 2·287, n = 81, P = 0·003, and (B) B = 0·22 ± 0·052, n = 81, P < 0·001. See text for GLM.

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These results were fully consistent with survival analysis of biparental care. First, the duration of biparental care increased with the time spent on the shore (Cox regression, proportion of time spent on the shore: B = −1·96 ± 0·586, Wald = 11·154, d.f. = 1, P < 0·001). Second, biparental care lasted longer in late broods than in early ones (observation date: B = −0·04 ± 0·012, Wald = 14·210, d.f. = 1, P < 0·001). As both time spent on shore and observation date were included in the same Cox regression model, both associations are significant after controlling for each other's effects.

Discussion

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

Our study of a precocial wader species with variable parental care provided two key results. First, Kentish plovers more often hatched young on the saltmarsh but subsequently moved with their young toward the shore, and this movement is fully consistent with the estimated changes in food availability. Food availability, as inferred by feeding efficiency of males, females and their chicks, was consistently higher on the shore than in the saltmarsh, and the difference between these habitats increased over the breeding season. As the saltmarsh has dried out by late June, this habitat ceased to be a profitable feeding site. Our results are thus consistent with observations in a congeneric wader species, the piping plover Charadrius melodus that rears brood in those habitats where arthropod abundance is high (Loegering & Fraser 1995; Elias, Fraser & Buckley 2000).

Second, females stayed longer with their brood on the shore than in the saltmarsh, and this effect was independent of timing in the season. This result is important, as it suggests that the distribution of resources influences the timing of female desertion. Resource abundance is often invoked to explain brood desertion (see Székely et al. 1996), and from a theoretical perspective we may expect less or more desertion with increasing food abundance (Webb et al. 2002). First, poor resources and low body condition may force a parent to abandon the nest and forgo breeding, as has frequently been inferred in seabirds and penguins (Chaurand & Weimerskirch 1994; Olsson 1997). Second, parents may capitalize from rich food resources as these allow them to secure a new mate and renest (Oring 1986; Székely et al. 1996), thus more desertion may be expected with increasing food abundance.

Our results in the current study suggest a third route for the relationship between offspring desertion and resources, as enhanced resources lead to increased competition between families. Fights between neighbouring families can be vicious, so that the chicks are injured or killed occasionally (A. Kosztolányi & T. Székely, pers. obs.). Therefore theoretical analyses of parental care should not only take into account how resources may influence the behaviour of parents in a given brood, but should consistently analyse the effects of resources on all broods in the population.

Furthermore, food abundance may also affect the number of breeders in a population. The deserting parents often remate and renest in the Kentish plover (Warriner et al. 1986; Székely & Lessells 1993; Amat et al. 1999), thus desertion influences the number of unmated individuals in the population. Therefore resources not only influence brood desertion, but they also induce changes in local mating opportunities (Székely, Webb & Cuthill 2000).

Our results are consistent with a cross-population study of R.F. Freudenthal & C.M. Lessells (unpublished manuscript) that compared the time-budget and duration of biparental care between a high-density population of Kentish plover in Portugal and a low-density one in Hungary. They noted that the frequency of fighting is higher in Portugal, and the duration of biparental care is significantly longer. Taken together, the consistent results between our results from the Turkish population and the cross-population study of Freudenthal & Lessells (unpubl.) strengthen the notion that brood desertion relates to density of plovers on the brood-rearing site. Similarly, in semipalmated plovers Charadrius semipalmatus those parents that raised their chicks on the coast fought more than inland parents, probably because the density of broods was higher on the coast (Blanken & Nol 1998).

Parents spent more time on brooding in the saltmarsh than on the shore, and the saltmarsh chicks received more brooding from their parents. We suggest two explanations for this result. First, the chicks in the saltmarsh were undernourished and grew more slowly than chicks on the shore (A. Kosztolányi, T. Székely & I.C. Cuthill, unpublished data). As the ability of chicks to regulate their body temperature increases with their body mass (Visser & Ricklefs 1993), these small undernourished chicks needed more brooding. Second, if a brood was only temporarily on the shore, it was more profitable both for the parents and their chicks to feed while on the shore, and defer brooding until they went back to the saltmarsh.

Our results raise two questions. First, are the growth and/or survival of plover chicks influenced by the brood-rearing habitat? For instance, we may predict that the enhanced food supply allows rapid growth and better brood survival. However, this effect may be counter balanced by enhanced crowding so that brood survival may be actually lower on the shore than in saltmarsh. In a different study we test these predictions (A. Kosztolányi, T. Székely & I.C. Cuthill, unpublished data). Second, if more food is available on the shore than in the saltmarsh, then why do some broods grow up in the saltmarsh? The competitive ability of parents may not be equal, so some parents may be unable to take their chick to the shore, even if this would be beneficial for the young (A. Kosztolányi, T. Székely & I.C. Cuthill, unpublished data). Alternatively, the costs of habitat change may simply be high. For example, when Kentish plovers move between territories, they are attacked by residents. Such attacks may lead to chick loss, or injury (A. Kosztolányi & T. Székely, pers. obs.).

In conclusion, we show that broods of a precocial wader, the Kentish plover, frequently move between habitats in search of food, and these movements profoundly influence plover densities. These results, we believe, strongly suggest that rich food actually constrains female desertion and polyandry in the Kentish plover. Female behaviour appears to be a phenotypic response to local variations in food availability. Experimental studies, however, are needed to establish whether the relationships we report here between brood movements and parental care are indeed driven by changes in resource abundance.

Acknowledgements

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

The project was funded by grants from NERC (GR3/10957, to A.I. Houston, ICC & J.M. McNamara), BBSRC (BBS/B/05788 to TS, ICC, A.I. Houston & J.M. McNamara), Hungarian Scientific Research Fund (T043390, to TS), by Eötvös Scholarship of the Hungarian State to AK, by Royal Society/NATO Postdoctoral Fellowship to AK. Rings were provided by the late P. Evans (Durham University, UK) and Radolfzell Vogelwarte (Germany). We thank the assistance of J. Kis, Á. Lendvai, I. Szentirmai and Y. Demirhan in the field. Permission for the field work was provided by the Turkish Ministry of National Parks, Tuzla Municipality and Mr E. Karakaya, the Governor of Karatas District.

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