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Keywords:

  • clutch manipulation;
  • clutch removal;
  • conservation management;
  • egg harvest;
  • Falco punctatus;
  • fecundity;
  • fostering;
  • Mauritius kestrel;
  • supplemental feeding;
  • survival

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Life-history theory assumes that trade-offs exist between an individual's life-history components, such that an increased allocation of a resource to one fitness trait might be expected to result in a cost for a conflicting fitness trait. Recent evidence from experimental manipulations of wild individuals supports this assumption.
  • 2
    The management of many bird populations involves harvesting for both commercial and conservation purposes. One frequently harvested life-history stage is the egg, but the consequences of repeated egg harvesting for the individual and the long-term dynamics of the population remain poorly understood.
  • 3
    We used a well-documented restored population of the Mauritius kestrel Falco punctatus as a model system to explore the consequences of egg harvesting (and associated management practices) for an individual within the context of life-history theory.
  • 4
    Our analysis indicated that management practices enhanced both the size and number of clutches laid by managed females, and improved mid-life male and female adult survival relative to unmanaged adult kestrels.
  • 5
    Although management resulted in an increased effort in egg production, it reduced parental effort during incubation and the rearing of offspring, which could account for these observed changes.
  • 6
    Synthesis and applications. This study demonstrates how a commonly applied harvesting strategy, when examined within the context of life-history theory, can identify improvements in particular fitness traits that might alleviate some of the perceived negative impact of harvesting on the long-term dynamics of a managed population.

Introduction

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

Life-history theory assumes that trade-offs exist between different measures of an individual's life-history components (Stearns 1992). On this basis individuals are assumed to allocate resources to competing activities based on evolutionary strategies that maximize lifetime fitness (Roff 1992; Stearns 1992). Therefore an increased allocation of resources to one fitness trait might be expected to result in a cost for a conflicting fitness trait. Conversely a reduction in resource allocation to one fitness trait could lead to an increase in resource allocation to another. Recent evidence from experimental manipulations of wild bird populations supports this concept. An increased investment in current fecundity in lesser black-backed gulls Larus fuscus and great tits Parus major (Nager, Monaghan & Houston 2001; Visser & Lessells 2001) and increased reproductive effort in Eurasian kestrels Falco tinnunculus (Daan, Deerenberg & Dijkstra 1996; Dijkstra et al. 1990) resulted in reduced future survival. A reduction in current reproductive effort was found to benefit future survival in black-legged kittiwakes Rissa tridactyla (Golet, Irons & Estes 1998).

Wildlife management is now a global activity, with wild bird populations managed for the purposes of commercial exploitation, biodiversity conservation and pest control. Notable examples include the harvesting of sooty shearwaters Puffinus griseus (Hamilton & Moller 1995), common murres Uria aalgae, eider Somateria mollissima and Atlantic puffins Fratercula arctica (Denlinger & Kenton 2001), the restoration of the kakapo Strigops habroptilus (Bell & Merton 2002), pink pigeon Columba mayerii (Jones et al. 1999) and Californian condor Gymnogyps californianus (Meretsky et al. 2000) and the control of populations of red billed quelea Quelea quelea (Jones et al. 2000) and great cormorants Phalocrocorax carbo sinensis (Frederiksen, Lebreton & Bregnballe 2001). Frequently management activities focus on the exploitation/manipulation of a particular life-history stage. The common practice of egg harvesting is a good example of this, as it is implemented both for commercial (Denlinger & Kenton 2001; Feare & Doherty 2004) and conservation (Jones et al. 1999; Kuehler & Lieberman 2000; Litzbarski 2000; Armstrong 2001) purposes. In the short term both the benefits, i.e. an economically valuable resource and additional stock, and costs to the population, i.e. a reduced birth rate, of egg harvesting are immediately apparent. However, the long-term consequences of this repeated activity, at the level of the individual and the population, remain poorly understood. Life-history theory indicates that if a particular fitness trait, such as fecundity, is manipulated (via egg harvesting) then there will be benefits or costs applicable to other fitness traits. By adjusting the natural balance of resource allocation, management potentially alters not only individuals’ life histories but also the long-term dynamics of the population. Currently, documented examination of the consequences of repeated egg harvesting is limited but required if wildlife managers are to improve their management strategies and practices.

In our study we explored the consequences of repeated harvesting of eggs and associated management, conducted as part of a species’ recovery programme, on an individual's future fitness within the context of life-history theory. Egg harvesting might be expected to increase the reproductive cost through the production of additional eggs. In contrast, brood-rearing costs might be reduced through reduced reproductive success and manipulation of the parental investment in the brood. If the latter effect was to predominate then we would expect improved performance in managed (harvested) individuals, but if the costs of additional eggs were prevalent then we would expect poorer performance in managed individuals. We used empirical data collected (1987–2004) on a successfully reintroduced population of the Mauritius kestrel Falco punctatus Temmink, which experienced a suite of management practices, including egg harvesting, for a period of 6 years during the population's establishment. As management activities occurred during the population's early development, the majority of the data come from kestrels that (i) did not experience management and (ii) had no temporal overlap with those that did. There was therefore potential for temporal variation in fitness traits in response to the increasing population density (0–45 breeding pairs). In order to explore this we examined two subsets of data: (i) kestrels that either experienced management or did not but overlapped temporally and (ii) all kestrels independent of management or temporal distribution within the study period. The former allowed us to explore temporal vs. management-induced variation in fitness traits, while the latter maximized the use of all complementary data. The kestrels in the Bambous mountains in Mauritius are an ideal model study system for this work because the management they experienced is typical of that employed in threatened species’ recovery programmes and the demographic detail on the population is so complete.

Methods

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

study species and population

The study was conducted in the Bambous mountain range (57°42′ E, 20°20′ S) on the east coast of the Indian Ocean island of Mauritius. As part of a recovery programme the Mauritius kestrel was reintroduced into this mountain range in 1987, more than 30 years after its last documented sighting (Jones et al. 1995). The reintroduction was a success and the population developed rapidly throughout the 1990s, and has remained relatively stable at 40–45 breeding pairs since 2000. Mauritius kestrels are principally monogamous, nesting in traditional natural and artificial cavities. The male provides all food for the pair throughout courtship, egg laying and incubation, and provisions the chicks and female during the first 3 weeks of the nestling period. The earliest eggs (clutch size two–five) are laid in September and the latest fledglings (brood size one–four) leave the nest in late February. Breeding seasons are therefore referred to by the two calendar years they span, for example 1987–88. Mauritius kestrels fledge at around 35 days old, achieve independence at around 85 days old and are capable of breeding at 1 year old. Kestrel pairs are capable of laying a replacement clutch (second) of eggs should the first clutch ‘fail’ early enough in the breeding season.

conservation management

As part of the species’ recovery programme the Bambous kestrel population was intensively managed between 1987–88 and 1993–94. Conservation management techniques applied to the population included nest box provision, predator control, egg harvesting, chick fostering and supplemental feeding (Jones et al. 1991, 1995). Following the initial releases of captive-reared individuals into the Bambous mountains, breeding pairs quickly became established. These pairs acted as a source for the release stock, through the harvesting of clutches, for additional supplementations and reintroductions elsewhere on Mauritius. The annual selection of pairs from which clutches were harvested was based on the required release stock for that year. The age, experience, prior fecundity and management history did not influence the selection of pairs for harvesting. The only apparent trend in the harvesting strategy was in the proportion of first clutches harvested as the population developed. This declined from 80–100% (1988–89 to 1991–92) to 40–55% (1992–93 to 1993–94). A total of 12 females and 12 males had at least one clutch harvested. Thirty-five first clutches were harvested after experiencing at least 10–12 days of natural incubation. In nine instances the harvested clutch was either substituted with ‘dummy’ eggs (which were later exchanged for fostered chicks) or chicks >10- days old were ‘fostered’ immediately. Where dummy eggs were substituted for a harvested clutch, they were incubated for no longer than the natural incubation period. Mauritius kestrel chicks were hatched and hand-reared in captivity prior to being fostered. Part of the management technique of fostering chicks to wild pairs included the provision of supplemental food on a regular basis. All pairs that raised fostered chicks received supplemental feed (equivalent, at least, to the requirement of the brood) during both the nestling and post-fledging periods up to independence. Where fostering did not occur pairs usually laid a second clutch. Twenty-five second clutches were laid following the harvesting of the first clutch. Only one of these was harvested and substituted with fostered chicks. The progress of 24 second clutches was allowed to proceed naturally, i.e. without any management. In any one breeding season a pair therefore experienced (with one exception) either the harvesting of the first clutch or harvesting of the first clutch followed by the fostering of chicks or no management at all. For 11 out of 12 females the initial clutch laid during their documented reproductive life span was harvested and chicks fostered in nine instances. For seven females this occurred at the age of 1 and for two at the age of 2 years old. For managed female kestrels these data, in relation to age, are summarized in Table 1a. For managed male kestrels (with one exception) fostering occurred at the age of 1 and 2 years old (Table 1b). Managed kestrels were therefore hindered, in most cases, from natural successful reproduction (i.e. fledging wild-bred young) while aged 1 and 2 years old.

Table 1.  The number of clutches laid by and the type and frequency of conservation management activities experienced by Mauritius kestrels at different ages; (a) females and (b) males
Clutch number and management actionAge (years)
1 23456–8
(a) Females
First clutches laid711119612
First clutches: unmanaged0 2 22311
First clutches: natural fledglings0 0 112 6
First clutches: harvested only0 7 973 0
First clutches: harvested and chicks fostered7 2 000 1
Second (replacement) clutches laid0 7 974 4
Second clutches: unmanaged 6 974 4
Second clutches: natural fledglings 1 742 0
Second clutches: harvested and chicks fostered 1 000 0
(b) Males
First clutches laid6 91197 5
First clutches: unmanaged0 2 223 4
First clutches: natural fledglings0 1 123 4
First clutches: harvested only1 5 774 0
First clutches: harvested and chicks fostered5 2 200 1
Second (replacement) clutches laid1 6 874 3
Second clutches: unmanaged1 5 874 3
Second clutches: natural fledglings0 2 542 0
Second clutches: harvested and chicks fostered0 1 000 0

data collection

Since the initial reintroduction the population was intensively monitored each breeding season (September–March). Each breeding season, pairs were located and data on the breeding biology of the kestrel were collected through repeat visits to breeding pairs at specific points within the breeding cycle. This enabled data to be collected on clutch details (size and laying pattern), productivity (i.e. number of fledglings produced) and management practices. Identities of breeding birds were recorded. Where cavities were inaccessible and provided that the breeding attempt was successful, only productivity could be ascertained with any level of accuracy.

Individual kestrels were identifiable through a unique combination of colour rings on one tarsus and a numbered aluminium ring on the other. All released individuals and more than 93% of wild-bred fledglings were marked accordingly while still in the nest. Data on survival were collated from resightings of marked individuals, recorded each breeding season over the course of the study period (1987–88 to 2003–04). Individual encounter histories were constructed from these resightings, with the breeding season as the time interval. The starting point for an individual's history was fledging. Over the course of the study some individuals lost one or both colour rings. In order to confirm their identities they were trapped, identified from the numbered aluminium ring, and the colour rings replaced.

sex determination

As part of the annual monitoring programme of the kestrel in the Bambous mountains, chicks were ringed in the nest cavity (where accessible) and a series of morphometric measurements taken. The Mauritius kestrel exhibits reverse sexual size dimorphism (Jones 1987) and this can be seen in chicks at age 2 weeks and older (M. A. C. Nicoll, C. G. Jones & K. Norris, personal observations). Using principally body mass (g) and tarsus length (mm) and comparing the within-brood variation it was feasible for the observer to sex individuals while still in the nest. Sex was recorded for 319 wild-hatched fledglings (while still in the nest) between 1991–92 and 2000–01. One-hundred and thirty-nine of these were observed as adults and their sexes reconfirmed (1991–92 to 2003–04). Of these 139 kestrels, the sex of 132 was the same as that recorded as a chick (94·97% accuracy). Using a Cohen's Kappa test statistic (Titus, Mosher & Williams 1984), thereby accounting for the proportion of agreement that might occur by chance, the accuracy was estimated at 90·0% (Kappa = 0·899). For kestrels released as part of the reintroduction, sex was known prior to the release.

Additional details of the study area, study population, monitoring programme and biology of the kestrel are provided in Jones & Owadally (1985) and Nicoll, Jones & Norris (2003, 2004).

statistical analyses

Our analysis was designed to explore the long-term effects of management on survival and fecundity. To do this we divided kestrels into two groups, according to the two definitions below, those that experienced management (managed) and those that had not (unmanaged), split them according to sex and then compared certain life-history components of individuals that received management with those that did not.

For the purpose of this study the following two important definitions were made. Managed females were those female kestrels that experienced the harvest of at least one first clutch of their eggs, between 1988–89 and 1993–94. Managed males were those male kestrels that were part of a pair that experienced the harvest of at least one first clutch of eggs, between 1988–89 and 1993–94.

Survival

Resighting histories for 264 female and 256 male Mauritius kestrels were constructed from the survival data collected between 1987–88 and 2003–04. For each sex resightings for released and wild-bred kestrels were pooled as an individual's origin was found to have no significant effect on its subsequent survival (Nicoll, Jones & Norris 2004). Individual kestrels were assigned to one of two groups, those that were managed, i.e. experienced egg harvesting, and those that were not. An initial global model was constructed, which included age-structured (e.g. juveniles and adults) survival (Φ) and resighting probability (p) and a management (mg) effect whereby Φ and p varied between managed and unmanaged kestrels. The model was as follows:

  • Φ(1: 2−5+ · mg)p(1: 2−5+ · mg)

The age structure was based on prior analyses that indicated juvenile (first year) survival was time dependent while adult survival was on average higher and constant (Nicoll, Jones & Norris 2003, 2004). Four adult age classes (2, 3, 4 & 5 years old), denoted by 2–5+ in the model, were included based on the average life span of 67 kestrels for which the maximum age was known, 4·1 ± 0·32 years. Juvenile survival (1 year old) was independent of management actions as these were only applied to breeding kestrels. We tested for a gross effect of management on adult survival by comparing the global model with a reduced model, which excluded the management effect in adult survival.

Egg harvesting and fostering occurred principally during the early part of an individual's reproductive lifetime. It is therefore plausible that any influence of management actions on adult survival may be age-specific rather than generic. For example, survival could be management-dependent in mid-life (3–4 years) but management-independent for 2 and 5 year olds. In order to test this hypothesis we graphed the survival estimates for the two groups of adult kestrels generated from the global model and examined them for evidence of age-specific similarities in survival estimates (Fig. 1a,b). We then compared a reduced model, which incorporated any identified age-specific similarities in survival, with the global model. This approach was applied to each sex in turn.

image

Figure 1. Age-specific survival estimates for female and male kestrels; diamonds, managed; circles, unmanaged. (a) Estimates of survival generated from the global model (model 3, Table 2) for female kestrels; (b) Estimates of survival generated from the global model (model 5, Table 3) for male kestrels; (c) Estimates of survival generated from the model (model 1, Table 2) exploring age-specific management effects in female kestrels; (d) Estimates of survival generated from the model (model 4, Table 3) exploring age-specific management effects in male kestrels. Vertical error bars represent 95% confidence intervals.

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All model fitting and parameter estimation was conducted using Program mark (version 4.1; White & Burnham 1999). Corrected Akaike's information criterion (AICc) in mark (Cooch & White 2001) and likelihood ratio tests (LRT) were used to select the most parsimonious model from a set of candidate models and test hypotheses, respectively. For one model to be selected above another the difference in AICc (ΔAICc in MARK) should be > 2 (Anderson & Burnham 1999) but if ΔAICc is < 2 then the suite of models with similar AICc should be considered. LRT were used to test between models that excluded a management effect and those with an age-specific management effect.

Clutch size

Clutch size (referring to first clutches only from this point on) was normally distributed (Kolmogorov–Smirnov normality test, P > 0·15) and its relation to age of female and management practices explored using a general linear modelling (GLM) framework in the statistical software minitab (version 14.1; minitab 2003). We used a GLM framework to (i) build an age-dependent model for clutch size and (ii) use the resulting model to test hypotheses on the influence of management practices on clutch size. Data on 12 managed females and 95 unmanaged females were used. The analysis was repeated for clutch size and male kestrels. Egg harvesting occurred during a period in the population's development when kestrel density was relatively low (< 20 monitored pairs). To explore the possibility that a difference in clutch size between managed and unmanaged female kestrels might be density-dependent we repeated the analysis using a subset of seven unmanaged individuals that temporally overlapped with harvested kestrels (i.e. were hatched 1991–92). No significant variation in clutch size between managed and unmanaged females would be indicative of density-dependence.

Results

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

conservation management

The impact of the management activities on the frequency of clutches laid and the duration of the incubation and nestling period are shown in Table 2. Management, in general, appeared to increase the number of clutches laid (per individual per season) and reduced both the incubation and nestling period.

Table 2.  The impact of management activities experienced by Mauritius kestrels on the number of clutches laid, the incubation period, the nestling period and fledgling production for clutches laid between 1988–89 and 1993–94
Management activityClutches female−1 year−1Incubation period (days)Nestling period (days)Productivity clutch−1 (fledglings)
  • *

    Fostering was accompanied by supplemental feeding during the nestling and fledgling dependency period at the nest site.

  • This value represents the incubation period only for those clutches that were harvested. All clutches that were laid following harvesting were left unmanaged.

  • Values taken from the literature (Jones 1987; Jones & Owadally 1988; Nicoll 2004); all other values are sample means ± 1 SE.

Harvesting and fostering*1·20 ± 0·1322·20 ± 3·4620·80 ± 0·592·30 ± 0·21
Harvesting only1·88 ± 0·1013·92 ± 1·3432–350·94 ± 0·20
None1·11 ± 0·0828–3032–350·90 ± 0·24

Where first clutches were harvested and pairs were allowed to recycle without interference from management, the incubation period for the first clutch was significantly reduced (Table 2) relative to the standard incubation period (one-sample t-test, t25 = −11·25, P < 0·001, against the null hypothesis of 29 days). Where first clutches were harvested and followed by the fostering of chicks, the incubation period was again reduced (Table 2) relative to the standard incubation period but not significantly (one-sample t-test, t9 = −1·96, P < 0·086, against the null hypothesis of 29 days). Where chicks were fostered (10–12 days old), following the harvesting of a clutch, the subsequent nestling period was significantly reduced by an average of 38% (Table 2) relative to the documented period (one-sample t-test, t10 = −21·43, P < 0·001, against the null hypothesis of 33·5 days). Note data on the incubation and nestling periods were not available with sufficient precision for the control nests (unmanaged) therefore documented estimates of both of these periods were used for comparison. Annually, females that experienced management laid significantly more clutches than those that did not experience management (two-sample t-test, t55 = −5·05, P < 0·001).

survival

Our analysis, conducted to explore the influence of management on adult female kestrel survival (Table 3), provided strong evidence for an age-specific effect (model 1; Table 3). The influence of management on survival between the ages of 1–2 and 2–3 was significant: likelihood ratio test, χ2 = 6·067, P= 0·048, general model – model 1 (Table 3) vs. the reduced model – model 2 (Table 3).

Table 3.  Models used to test the effect of management practices (egg harvesting and fostering) on survival (Φ) for adult female Mauritius kestrels using a Corwack-Jolly-Seber (CJS) modelling framework. Models are ordered according to Akaike's information criterion (AICc), with model parsimony increasing with decreasing AICc. The sin link function was used for the running of all models in program mark
ModelAICcΔAICcAICc weightsParametersDeviance
  • Φ, apparent survival of adults and juveniles; p, resighting probability; 1, represents time-dependent juvenile Φ and constant p for 1-year-old kestrels; 2, 3, etc., represent age classes; –, parameter estimates for the ages between the specified age limits are all different; × mg, management effect included in the model, but applies only to the ages in the specified age range.

  • *

    Global model with the full age structure and a management effect in both Φ and p.

1{Φ(1: 2–3 · mg, 4–5+) · p(1: 2–5+ · mg)}1109·4500·0000·61131354·511
2{Φ(1: 2–5+) · p(1 : 2–5+ · mg)}1111·0701·6200·27229360·578
3{Φ(1: 2–5+ · mg) × p(1: 2–5+ · mg)}*1112·9633·5140·10533353·546

Management practices appeared to have a positive effect on the age-specific estimates of adult female survival (Fig. 1c). Female kestrels that experienced management showed a greater improvement in survival compared with unmanaged females between the ages of 1 and 2. This was followed by a decline in survival with increasing age. Unmanaged females’ survival also improved between the ages of 1 and 2. The survival of 4- and 5-year-old female kestrels was not influenced by management. For a 1-year-old female kestrel the probability of reaching 5 years of age was 0·575 if it experienced management and 0·378 if it did not.

Our analysis conducted to explore the influence of management on adult male kestrel survival (Table 4) provided strong evidence for an age-specific effect (model 4; Table 4). The influence of management on survival between the ages of 1–2 and 2–3 years was significant: likelihood ratio test χ2 = 8·872, P= 0·012, general model – model 4 (Table 4) vs. the reduced model – model 6 (Table 4). Management practices appeared to have a positive effect on age-specific adult male survival (Fig. 1d). Male kestrels that experienced management showed a greater improvement in survival compared with unmanaged males between the ages of 1 and 2. Survival reached a peak at 3 years of age, before declining with increasing age for managed males. Unmanaged males’ survival improved between the ages of 1 and 4. The survival of 4- and 5-year-old male kestrels was not influenced by management. For a 1-year-old male kestrel the probability of reaching 5 years of age was 0·542 if it experienced management and 0·296 if it did not.

Table 4.  Models used to test the influence of management practices (egg harvesting and fostering) on survival (Φ) for adult male Mauritius kestrels using a CJS modelling framework. Models are ordered according to Akaike's information criterion (AICc), with model parsimony increasing with decreasing AICc. The sin link function was used for the running of all models in program mark
ModelAICcΔAICcAICc weightsParametersDeviance
  • Φ, apparent survival of adults and juveniles; p, resighting probability; 1, represents time-dependent juvenile Φ and constant p for 1-year-old kestrels; 2, 3, etc., represent age classes; –, parameter estimates for the ages between the specified age limits are all different; · mg, management effect included in the model, but applies only to the ages in the specified age range.

  • *

    Global model with the full age structure and management effect in both Φ and p.

4{Φ(1: 2–3 · mg, 4–5+ =) · p(1: 2–5+ · mg)}892·82800·70131275·689
5{Φ(1: 2–5+ · mg) × p(1: 2–5+ · mg)}*896·7143·8860·10033275·029
6{Φ(1: 2–5+) · p(1: 2–5+ · mg)}897·1914·3630·07929284·561

clutch size

Managed v. unmanaged

For all female kestrels, clutch size exhibited an age-dependent improvement in size (age, F1,302 = 5·76, P < 0·017) followed by an age-dependent decline in size (age2, F1,302 = 4·21, P < 0·041). There was strong evidence for variation between managed and unmanaged female kestrels in their age-dependent patterns of clutch size. For managed female kestrels the pattern of age-dependent improvement in clutch size differed significantly from that for unmanaged female kestrels (age * management interaction term, F2,302 = 5·26, P < 0·006), as did the ensuing age-dependent decline (age2 * management interaction term, F2,302 = 4·95, P < 0·008). Mean age-specific clutch sizes for both managed and unmanaged females are shown in Fig. 2.

image

Figure 2. Mean age-specific clutch sizes for all female kestrels that experienced egg harvesting (closed circles, managed) (quadratic model y=−0·099x2 + 0·776x + 2·445) and those that did not (open circles, unmanaged) (quadratic model y=−0·008x2 + 0·111x + 2·943). For managed females, age 7 represents 7–8 years olds, and for unmanaged females, age 8 represents 8–11 years olds. Sample sizes are in parentheses. Vertical error bars represent ± 1 SE.

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An age-dependent pattern in clutch size was clearly evident for male kestrels. As for females there was an increase in size with age (age, F1,272 = 26·62, P < 0·001) followed by a decline in size with age (age2, F1,272 = 20·64, P < 0·001). For managed male kestrels the pattern of age-dependent improvement in clutch size differed significantly from that for unmanaged male kestrels (age * management interaction term, F2,272 = 13·36, P < 0·005), as did the ensuing age-dependent decline (age2 * management interaction term, F2,272 = 10·06, P < 0·006). Mean age-specific clutch sizes for both managed and unmanaged males are shown in Fig. 3.

image

Figure 3. Mean age-specific clutch sizes for male kestrels that experienced egg harvesting (closed circles, managed) (quadratic model y=−0·091x2 + 0·788x + 2·299) and those that did not (open circles, unmanaged) (quadratic model y=−0·025x2 + 0·256x + 2·765). For managed males, age 6 represents 6–8 years olds, and for unmanaged males, age 9 represents 9–11 years olds. Sample sizes are in parentheses. Vertical error bars represent ± 1 SE.

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Density-independence

For the 19 female kestrels (hatched 1991–92) that temporally overlapped in their reproductive life span, clutch size was strongly age-dependent, increasing with age (age, F1,75 = 10·13, P < 0·002) before declining in older age classes (age2, F1,75 = 9·17, P < 0·003). There was strong evidence for variation between managed (12) and unmanaged (seven) kestrels in their age-dependent patterns of clutch size. For managed kestrels the pattern of age-dependent improvement in clutch size differed significantly from that for unmanaged kestrels (age * management interaction term, F2,75 = 5·64, P < 0·005), as did the ensuing age-dependent decline (age2 * management interaction term, F2,75 = 5·57, P < 0·006). This significant variation between age-specific clutch size patterns provided compelling evidence that management effects were not confounded with density. Mean age-specific clutch sizes for both managed and unmanaged females are shown in Fig. 4.

image

Figure 4. Mean age-specific clutch sizes for a subset of female kestrels (hatched 1991–92) that experienced egg harvesting (closed circles, managed) (quadratic model y=− 0·099x2 + 0·776x + 2·445) and those that did not (open circles, unmanaged) (quadratic model y=−0·208x2 + 1·809x − 0·399). For managed females, age 7 represents 7–8 years olds, and for unmanaged females, age 6 represents 6–11 years olds. Sample sizes are in parentheses. Vertical error bars represent ± 1 SE.

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Discussion

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

Our analysis has provided strong evidence for an age-dependent improvement in clutch size and survival for kestrels breeding naturally in the Bambous mountains, Mauritius. For female kestrels these improvements occurred principally between the ages of 1 and 2, while for males there was a more age-structured pattern for clutch size and survival. Kestrels that experienced management also exhibited improvements in both clutch size and survival with age. However, relative to unmanaged individuals these improvements were of a greater magnitude. Managed individuals of both sexes tended to demonstrate an improvement in clutch size and survival with age followed by a decline in older age classes to a level experienced by unmanaged kestrels of a similar age.

life-history trade-offs

The age-specific patterns exhibited by Mauritius kestrels arise from data collected as part of a long-term (ongoing) post-release monitoring programme. The detail of the data collected and the consistency of management practices employed between 1988–89 and 1993–94 provide a rare opportunity to examine the consequences of widely employed management techniques on future fitness traits of individuals.

The mark–recapture data collected between 1987–88 and 2001–02 has been used previously to explore the influence of endogenous and exogenous factors on the survival of kestrels in the Bambous mountains (Nicoll, Jones & Norris 2003, 2004). While this study incorporates an additional 2 years of mark–recapture data, the limitations and potential violations of the assumptions associated with capture–mark–recapture models (Lebreton et al. 1992) by the data remain the same and have been discussed thoroughly previously (Nicoll, Jones & Norris 2003, 2004). In brief, the data set potentially conflicts with the following assumptions: (i) every marked animal in the population at time (t) has the same probability of recapture; (ii) marks are not lost or missed; (iii) samples are instantaneous, relative to the interval between occasions (t) and (t + 1). However, the potential violation of these three assumptions is considered to have a negligible influence on the results of the modelling procedure and outcome.

The selection of pairs for management was independent of a female's prior fecundity but dependent on accessibility and the timing of the clutch in relation to conservation management actions across any given breeding season. Therefore during the period of management (1988–89 to 1993–94) some females did not have any clutches harvested or chicks fostered. It is clear from the duration of the study that management occurred during the formative period (1993–94) in the population's development, while the majority of the data collected on unmanaged kestrels comes from 1994–95. It is therefore plausible that any observed differences in fecundity between managed and unmanaged kestrels could be attributable to temporal variation in fecundity rather than management actions. For this kestrel population a negative density-dependent effect has already been documented for juvenile survival (Nicoll, Jones & Norris 2003) and the probability of breeding (Nicoll 2004). As some kestrels from the 1987–88 to 1991–92 cohorts, did not experience management, we were able to conduct an assessment of the effects of management on kestrels from within the same group of cohorts. Evidence for an effect of management on fecundity was strong, indicating that density-dependence was unlikely to account for the observed variation in age-specific fecundity between managed and unmanaged kestrels.

Life-history theory is based on the premise that trade-offs exist between an investment in current reproductive effort and other fitness traits (Stearns 1992). If current reproductive effort is increased individuals incur fitness costs and, conversely, if reproductive effort is reduced the expectation is for fitness benefits within other traits of an individual's life history. The patterns identified in the data collected on the Bambous kestrels are consistent with the trade-offs in life-history theory, with reduced current reproductive effort leading to a benefit in other fitness traits: future fecundity and survival. This is despite compensatory egg laying in response to egg harvesting. Our interpretation of these observed patterns is based on the plausible premise that the implemented management practices could manipulate an individual's reproductive effort. Did management achieve this and, if so, how?

Management applied to some Mauritius kestrels between 1988–89 and 1993–94 was geared towards a sustainable harvest of eggs to provide stock for reintroductions and population supplementations. Management increased the number and size of clutches laid per individual, thereby resulting in increased egg-laying costs for managed females relative to unmanaged females. In contrast, brood- and fledgling-rearing costs (time and food) were likely to be reduced by management, i.e. through the rearing of fostered broods or no broods at all. If egg-laying costs, relative to brood-rearing costs, were high then we might have expected a decrease in future fecundity and survival. If egg-laying costs were small, relative to brood-rearing costs, then improved future fecundity and survival might be expected. Our results were consistent with the latter and agreed with related data from experimental manipulations of Eurasian kestrels. While Daan, Deerenberg & Dijkstra (1996) found that increased brood-rearing costs reduced future adult survival, Wiehn & Korpimaki (1997) found that increased food availability during the nestling and fledgling periods reduced parental effort and benefited adult condition. In our study all managed kestrels breeding as 1 year olds (with the exception of one male) had their first clutch harvested and chicks fostered. Thus the reproductive effort for managed kestrels was reduced relative to that for unmanaged 1 year olds and could account for the elevated survival probabilities between the ages of 1 and 2 years and improved fecundity at the age of 2. Any differences in fecundity and survival between managed and unmanaged kestrels later in life could be attributed to the combined reduced reproductive effort experienced as 1 year olds and any subsequent management as 2 year olds. The majority of managed 2 year olds either failed to rear chicks (naturally) to fledging from second clutches or reared fostered chicks successfully.

implications

From the perspective of a wildlife manager the product of a harvest has either some commercial or conservation value. While studies often examine the impact of harvesting at a particular stage in the reproductive cycle, they are typically conducted without considering the consequences in terms of life-history trade-offs. Although not conducted as a formal experiment, our results are consistent with life-history theory. As a consequence of harvesting, populations are likely to experience short-term potential costs in productivity but our data suggest potential compensation (at best in part) by benefits to future survival and fecundity. However, as eggs are relatively easy units to harvest it may be possible to exceed accidentally the limit that a population can sustain (Kokko, Lindstrom & Ranta 2001) despite any compensation associated with the harvesting regime. Therefore harvesting strategies might benefit from carefully considering the consequences of their actions within the context of life-history theory.

Acknowledgements

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

The Mauritius kestrel recovery programme was funded by The Mauritian Wildlife Foundation, The Peregrine Fund, The Durrell Wildlife Conservation Trust and The National Parks and Conservation Service, Government of Mauritius. We would like to recognize the contributions made by the many individuals who facilitated and carried out the management of both the wild and captive kestrel populations. Invaluable comments and suggestions were made on an earlier draft of the manuscript by Tim Benton and Hanna Kokko. Further comments were provided by Tim Coulson and an anonymous referee. M. A. C. Nicoll was supported by funding from the Natural Environmental Research Council, UK.

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  6. Discussion
  7. Acknowledgements
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