Apparent Monthly Survival Research Articles

Environmental heterogeneity modifies the link between personality and survival in fluctuating small mammal populations.

Despite numerous studies examining the fitness consequences of animal personalities, predictions concerning the relationship between personality and survival are not consistent with empirical observations. Theory predicts that individuals who are risky (i.e. bold, active and aggressive) should have higher rates of mortality; however, empirical evidence shows high levels of variation in behaviour-survival relationships in wild populations. We suggest that this mismatch between predictions under theory and empirical observations results from environmental contingencies that drive heterogeneity in selection. This uncertainty may constrain any universal directional relationships between personality traits and survival. Specifically, we hypothesize that spatiotemporal fluctuations in perceived risk that arise from variability in refuge abundance and competitor density alter the relationship between personality traits and survival. In a large-scale manipulative experiment, we trapped four small mammal species in five subsequent years across six forest stands treated with different management practices in Maine, United States. Stands all occur within the same experimental forest but contain varying amounts of refuge and small mammal densities fluctuate over time and space. We quantified the effects of habitat structure and competitor density on the relationship between personality traits and survival to assess whether directional relationships differed depending on environmental contingencies. In the two most abundant species, deer mice and southern red-backed voles, risky behaviours (i.e. higher aggression and boldness) predicted apparent monthly survival probability. Mice that were more aggressive (less docile) had higher survival. Voles that were bolder (less timid) had higher survival, but in the risky forest stands only. Additionally, traits associated with stress coping and de-arousal increased survival probability in both species at high small mammal density but decreased survival at low density. In the two less abundant study species, there was no evidence for an effect of personality traits on survival. Our field experiment provides partial support for our hypothesis: that spatiotemporal fluctuations in refuge abundance and competitor density alter the relationship between personality traits and survival. Our findings also suggest that behaviours associated with stress coping and de-arousal may be subject to density-dependent selection and should be further assessed and incorporated into theory.

Eco-evolutionary rescue promotes host-pathogen coexistence.

Emerging infectious pathogens are responsible for some of the most severe host mass mortality events in wild populations. Yet, effective pathogen control strategies are notoriously difficult to identify, in part because quantifying and forecasting pathogen spread and disease dynamics is challenging. Following an outbreak, hosts must cope with the presence of the pathogen, leading to host-pathogen coexistence or extirpation. Despite decades of research, little is known about host-pathogen coexistence post-outbreak when low host abundances and cryptic species make these interactions difficult to study. Using a novel disease-structured N-mixture model, we evaluate empirical support for three host-pathogen coexistence hypotheses (source-sink, eco-evolutionary rescue, and spatial variation in pathogen transmission) in a Neotropical amphibian community decimated by Batrachochytrium dendrobatidis (Bd) in 2004. During 2010-2014, we surveyed amphibians in Parque Nacional G. D. Omar Torríjos Herrera, Coclé Province, El Copé, Panama. We found that the primary driver of host-pathogen coexistence was eco-evolutionary rescue, as evidenced by similar amphibian survival and recruitment rates between infected and uninfected hosts. Average apparent monthly survival rates of uninfected and infected hosts were both close to 96%, and the expected number of uninfected and infected hosts recruited (via immigration/reproduction) was less than one host per disease state per 20-m site. The secondary driver of host-pathogen coexistence was spatial variation in pathogen transmission as we found that transmission was highest in areas of low abundance but there was no support for the source-sink hypothesis. Our results indicate that changes in the host community (i.e., through genetic or species composition) can reduce the impacts of emerging infectious disease post-outbreak. Our disease-structured N-mixture model represents a valuable advancement for conservation managers trying to understand underlying host-pathogen interactions and provides new opportunities to study disease dynamics in remnant host populations decimated by virulent pathogens.

Estimation of apparent survival probability of the harvestman Paranemastoma sillii sillii (Herman, 1871) from two caves

Reliable estimates of population parameters are lacking for most cave-dwelling species. This lack of knowledge may hinder the appropriate management of caves and populations of cave-dwelling species. Using monthly capture-recapture data and Cormack-Jolly-Seber models, we (i) estimated the apparent survival of individuals in two cave populations of the harvestman Paranemastoma sillii sillii (Herman, 1871) from the Mehedinti Mountains in south-western Romania; (ii) investigated temporal variation in apparent survival; (iii) tested if surface weather conditions affect apparent survival of cave-dwelling harvestmen through their influence upon cave environmental conditions and (iv) tested for sex differences in apparent survival. Our results show that the apparent monthly survival estimates were high for both studied cave populations and there was a significant sex effect on survival. Males had lower survival than females, and the survival difference between caves was larger in males than in females. Temporal (i.e., monthly) variation in apparent survival was low and the weather conditions at the surface had little influence on apparent survival as the environment inside the caves is well buffered against weather fluctuations outside the caves. Our results indicate that caves stabilize survival of facultative cave-dwelling species and may serve as microrefugia for epigean species. We suggest that caves should be considered for conservation because they may serve as a refuge for some epigean species during harsh weather conditions.

Do American dippers obtain a survival benefit from altitudinal migration?

Studies of partial migrants provide an opportunity to assess the cost and benefits of migration. Previous work has demonstrated that sedentary American dippers (residents) have higher annual productivity than altitudinal migrants that move to higher elevations to breed. Here we use a ten-year (30 period) mark-recapture dataset to evaluate whether migrants offset their lower productivity with higher survival during the migration-breeding period when they occupy different habitat, or early and late-winter periods when they coexist with residents. Mark-recapture models provide no evidence that apparent monthly survival of migrants is higher than that of residents at any time of the year. The best-supported model suggests that monthly survival is higher in the migration-breeding period than winter periods. Another well-supported model suggested that residency conferred a survival benefit, and annual apparent survival (calculated from model weighted monthly apparent survival estimates using the Delta method) of residents (0.511 ± 0.038SE) was slightly higher than that of migrants (0.487 ± 0.032). Winter survival of American dippers was influenced by environmental conditions; monthly apparent survival increased as maximum daily flow rates increased and declined as winter temperatures became colder. However, we found no evidence that environmental conditions altered differences in winter survival of residents and migrants. Since migratory American dippers have lower productivity and slightly lower survival than residents our data suggests that partial migration is likely an outcome of competition for limited nest sites at low elevations, with less competitive individuals being forced to migrate to higher elevations in order to breed.

Population stability in the endangered Fleay’s barred frog (Mixophyes fleayi) and a program for long-term monitoring

In the wake of the global decline in amphibians there is a need for long-term population monitoring. Previous research suggested that the endangered Fleay’s barred frog (Mixophyes fleayi) had recovered after a severe decline. We aimed to determine whether this recovery has been sustained and to test an example of a monitoring program that could be employed at intervals of five or more years to assess long-term population stability. We conducted capture–mark–recapture five years after the last detailed census at Brindle Creek in Border Ranges National Park, New South Wales. Frogs were captured along a 200 m creek transect between September 2013 and February 2014. We used program Mark to estimate demographic parameters of adult male frogs using two modelling approaches: robust design (RD) and the POPAN formulation of the Jolly–Seber model. Abundance was estimated at 38.2 ± 0.5 (s.e.) (RD) and 46.0 ± 2.7 (POPAN). Abundance in 2008 was estimated at 53.2 ± 10.0 (POPAN) male frogs. Estimates of apparent monthly survival over our five-month-long study were very high (RD: 1.0 ± 0.0; POPAN: 1.0 ± 0.02). Recapture estimates were also high (RD: 0.40 ± 0.07 to 0.72 ± 0.05 per session; POPAN: 0.84 ± 0.05 per month). These data suggest that the Brindle Creek population has remained relatively stable over a period of ~10 years.

Estimated Survival of Subadult and Adult Atlantic Sturgeon in Four River Basins in the Southeastern United States

AbstractPrompted by concerns about the status of Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus, in 2012 the National Oceanic and Atmospheric Administration listed one distinct population segment (DPS) as threatened (Gulf of Maine) and listed the remaining four DPSs as endangered (New York Bight, Chesapeake Bay, Carolina, and South Atlantic). To provide information for recovery planning, we estimated the survival of subadult and adult Atlantic Sturgeon in two river basins within the Carolina DPS (Roanoke and Cape Fear rivers, North Carolina) and two basins within the South Atlantic DPS (Ashepoo–Combahee–Edisto rivers [ACE], South Carolina; Altamaha River, Georgia). Estimated detection probability varied strongly by season but was similar among river basins, likely reflecting a winter migration into marine waters with minimal receiver coverage. Apparent monthly survival was very high and precisely estimated for the Roanoke River (0.985; 95% credible interval [CI] = 0.970–0.995), Cape Fear River (0.979; 95% CI = 0.971–0.986), ACE (0.989; 95% CI = 0.979–0.993), and Altamaha River (0.985; 95% CI = 0.973–0.994) basins. A pooled estimate for 87 adults from all four basins was 0.988 (95% CI = 0.982–0.992). The monthly rates implied annual apparent survival rates of 0.839 (Roanoke River basin), 0.778 (Cape Fear River basin), 0.871 (ACE basin), and 0.842 (Altamaha River basin); the pooled estimate for adults was 0.860. Our estimated survival rates were similar to other recent estimates for Atlantic Sturgeon but lower than recent estimates for several populations of Gulf Sturgeon A. oxyrinchus desotoi. Recovery of Atlantic Sturgeon in these southeastern rivers will occur more quickly if survival can be increased to a level that is consistent with published estimates of true natural mortality (0.03–0.07; annual survival ≥ 0.93).Received March 18, 2015; accepted August 26, 2015

Using novel spatial mark–resight techniques to monitor resident Canada geese in a suburban environment

Context Over the past two decades, an increase in the number of resident (non-migratory) Canada geese (Branta canadensis) in the United States has heightened the awareness of human–goose interactions. Aims Accordingly, baseline demographic estimates for goose populations are needed to help better understand the ecology of Canada geese in suburban areas. Methods As a basis for monitoring efforts, we estimated densities of adult resident Canada geese in a suburban environment by using a novel spatial mark–resight method. We resighted 763 neck- and leg-banded resident Canada geese two to three times per week in and around Greensboro, North Carolina, over an 18-month period (June 2008 – December 2009). We estimated the density, detection probabilities, proportion of male geese in the population, and the movements and home-range radii of the geese by season ((post-molt I 2008 (16 July – 31 October), post-molt II 2008/2009 (1 November – 31 January), breeding and nesting 2009 (1 February – 31 May), and post-molt I 2009). Additionally, we used estimates of the number of marked individuals to quantify apparent monthly survival. Key results Goose densities varied by season, ranging from 11.10 individuals per km2 (s.e. = 0.23) in breeding/nesting to 16.02 individuals per km2 (s.e. = 0.34) in post-molt II. The 95% bivariate normal home-range radii ranged from 2.60 to 3.86 km for males and from 1.90 to 3.15 km for females and female home ranges were smaller than those of male geese during the breeding/nesting and post-molt II seasons. Apparent monthly survival across the study was high, ranging from 0.972 (s.e. = 0.005) to 0.995 (s.e. = 0.002). Conclusions By using spatial mark–resight models, we determined that Canada goose density estimates varied seasonally. Nevertheless, the seasonal changes in density are reflective of the seasonal changes in behaviour and physiological requirements of geese. Implications Although defining the state–space of spatial mark–resight models requires careful consideration, the technique represents a promising new tool to estimate and monitor the density of free-ranging wildlife. Spatial mark–resight methods provide managers with statistically robust population estimates and allow insight into animal space use without the need to employ more costly methods (e.g. telemetry). Also, when repeated across seasons or other biologically important time periods, spatial mark–resight modelling techniques allow for inference about apparent survival.

Survival of a Mountaintop Hummingbird, the Hyacinth VisorbearerAugastes scutatus, in Southeastern Brazil

The key demographic parameters of a population of Hyacinth Visorbearer, Augastes scutatus, an endemic hummingbird of the mountaintops of southeastern Brazil, are described for the first time. The study was conducted in the Alto do Palácio region, in the Serra do Cipó National Park, southeastern Brazil. Monthly, from August 2007 to July 2009, the Hyacinth Visorbearer individuals were captured in mist nets and banded. Using mark-recapture data, we modeled monthly apparent survival (φ;) and capture probability (p), with sex and time (month) as covariates. We considered two time-since-marking (TSM) models among candidate models to control for the possible effect of transient individuals on parameter estimates. The two best-supported models (gaining 74% of summed Akaike weight) included effects of transient birds on monthly survival, with constant capture probability. The model-averaged apparent survival estimates for the first month after capture (φ;1) were considerably lower than the values for the second and later months following capture (φ;2 ). The apparent monthly survival showed difference between sexes, with females' value (φ;2 = 0.93 ± 0.03) slightly higher than males' value (φ;2 = 0.90 ± 0.03). These differences translated into distinctly higher annual survival of females (0.418 ± 0.16) than males (0.318 ± 0.12). The estimated annual capture probability was high (75%). We strongly suggest that TSM effects should be evaluated in future humming bird population studies.

Longleaf pine management practices and their impact on small mammal populations

Rapid decline and degradation of longleaf pine ecosystems in the southeastern United States are conservation concerns. Prescribed fire is the primary management activity in this fire-dependent ecosystem, but prescribed fire is under increasing scrutiny, primarily due to air quality issues. There are concerns that prescribed fire may be removed, or replaced by herbicide, as a forest management tool without adequate understanding of the ecological consequences associated with such a change in management. We conducted a capture-mark-recapture study from April 1999 to April 2002 to examine experimentally the effect of prescribed fire, herbicide application and herbicide-prescribed fire combination on apparent survival rates of cotton mouse (Peromyscus gossypinus) and cotton rat (Sigmodon hispidus) populations. There was no evidence that herbicide application affected survival of cotton mice. Evidence to support effects of prescribed fire and herbicide-fire combination on survival of cotton mice was weak, although apparent monthly survival generally increased after these treatments. There was strong evidence that prescribed fire and herbicide-fire treatments affected survival of cotton rats, but the evidence for the effect of herbicide alone on survival was weak; survival rates declined in response to all three treatments but most strongly in response to the prescribed fire treatment. Fire alone had a stronger effect than an herbicide-fire treatment in both species. Without clear understanding of their ecological impacts, alternatives to prescribed fire should be employed with caution.

Quantifying disturbance resistance in an ecologically dominant species: a robust design analysis

Disturbance is now recognized as a key ecosystem process but few studies have examined its indirect effects on individuals in a population or its relationship to ecological dominance in a community. Using an ecologically dominant small mammal population in experimentally burned habitat as a model, I empirically tested the effect of disturbance on survival, abundance and fecundity and investigated whether recently burned habitat is a population sink. I also examined the effect of fire on community diversity, particularly how fire influenced dominance by bushveld gerbils Tatera leucogaster (Peters 1852). Live trapping in the first year post-fire yielded a total of 4,774 captures of 1,076 individual bushveld gerbils in a tropical savanna in southern Africa. The robust design allowed for an investigation of the effects of fire, sex and temporal variation on survival while controlling for potential differences in detection and temporary emigration. Although there were fewer individuals in burned savanna during the first 6 months post-fire, their apparent monthly survival was not significantly lowered compared with the control, with males and females surviving equally well. Fecundity, represented by proportion of females lactating, was unaffected by fire and, overall, recently burned habitat does not appear to be sink habitat. The disturbance resistance exhibited by this species is likely a contributing factor to its ecological dominance in the area, which is subject to relatively frequent fires. Results of this study highlight the need to consider disturbance regimes when evaluating patterns of species richness and evenness in an ecosystem.

Effects of contrasting habitats on population parameters and diet of Apodemus sylvaticus (Rodentia) in south-western Iceland

Abstract Population parameters, body mass and diet of Apodemus sylvaticus were compared in two nearby study sites in south-western Iceland: open grassland and mixed woodland. Live trapping was conducted at approximately 5 week intervals from September 2004 to October 2005. In addition, mice were caught in snap-traps every month to estimate food composition from stomach contents. Estimated population density was up to 10 times higher in the woodland than in grassland but seasonal dynamics were the same, with peak density in autumn and low density in spring. Apparent monthly survival varied greatly throughout the winter in the grassland, ranging from 39% to 78%, but was stable at around 60% in the woodland. Mice fed predominantly on seeds in all seasons, with an exception during the spring when arthropod larvae were the most common food type. Food varied more by seasons than by habitats, but evidently mice from the woodland were less likely to feed on green material and showed more seasonal fluctuations in seed consumption. During mid-winter, woodland mice of both sexes were heavier than mice in the grassland. To some extent, the observed differences in density, survival, weight and diet could be reflecting differences in habitat quality in the two contrasting habitat types.

Evidence of Effects of Endemic Chytridiomycosis on Host Survival, Behavior, and Emigration: Reply to Schmidt

[Extract] In our recent publication (Murray et al. 2009), we documented a marked disease-associated reduction in apparent survival of threatened frogs infected with pathogen Batrachochytrium dendrobatidis (Bd), despite a long period since its invasion. This result raises concern for hundreds of species of amphibians globally because it demonstrates that endemic chytridiomycosis can remain virulent for long periods, potentially placing ongoing pressure on amphibian populations that may elevate extinction risk. In response, Schmidt claims the apparent monthly survival estimates of Murray et al. are likely confounded with permanent emigration from study site… . If infected and uninfected frogs have different emigration probabilities, then one would obtain different apparent survival estimates from mark-recapture analysis for Bd-positive and Bd-negative frogs. Schmidt concludes, I believe that most of difference [in survival rate between infected and uninfected animals] is caused by permanent emigration from study site.

Estimating the Impact of Disease in Species Threatened by Amphibian Chytrid Fungus: Comment on Murray et al.

Chytridiomycosis is an emerging disease of amphibians caused by the chytrid fungus Batrachochytrium dendrobatidis (hereafter Bd) (Fisher et al. 2009). Chytridiomycosis is thought to be contributing significantly to global amphibian declines (Berger et al. 1998; Stuart et al. 2004; Skerratt et al. 2007). In the Neotropics the disease has spread in a wave-like fashion (Lips et al. 2008), and arrival at previously pathogen-free sites in this region results in mass mortalities and extinctions (Lips et al. 2008). Little is known, however, about the effects of Bd after population declines have occurred. In some places, Bd persists where it has invaded (Retallick et al. 2004). Thus, amphibians may evolve resistance to this novel pathogen such that the negative effects of Bd are attenuated over time. Murray et al. (2009) address this very important question. They used a multistate mark-recapture model to estimate survival of Bd-infected and Bd-free frogs (Litoria pearsoniana) in Queensland, where Bd has been present for 30 years. The use ofmark-recapturemethods is a clear improvement over earlier studies in which survival was estimated but did not account for imperfect detectability of animals in field studies (Faustino et al. 2004; Schmidt 2004). Murray et al. found that Bd causes a substantial reduction in survival. This matters because a reduction in adult survival can lead to amphibian population declines (Schmidt et al. 2005) and because it implies that Bd remains a dangerous pathogen, such that amphibian populations are unlikely to recover from population declines caused by Bd for many decades, if ever. Here, I point out that the apparent monthly survival estimates ofMurray et al. are likely confoundedwith perma- Paper submitted November 11, 2009; revised manuscript accepted December 14, 2009. nent emigration from the study site (Sandercock 2006). If infected and uninfected frogs have different emigration probabilities, then one would obtain different apparent survival estimates from the mark-recapture analysis for Bd-positive and Bd-negative frogs.

Impact and Dynamics of Disease in Species Threatened by the Amphibian Chytrid Fungus,Batrachochytrium dendrobatidis

Estimating disease-associated mortality and transmission processes is difficult in free-ranging wildlife but important for understanding disease impacts and dynamics and for informing management decisions. In a capture-mark-recapture study, we used a PCR-based diagnostic test in combination with multistate models to provide the first estimates of disease-associated mortality and detection, infection, and recovery rates for frogs endemically infected with the chytrid fungus Batrachochytrium dendrobatidis (Bd), which causes the pandemic amphibian disease chytridiomycosis. We found that endemic chytridiomycosis was associated with a substantial reduction (approximately 38%) in apparent monthly survival of the threatened rainforest treefrog Litoria pearsoniana despite a long period of coexistence (approximately 30 years); detection rate was not influenced by disease status; improved recovery and reduced infection rates correlated with decreased prevalence, which occurred when temperatures increased; and incorporating changes in individuals' infection status through time with multistate models increased effect size and support (98.6% vs. 71% of total support) for the presence of disease-associated mortality when compared with a Cormack-Jolly-Seber model in which infection status was restricted to the time of first capture. Our results indicate that amphibian populations can face significant ongoing pressure from chytridiomycosis long after epidemics associated with initial Bd invasions subside, an important consideration for the long-term conservation of many amphibian species worldwide. Our findings also improve confidence in estimates of disease prevalence in wild amphibians and provide a general framework for estimating parameters in epidemiological models for chytridiomycosis, an important step toward better understanding and management of this disease.

Migratory songbird use of shade coffee in the Venezuelan Andes with implications for conservation of cerulean warbler

Although previous studies have demonstrated high use of shade coffee plantations by Neotropical migratory birds, we still have a poor understanding of the suitability of shade coffee plantations as habitat on the wintering grounds. We studied density, body condition, and survivorship of Neotropical migrants, with emphasis on cerulean warbler ( Dendroica cerulea), in primary forest and shade coffee plantations in Venezuela. We worked in three primary forest sites and three shade coffee plantations on the western slope of the Cordillera de Mérida of the Andes Mountains. At each site, we surveyed migrants with distance-based line transects and mist-netted and banded migrants during November–February 2005/06 and 2006/07. In addition, we estimated apparent monthly survival for cerulean warbler based on 29 color-banded individuals. Densities of migrants were 3–14× higher in shade coffee plantations than primary forest sites, even after accounting for differences in detectability. Apparent monthly survival of cerulean warblers was estimated at 97% and overwinter persistence was similarly high. In addition, cerulean warblers demonstrated high between-season fidelity, with 65% of the birds banded the first year being resighted during the second year. Interestingly, immature birds returned at nearly half the rate as did adults. Banding data indicated that body condition increased significantly as the season progressed for cerulean warblers, Tennessee warblers ( Vermivora peregrina), and American redstarts ( Setophaga ruticilla). Collectively, these data provide evidence that shade coffee plantations offer high quality wintering habitat for Neotropical migrants, including cerulean warblers.

Surviving north of the natural range: the importance of density independence in determining population size

AbstractThe relative importance of density‐dependent and ‐independent processes in determining population density has been predicted to vary according to whether the population concerned is located near the centre or the periphery of the species' range. Thus, density‐independent processes should be more pronounced near the periphery. The long‐tailed wood mouse Apodemus sylvaticus in Iceland is at the northern and western edge of its geographical range. We estimated the autumn population density in an open habitat in south‐western Iceland in 9 years out of 10 during 1996–2005 in order to monitor the annual maximum population size. Furthermore, we estimated population density and survival at c. 5‐week intervals from September 2001 to October 2003 and from September 2004 to November 2005 in order to reveal the causes of variation in maximum population size. The estimated autumn population density was low, ranging from 2.7 to 8.9 mice ha−1 while spring densities ranged from 0.4 to 0.8 mice ha−1. Apparent monthly survival probabilities ranged from 0.4 to 0.7 per month in autumn and 0.7 to 0.9 in winter. Our results suggest that low temperature in early winter (October–December) is the major determinant of population density in the following autumn, explaining 74% of the variation in autumn population density. No significant correlation was found between either the NAO index or the NAO winter index and variation in wood mouse population density in autumn. Differential mortality in early winter results in variation in spring population size. This study shows clear evidence of density‐independent control of a mammal population at the edge of its geographical range as opposed to the mostly density‐dependent control previously recorded near its centre of distribution.

Survival of Marbled Teal ( Marmaronetta angustirostris) released back into the wild

Reintroduction or re-enforcement programmes are major tools in species conservation, but there is a need for more studies that assess the influence of different husbandry and release methods on the survival of released animals. We investigated the survival of globally threatened Marbled Teal ( Marmaronetta angustirostris) taken into captivity as ducklings when they became trapped in an irrigation channel, then released again after fledging. We used wing tags and mark–recapture models to estimate the survival of released teal. Ducklings rescued in 1996 ( n = 53) were released soon after fledging in September and their survival was modelled for seven months until April 1997. Their apparent monthly survival rate (lower than true survival owing to loss of wing tags) was 0.85 ± 0.12 (±s.e.). Ducklings rescued in 1997 ( n = 44) were released together in February 1998 over five months after fledging, and their survival was modelled for six months from February until August. Their apparent monthly survival rate was 0.54 ± 0.06. Ducklings rescued in 1998 ( n = 159) were released in August–September soon after fledging and their survival was modelled for 10 months from August until June. Their apparent monthly survival rate was 0.83 ± 0.07. Monthly survival was significantly higher for the 1996 and 1998 cohort, suggesting that retaining birds in captivity after fledging had a negative impact on post-release survival. When birds were released in February, a lower proportion survived until the breeding season three months later than when they were released five months earlier in September.