Lower Apparent Survival Research Articles

Relationship between wintering site and survival in a migratory waterbird using different migration routes

When wintering at different sites, individuals from the same breeding population can experience different conditions, with costs and benefits that may have implications throughout their lifetime. Using a dataset from a longitudinal study on Eurasian Spoonbills from southern France, we explored whether survival rate varied among individuals using different wintering sites. In the last 13 years, more than 3000 spoonbills have been ringed as chicks in Camargue. These birds winter in five main regions that vary in both migratory flyway (East Atlantic vs. Central European) and migration distance (long-distance vs. short-distance vs. resident). We applied Cormack–Jolly–Seber models and found evidence for apparent survival to correlate with migration distance, but not with flyway. During the interval between the first winter sighting and the next breeding period, long-distance migrants had the lowest survival, independently of the flyway taken. Additionally, as they age, spoonbills seem to better cope with migratory challenges and wintering conditions as no differences in apparent survival among wintering strategies were detected during subsequent years. As dispersal to other breeding colonies was rarely observed, the lower apparent survival during this period is likely to be partly driven by lower true survival. This supports the potential role of crossing of natural barriers and degradation of wintering sites in causing higher mortality rates as recorded for a variety of long-distance migrants. Our work confirms variation in demographic parameters across winter distribution ranges and reinforces the importance of longitudinal studies to better understand the complex demographics of migratory species.

Body size constrains the annual apparent survival of lekking Great Snipe Gallinago media males of eastern, lowland population

Survival of adult individuals is a key demographic parameter, that is critical for conservation programs of species and evolutionary research focused on life-history traits. Here, we present the estimates of the apparent annual survival of lekking male Great Snipes of the lowland population, breeding in wet meadow habitats in the valley of the Pripyat River. We also investigate individual traits such as body size and body condition, that may influence birds’ survival. The annual apparent survival in the studied population was low (Φ (.) = 0.43), but no change in this parameter was found over the period of 22 years. The chances for survival increased with the body size of an individual. We hypothesize that smaller individuals with shorter bills might face limited foraging options compared to larger birds, which may result in insufficient nutrition during challenging life cycle events such as moulting of primaries combined with preparation for long-distance migration conducted in one non-stop flight. Additionally, the presented results indicate that the wet meadow in the Pripyat River Valley provides suitable breeding habitat for this species with sufficient conservation practices held there, and the low annual apparent survival is probably a life-history trait of this species resulting from a “high pace of life” and elevated metabolic rate. To our knowledge, this study provides the first estimates of the long-term population characteristic for the eastern, lowland population of this species, which could prove helpful in the assessment of population viability for this globally declining species.

Sex-biased infections scale to population impacts for an emerging wildlife disease.

Demographic factors are fundamental in shaping infectious disease dynamics. Aspects of populations that create structure, like age and sex, can affect patterns of transmission, infection intensity and population outcomes. However, studies rarely link these processes from individual to population-scale effects. Moreover, the mechanisms underlying demographic differences in disease are frequently unclear. Here, we explore sex-biased infections for a multi-host fungal disease of bats, white-nose syndrome, and link disease-associated mortality between sexes, the distortion of sex ratios and the potential mechanisms underlying sex differences in infection. We collected data on host traits, infection intensity and survival of five bat species at 42 sites across seven years. We found females were more infected than males for all five species. Females also had lower apparent survival over winter and accounted for a smaller proportion of populations over time. Notably, female-biased infections were evident by early hibernation and likely driven by sex-based differences in autumn mating behaviour. Male bats were more active during autumn which likely reduced replication of the cool-growing fungus. Higher disease impacts in female bats may have cascading effects on bat populations beyond the hibernation season by limiting recruitment and increasing the risk of Allee effects.

Annual captures and low apparent survival rates in two tit species in western Hungary

AbstractAdult and juvenile survival are important factors affecting the population dynamics of small passerines. Understanding variation in the population dynamics and survival rates is critical for ecological studies and nature conservation. The aim of this study was to investigate the annual capture-recapture, apparent survival and capture probabilities of the Blue TitCyanistes caeruleusand the Great TitParus majoroccurring in western Hungary. Data from 8,628 Blue Tits and 7,727 Great Tits came from a constant-effort ringing scheme, using three ringing periods, spanning 24 years (1998 to 2021). The annual captures did not show a significant linear trend from 1998 to 2021 in the study site for both tit species. The temporal variation of annual captures and the annual capture-recapture proportions of different ages and sexes of the tit species were similar. This indicated that the migration strategies of these two partial migrant species did not differ significantly. According to the best standard Cormack-Jolly-Seber model, apparent survival of first-year birds was lower than that for adults. The CJS model selection for the dataset indicated that the time and sex had no effect on apparent survival probabilities for both tit species. Capture probability in the juvenile groups was not significantly higher than that in the adult groups for both species.

Long‐term apparent survival of a cold‐stunned subpopulation of juvenile green turtles

AbstractUnderstanding the effects of extreme weather on animal populations is fundamental to ecological and conservation sciences and species management. Climate change has resulted in both warm and cold temperature extremes, including an increased frequency of severe cold snaps at middle latitudes in North America. These unusually cold air masses cause rapid declines in nearshore ocean temperatures in coastal areas, with detrimental effects on marine organisms. Acute cold‐stun events (hereafter cold stuns) occur when hundreds to thousands of resident juvenile sea turtles fail to escape shallow water during cold snaps. Human intervention through rescue and recovery largely mitigates direct juvenile sea turtle mortality, but delayed effects of cold stuns on rescued individuals are not well understood. Our objective was to examine long‐term juvenile green turtle (Chelonia mydas) survival across four cold stuns of varying severity in St. Joseph Bay, Florida, between 2010 and 2018. We used the classic Cormack–Jolly–Seber model in a hierarchical Bayesian framework to estimate apparent survival (i.e., emigration and mortality) of rescued turtles at different time intervals. Our results indicated about half of a cohort rescued during a severe cold stun in January 2010 likely remained in the population 1 year later, with 10%–20% remaining 4 years later, and as few as 5% by 2018. The results also suggested higher apparent survival for cohorts rescued during two subsequent milder cold stuns. Emigration was a more plausible ecological explanation for low apparent survival than delayed mortality. Potential ecological mechanisms underlying emigration include a reduction in food availability and a behavioral response to either the severe weather event or handling during rescue (or both). However, the typical annual turnover of juvenile green turtles, though assumed low, is not well known in St. Joseph Bay. Thus, our apparent survival estimates may be reflective of higher‐than‐expected emigration in the broader population. Our study provides important baseline information about long‐term juvenile sea turtle survival after cold stuns in temperate regions. We also highlight the importance of strategic monitoring between cold stuns to examine additional ecological questions.

Population Status of the Suwannee Alligator Snapping Turtle (Macrochelys suwanniensis) in the Suwannee River, Florida

Freshwater megafauna populations, which are declining worldwide, are well known but often overlooked and understudied compared with marine and terrestrial megafauna. One species of freshwater megafauna is the Suwannee alligator snapping turtle (Macrochelys suwanniensis), which is endemic to the Suwannee River drainage in Georgia and Florida. Several trapping studies have examined M. suwanniensis distribution, body size, and population structure, but little information exists regarding its population status. The objectives of our study were to 1) estimate population size, 2) estimate apparent survival, and 3) model population growth rates (k) by conducting a capture–mark–recapture study of M. suwanniensis in the Suwannee River in Florida. From 2011 to 2013, we repeatedly sampled 12 randomly selected 5-km sites along the Suwannee River for M. suwanniensis using baited hoop-net traps. We captured 126 individuals and had 29 recaptures. Both adult males and adult females had very high apparent survival (0.99), whereas juveniles had lower apparent survival (0.32). We estimated a population density of 6.6 turtles/river km, indicating a population of 1709 (95% CI, 1205–2694) M. suwanniensis from the town of White Springs to the upper limit of the estuary in the main stem of the Suwannee River (approximately 259 river km). We constructed 2 postbreeding census matrix population models for M. suwanniensis and incorporated parameters from this study and from the literature. Both matrix population models suggested a slightly decreasing population (k = 0.99), but because of the uncertainty around our estimates, we consider the population trend to be unclear. Elasticity analysis revealed that k was most sensitive to changes in adult survival compared with other model components. This is a conservation concern because adult M. suwanniensis may be incidentally killed by fishing gear. Our study was short-term, and our analyses had limitations; therefore, we recommend future areas of research, including long-term population monitoring.

Age-specific, population-level pedigree of wild black bears provides insights into reproduction, paternity, and maternal effects on offspring apparent survival.

Wildlife pedigrees provide insights into ecological and evolutionary processes. DNA obtained from noninvasively collected hair is often used to determine individual identities for pedigrees and other genetic analyses. However, detection rates associated with some noninvasive DNA studies can be relatively low, and genetic data do not provide information on individual birth year. Supplementing hair DNA stations with video cameras should increase the individual detection rate, assuming accurate identification of individuals via video data. Video data can also provide birth year information for individuals captured as young of the year, which can enrich population‐level pedigrees. We placed video cameras at hair stations and combined genetic and video data to reconstruct an age‐specific, population‐level pedigree of wild black bears during 2010–2020. Combining individual birth year with mother–offspring relatedness, we also estimated litter size, interlitter interval, primiparity, and fecundity. We used the Cormack‐Jolly‐Seber model in Program Mark to evaluate the effect of maternal identity on offspring apparent survival. We compared model rankings of apparent survival and parameter estimates based on combined genetic and video data with those based on only genetic data. We observed 42 mother–offspring relationships. Of these, 21 (50%) would not have been detected had we used hair DNA alone. Moreover, video data allowed for the cub and yearling age classes to be determined. Mean annual fecundity was 0.42 (95% CI: 0.27, 0.56). Maternal identity influenced offspring apparent survival, where offspring of one mother experienced significantly lower apparent survival (0.39; SE = 0.15) than that of offspring of four other mothers (0.89–1.00; SE = 0.00–0.06). We video‐documented cub abandonment by the mother whose offspring experienced low apparent survival, indicating individual behaviors (e.g., maternal care) may scale up to affect population‐level parameters (e.g., cub survival). Our findings provide insights into evolutionary processes and are broadly relevant to wildlife ecology and conservation.

Age‐ and sex‐specific survival of the Gunnison's prairie dog (Cynomys gunnisoni)

AbstractAge‐ and sex‐specific survival rates are essential for understanding animal population dynamics and discerning environmental influences on population parameters. To date, little research has been done on age‐ and sex‐specific survival of the Gunnison's prairie dog (Cynomys gunnisoni), a colonial ground‐dwelling squirrel inhabiting the sagebrush ecosystem of the southwestern United States. To discern impacts of the year of study, age, sex, and reproductive status on survival, we conducted a capture‐mark‐recapture analysis on a previously collected long‐term dataset of a population of Gunnison's prairie dogs (n = 2508) from 1989 to 1995. We implemented two models: age‐cohort Cormack–Jolly–Seber (CJS) models (to examine survival by age, sex, and year) and multistate mark‐recapture models (to analyze survival by age and year, as well as breeding status). Females had higher apparent survival than males. Yearlings had a low apparent survival rate (0.288 ± 0.022, [±SE]) compared to juveniles (0.461 ± 0.019) and adults (0.482 ± 0.055). Less than 50% (48.0 ± 2.7) of female yearlings successfully weaned litters each year. Yearling survival rate was lower than other highly social prairie dog species that do not reproduce as yearlings, suggesting that there may be a cost to reproduction. We also constructed a postbreeding matrix population model and associated elasticity analysis, which indicated that the reproductive output of yearlings was the most significant potential driver of population growth. Our findings provide more nuanced estimates for age‐ and sex‐specific survival in this species than were previously available and also more broadly inform linkages between demography and sociality in prairie dogs.

Strong evidence supporting a relationship between colour pattern and apparent survival in common crossbills

Carotenoid staining has been repeatedly shown to serve as a sexually selected individual quality signal. In different species, individuals that show brighter carotenoid-based signals have been found to have superior feeding abilities, recover faster from disease, and generally enjoy better body condition. In the common crossbill (Loxia curvirostra), the colour has also been related to the different populations, with northern and central European populations being described as redder than those in the Mediterranean region. A study in the Pyrenees showed that long-winged individuals had lower apparent survival, and the proportion of red individuals was higher in long-winged birds, concluding that they could be nomadic birds (that travel long distances). A priori, if the red crossbills are more mobile than the yellow and orange ones, their apparent survival will be lower. However, in our study, red males showed a greater survival than males of other colours and almost double than that of the yellow ones. These results suggest that red coloration is linked to higher quality individuals regardless of their mobility.

Long‐term monitoring using DNA sampling reveals the dire demographic status of the critically endangered Gobi bear

AbstractInformation about population demography is crucial for developing and implementing conservation measures. The brown bear in the Gobi desert of southwestern Mongolia (referred to as the Gobi bear) is one of the smallest and most isolated brown bear populations in the world. We conducted genetic sampling (n = 2660 samples collected) using hair corrals around feeding sites at 13 water sources during 2009, 2013, and 2017 to evaluate population size, survival, and population trend. Bears were identified using 13 microsatellite loci and one sex marker. We detected 51 unique individuals (15F and 36M) from our targeted surveys in 2009, 2013, and 2017. Based on capture–mark–recapture robust design, population estimates were 23 (95% CI: 21–32) in 2009, 28 (95% CI: 25–35) in 2013, and 31 (95% CI: 29‐38) individuals in 2017. Spatial capture–recapture analysis suggested abundance was very low ( = 27; 95% CI: 22–35), and there was no significant change from 2009 to 2017. The population density was 0.93 bears/1000 km2 (95% CI: 0.74–1.17). Our population estimates suggested a stable population trend. However, the population is still very small, and the sex ratio is skewed toward males, raising concerns for future persistence. Annual survival based on Robust design CMR was 0.85. Low abundance and apparent survival for both sexes in this unhunted population coupled with a skewed sex ratio highlight the need for on‐the‐ground conservation action to conserve this isolated population of bears.

Movement, Survival, and Delays of Atlantic Salmon Smolts in the Piscataquis River, Maine, USA

AbstractMovement, delays, and survival of hatchery Atlantic Salmon Salmo salar smolts were evaluated through the Piscataquis River, a tributary of the Penobscot River in Maine, USA. We explored the effects of the river’s four dams (Guilford, Dover, Browns Mill, and Howland dams) from 2005 to 2019. During this period, the downstream‐most dam (Howland Dam) transitioned from full hydropower generation to seasonal turbine shutdowns and later was decommissioned with the construction of a nature‐like fish bypass in 2016. We estimated survival through open‐river reaches and at each dam using acoustic telemetry (n = 1,611). Dams decreased survival, with per‐river‐kilometer (rkm) apparent survival averages of 0.972, 0.951, and 0.990 for Guilford, Dover, and Browns Mill dams compared to a per‐rkm survival of 0.999 for open‐river reaches. Turbine shutdowns increased survival at Howland Dam (to around 0.95), which was further increased by the nature‐like fish bypass (0.99). We used radiotelemetry in 2019 (n = 75) and demonstrated that approximately one‐third of the fish used the bypass, while the remaining fish used alternative routes. Smolts successfully passing the three upstream dams had lower apparent survival through Howland Dam than smolts that were released upstream of Howland Dam. Although smolts passing Browns Mill Dam had high survival, the dam caused extended delays, with median delay times surpassing 48 h in most years. Most of the delays caused by Browns Mill Dam occurred after fish had passed the dam and may indicate a sublethal effect of passage. Overall, while survival through Howland Dam has improved, passage and delays at the three upstream dams in aggregate represent a critical impediment to the effective use of the high‐quality spawning habitat found upstream.

Using Acoustic Telemetry to Estimate Weakfish Survival Rates along the U.S. East Coast

AbstractThe Weakfish Cynoscion regalis, an economically important species, has declined over the last 30 years, corresponding with an increase in total mortality according to the most recent stock assessment. We estimated estuarine‐specific and coastwide apparent survival of Weakfish by using a Cormack–Jolly–Seber model to provide insights into the spatiotemporal component of mortality. Telemetered Weakfish (n = 342) were released across five estuaries ranging from North Carolina to New Jersey between 2006 and 2016. In estuaries from Delaware Bay and northward, egress peaked around the third week of September; in North Carolina, egress peaked by the first week of November. For three estuaries with adequate sample sizes, apparent survival estimates were similar and a joint model including all telemetered Weakfish estimated an extremely low annual apparent survival rate of 0.001 (95% credible interval [CrI] = 0.002–0.0003) or annual apparent instantaneous total mortality of 7.25 (95% CrI = 6.28–8.05). At a minimum, 61% of telemetered Weakfish emigrated in the fall, but only 2 of 149 fish with long‐lived transmitters were detected as returning to estuaries the following year. This is a small proportion for a fish that exhibits spawning site fidelity. We conclude that the disappearance of telemetered Weakfish represents mortality that occurs between emigration and the spring spawning period, potentially during overwinter periods on the continental shelf. Our study provides insights into the magnitude, timing, and location of Weakfish loss and facilitates an improved understanding of Weakfish population dynamics for use in stock rebuilding.

Personality-driven life history trade-offs differ in two subpopulations of free-ranging predators.

Consistent individual differences in behaviour (i.e. personality) can be explained in an evolutionary context if they are favoured by life history trade-offs as conceptualized in the pace-of-life syndrome (POLS) hypothesis. Theory predicts that faster-growing individuals suffer higher mortality and that this trade-off is mediated through exploration/risk-taking personality, but empirical support for this remains limited and ambiguous. Equivocal support to the POLS hypothesis suggests that the link between life history and personality may only emerge under certain circumstances. Understanding personality-driven trade-offs would be facilitated by long-term studies in wild populations experiencing different ecological conditions. Here, we tested whether personality measured in semi-captivity was associated with a growth-mortality trade-off via risk-taking in the wild in two subpopulations of juvenile lemon sharks Negaprion brevirostris known to differ in their predator abundance. We expected stronger personality-driven trade-offs in the predator-rich environment as compared to the predator-poor environment. Sharks were captured yearly from 1995 onwards allowing us to obtain long-term data on growth and apparent survival in each subpopulation. We then used a novel open-field assay to test sharks for exploration personality yearly from 2012 to 2017. A subset of the tested sharks was monitored in the field using telemetry to document risk-taking behaviours. We tested (a) if fast explorers in captivity took more risks and grew faster in the wild and (b) if natural selection acted against more explorative, faster-growing sharks. In the subpopulation with fewer predators, more explorative sharks in captivity took more risks in the wild and grew faster. In turn, larger, fast-growing sharks had lower apparent survival. In the predator-rich subpopulation, despite finding selection on fast growth, we found no link between exploration personality and the growth-mortality trade-off. Our study demonstrates that the association between personality and life history is favoured in some ecological contexts but not in others. We identify predator and resource abundance as two main potential drivers of the personality-mediated trade-off and emphasize that future work on the POLS hypothesis would benefit from an approach integrating behaviour and life history across ecological conditions.

Potential sex-dependent effects of weather on apparent survival of a high-elevation specialist

Mountain ecosystems are inhabited by highly specialised and endemic species which are particularly susceptible to climatic changes. However, the mechanisms by which climate change affects species population dynamics are still largely unknown, particularly for mountain birds. We investigated how weather variables correlate with survival or movement of the white-winged snowfinch Montifringilla nivalis, a specialist of high-elevation habitat. We analysed a 15-year (2003–2017) mark-recapture data set of 671 individuals from the Apennines (Italy), using mark-recapture models. Mark-recapture data allow estimating, forgiven time intervals, the probability that individuals stay in the study area and survive, the so called apparent survival. We estimated annual apparent survival to be around 0.44–0.54 for males and around 0.51–0.64 for females. Variance among years was high (range: 0.2–0.8), particularly for females. Apparent survival was lower in winter compared to summer. Female annual apparent survival was negatively correlated with warm and dry summers, whereas in males these weather variables only weakly correlated with apparent survival. Remarkably, the average apparent survival measured in this study was lower than expected. We suggest that the low apparent survival may be due to recent changes in the environment caused by global warming. Possible, non-exclusive mechanisms that potentially also could explain sexual differential apparent survival act via differential breeding dispersal, hyperthermia, weather-dependent food availability, and weather-dependent trade-off between reproduction and self-maintenance. These results improve our current understanding of the mechanisms driving population dynamics in high-elevation specialist birds, which are particularly at risk due to climate change.

Unraveling the dynamics of a ground‐dwelling beetle population exposed to quarry exploitation and restoration practices

Quarry exploitation and restoration practices are expected to have overarching and contrasting impacts on animal communities. Although many studies describe these impacts, they generally overlook the effects on population dynamics and individual movements. We assessed the impacts of quarry exploitation and restoration activity on population dynamics, individual movement, and habitat use of a sand‐dwelling specialist beetle (Scarites cyclops). The study was performed on three plots: one adjacent to the margin of quarry exploitation, another subjected to restoration practices, and a control plot with no disturbance. A capture‐mark‐recapture approach was undertaken to estimate population parameters, movement, and habitat use. In the exploitation plot S. cyclops exhibited lower probability of recapture and lower apparent survival, as well as many movements fleeing away from quarry limits. Habitat suitability modeling showed that the exploitation plot provided better habitat conditions for the species than the restoration plot. It exhibited higher bare ground cover with scattered clumps of vegetation and higher proportions of fine sand (<0.4 mm). In the restoration plot, S. cyclops population showed a lower abundance, with a higher rate of recaptures, and a more limited dispersal ability of the individuals. There is an apparent early stage of colonization by S. cyclops in the restoration plot, but movements may already be hampered by unsuitable habitat restoration (higher herbaceous cover and different soil texture). We suggest preserving suitable habitat patches in the vicinity of the impacted areas and providing dispersal routes. Beyond vegetation, soil texture must be considered to allow local animal communities to establish in restored areas.

Low first-year apparent survival of passerines in abandoned fields in northwestern Russia

Abstract First-year survival probability of migratory passerines during the period between fledging and first reproduction is a highly variable parameter that has a major effect on population dynamics. We used a long-term mark–recapture dataset (2002–2018) to examine first-year survival of 3 passerine species breeding in abandoned agricultural fields of northwestern Russia: Booted Warbler (Iduna caligata), Whinchat (Saxicola rubetra), and Western Yellow Wagtail (Motacilla flava). We banded 3,457 nestlings, including 1,363 Booted Warblers, 1,699 Whinchats, and 395 Western Yellow Wagtails, and resighted 12 Booted Warblers, 29 Whinchats, and 13 Western Yellow Wagtails in the year after fledging. We evaluated first-year apparent survival rates using Cormack-Jolly-Seber models in MARK program within the multispecies approach. We tested effect of fledge date on the first-year apparent survival. In all focal species, first-year apparent survival rates were low and reached the lower limits known for migratory passerines. We found no differences in first-year survival rates among the 3 species: the estimated average first-year apparent survival rate of all species was 0.05 ± 0.01. The fledge date had a considerable impact on first-year survival rate: later fledge dates negatively affected first-year survival. We suggest that first-year apparent survival rates in our study were low due to low natal philopatry and high mortality in the post-fledging period. Low apparent first-year survival may be a specific feature of open-nesting birds breeding in abandoned fields that are low-quality habitats because of high predation pressure.

Population fitness has a concave relationship with migration distance in Sanderlings

In Focus: Reneerkens, J., Versluijs, T. S. L., Piersma, T., Alves, J. A., Boorman, M., Corse, C., … Lok, T. (2020). Low fitness at low latitudes: wintering in the tropics increases migratory delays and mortality rates in an Arctic breeding shorebird. Journal of Animal Ecology, 89, 691-703. A central question in migratory ecology has been to understand the fitness consequences of individual variation in migration distance among different species and populations. Reneerkens et al. (2020) investigated the demographic consequences of long-distance migration for Sanderlings Calidris alba, an Arctic-breeding species of sandpiper. Their study population has a remarkable geographic distribution with a breeding range that is concentrated in northeast Greenland and Ellesmere Island, Canada but a nonbreeding range that extends across 85° of latitude from Scotland to Namibia. The authors report on unexpected patterns of latitudinal variation in three demographic parameters: timing of passage on northward migration, probability of juvenile migration and apparent survival of adults. Sanderlings travelling 1,800-2,800km to settle at north temperate sites during the nonbreeding season hadearlier passage dates, and also higher probabilities of migration and apparent survival. In contrast, birds travelling 6,000-7,800km to equatorial sites experienced later passage dates, delayed maturity and lower apparent survival. However, if Sanderlings migrated even farther and flew over 11,000km to nonbreeding sites in Namibia, then their performance was restored to early passage dates and higher survival. Movement tracks from birds tagged with geolocators showed that birds wintering in Namibia make nonstop flights of 7,500km that bypass West Africa during northward migration. Thus, all lines of evidence suggest that Sanderlings face adversity when spending the nonbreeding season at equatorial latitudes. Moreover, the central finding that components of fitness can have nonlinear relationships with migration distance is a novel discovery that leads to many additional questions. The new findings have broader implications for theoretical models of migration, and for understanding how different patterns of movements may arise or be maintained in migratory species.

Testing four hypotheses to explain partial migration: balancing reproductive benefits with limits to fasting endurance

Seasonal migration is ubiquitous in animals, and yet its underlying cause(s) remain poorly known. Species exhibiting short-distance altitudinal migration and intraspecific variation in migratory behavior (partial or differential migration) are ideal study systems for examining the selective pressures that affect individual migratory decisions. We used an individually marked population of yellow-eyed juncos, breeding along a 1000-m elevational gradient and migrating up and down that gradient, to examine the morphological, behavioral, and reproductive traits associated with migratory behavior. We tested the four most well-known hypotheses proposed to explain partial migration: the thermal tolerance, fasting endurance, dominance, and arrival time hypotheses. Our results indicate that: (1) limits to juncos’ fasting endurance constrain their ability to overwinter at high elevations, in support of the fasting endurance hypothesis, (2) differences in body size mediate fasting ability and are associated with variation in migratory behavior and overwinter apparent survival, (3) migratory behavior interacts with reproductive success, in partial support of the arrival time hypothesis, and (4) additional mechanisms that are not captured by the four well-known hypotheses might better explain individual variation in migratory behavior. Less migratory females achieved greater nesting success the following breeding season. Among males, nesting success influenced migratory tendency the following winter. Successful males may either migrate to a more benign winter climate without paying reproductive costs, or high levels of parental effort might physiologically constrain their ability to overwinter in harsh climates. Migration is a costly behavior, so migration must yield fitness benefits, at least for some individuals. Species in which migratory behavior varies among individuals in the same population provide an under-utilized opportunity to understand the costs and benefits that mediate individual migratory decisions. By marking and tracking a population of yellow-eyed juncos, we found that larger individuals were less likely to migrate, and this held true even within a sex class (males). Smaller individuals were not only more likely to migrate but also had lower apparent survival, and these differences were driven by size-related differences in fasting ability. Less migratory females accrued reproductive benefits, but males with the highest reproductive success were not less likely to migrate as predicted by the arrival time hypothesis. Our study demonstrates that migratory decisions balance the potential reproductive benefits and survival risks of overwintering on high-elevation breeding grounds.

Population dynamics of an endangered forest bird using mark–recapture models based on DNA-tagging

Populations of vertebrates are built of individuals of different sex, age class or stage, which often affect distinctly the population dynamics. Such intrapopulation partitioning of vital rates needs to be identified to develop efficient conservation actions. Using DNA extracted from feces and feathers we combined DNA-tagging and mark–recapture analyses to evaluate sex-specific population dynamics of an endangered population of capercaillie (Tetrao urogallus; Phasianidae). We built encounter histories for 120 individuals in the mating seasons of 2009–2011, in a study area of about 424 km2. Minimum number of individuals per mating season and estimates of population size ranged 56–67 and 76–115, respectively. Estimates of population size were consistently lower in multiple-season, open-population models than in single-season closed-population models. The super-population in the study area was 149 individuals for the whole study period. Sex-ratio was notably male-biased. Probability of recapture p ranged 0.62–0.70, and was similar for males and females. Female apparent survival φ was lower than expected, and much lower than male apparent survival. It includes however movements in and out of the sampled population, thus comparison with previously reported values based on conventional tagging should be cautious. Females showed higher turnover, indicated by higher probability β of entering the sampled population, and higher number of entries from the super-population, Bgross. Realized population growth rate ʎ was > 1 for both females and males. The combination of non-intrusive DNA-tagging and the analytical framework of mark–recapture models provided inferences on population dynamics that would have been hardly feasible with conventional methods. Male-biased sex ratios, higher female turnover and seemingly low female apparent survival were our key findings. While the whole population needs continuous monitoring, we believe that adult females deserve priority attention in evaluation and design of conservation actions.

Survival and Density of a Dominant Fish Species Across a Gradient of Urbanization in North Carolina Tidal Creeks

Development in the southeastern US coastal plain generates the need for a better understanding of how survival and abundance of estuarine nekton respond to urbanization. Apparent survival and density of the dominant Atlantic coast salt marsh fish, the mummichog Fundulus heteroclitus, were estimated in four North Carolina tidal creeks using a model simultaneously fitted to mark-resight and mark-recapture data. Rates of weekly loss (mortality plus emigration) were high (~ 10%). Sampling for tagged fish within and outside of study creeks showed high site fidelity to each creek, indicating that loss largely resulted from mortality rather than emigration. Estimated rates of apparent survival were lowest in the creek with the least instream- and watershed-level impacts. This creek has direct (non-culvert) access downstream to a larger waterbody, suggesting that enhanced access by predators and/or greater rates of permanent emigration may have contributed to lower apparent survival in this creek. There was a positive relation between minnow trap catch-per-unit-effort (CPUE) and density allowing the relationship between CPUE and habitat and urbanization to be examined in a total of six creeks. The largest CPUE estimates occurred early in each growing season and were associated with creeks possessing characteristics most representative of undisturbed salt marsh mosaics: high percentage of marsh coverage instream and downstream and high percentage of marsh edge. Given generally limited movement outside of creeks, differences in abundance among creeks likely result from different levels of recruitment that are related to salt marsh availability. Priority preservation of salt marsh habitats may be warranted by natural resource planners to maintain abundance levels of this trophically important species.