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1. Predation risk drives long‐term shifts in migratory behaviour and demography in a large herbivore population

   https://doi.org/10.1111/1365-2656.14022  

   Williams, S. ; Hebblewhite, M. ; Martin, H. ; Meyer, C. ; Whittington, J. ; Killeen, J. ; Berg, J. ; MacAulay, K. ; Smolko, P. ; Merrill, E. H. ( November 2023 , Journal of Animal Ecology)

   Migration is an adaptive life‐history strategy across taxa that helps individuals maximise fitness by obtaining forage and avoiding predation risk. The mechanisms driving migratory changes are poorly understood, and links between migratory behaviour, space use, and demographic consequences are rare.

   Here, we use a nearly 20‐year record of individual‐based monitoring of a large herbivore, elk (Cervus canadensis) to test hypotheses for changing patterns of migration in and adjacent to a large protected area in Banff National Park (BNP), Canada.

   We test whether bottom‐up (forage quality) or top‐down (predation risk) factors explained trends in (i) the proportion of individuals using 5 different migratory tactics, (ii) differences in survival rates of migratory tactics during migration and whilst on summer ranges, (iii) cause‐specific mortality by wolves and grizzly bears, and (iv) population abundance.

   We found dramatic shifts in migration consistent with behavioural plasticity in individual choice of annual migratory routes. Shifts were inconsistent with exposure to the bottom‐up benefits of migration. Instead, exposure to landscape gradients in predation risk caused by exploitation outside the protected area drove migratory shifts. Carnivore exploitation outside the protected area led to higher survival rates for female elk remaining resident or migrating outside the protected area.

   Cause‐specific mortality aligned with exposure to predation risk along migratory routes and summer ranges. Wolf predation risk was higher on migratory routes than summer ranges of montane‐migrant tactics, but wolf predation risk traded‐off with heightened risk from grizzly bears on summer ranges. A novel eastern migrant tactic emerged following a large forest fire that enhanced forage in an area with lower predation risk outside of the protected area.

   The changes in migratory behaviour translated to population abundance, where abundance of the montane‐migratory tactics declined over time. The presence of diverse migratory life histories maintained a higher total population abundance than would have been the case with only one migratory tactic in the population.

   Our study demonstrates the complex ways in which migratory populations change over time through behavioural plasticity and associated demographic consequences because of individuals balancing predation risk and forage trade‐offs.

    

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2. Territoriality drives preemptive habitat selection in recovering wolves: Implications for carnivore conservation

   https://doi.org/10.1111/1365-2656.13199  

   O'Neil, Shawn T. ; Vucetich, John A. ; Beyer, Jr., Dean E. ; Hoy, Sarah R. ; Bump, Joseph K. ; Vander Wal, ed., Eric ( March 2020 , Journal of Animal Ecology)

   According to the ideal‐free distribution (IFD), individuals within a population are free to select habitats that maximize their chances of success. Assuming knowledge of habitat quality, the IFD predicts that average fitness will be approximately equal among individuals and between habitats, while density varies, implying that habitat selection will be density dependent. Populations are often assumed to follow an IFD, although this assumption is rarely tested with empirical data, and may be incorrect when territoriality indicates habitat selection tactics that deviate from the IFD (e.g. ideal‐despotic distribution or ideal‐preemptive distribution).

   When territoriality influences habitat selection, species' density will not directly reflect components of fitness such as reproductive success or survival. In such cases, assuming an IFD can lead to false conclusions about habitat quality. We tested theoretical models of density‐dependent habitat selection on a species known to exhibit territorial behaviour in order to determine whether commonly applied habitat models are appropriate under these circumstances.

   We combined long‐term radiotelemetry and census data from grey wolvesCanis lupusin the Upper Peninsula of Michigan, USA to relate spatiotemporal variability in wolf density to underlying classifications of habitat within a hierarchical state‐space modelling framework. We then iteratively applied isodar analysis to evaluate which distribution of habitat selection best described this recolonizing wolf population.

   The wolf population in our study expanded by >1,000% during our study (~50 to >600 individuals), and density‐dependent habitat selection was most consistent with the ideal‐preemptive distribution, as opposed to the ideal‐free or ideal‐despotic alternatives.

   Population density of terrestrial carnivores may not be positively correlated with the fitness value of their habitats, and density‐dependent habitat selection patterns may help to explain complex predator–prey dynamics and cascading indirect effects. Source–sink population dynamics appear likely when species exhibit rapid growth and occupy interspersed habitats of contrasting quality. These conditions are likely and have implications for large carnivores in many systems, such as areas in North America and Europe where large predator species are currently recolonizing their former ranges.

    

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3. Sex, body size, and winter weather explain migration strategies in a partial migrant population of American Kestrels

   https://doi.org/10.1093/ornithology/ukad019  

   Ranck, Sadie C. ; Garsvo, Christina M. ; Schwartz, Darin M. ; Reynard, Linda M. ; Kohn, Matthew J. ; Heath, Julie A. ( April 2023 , Ornithology)

   Given increasing evidence that climate change affects the annual cycles of birds, it is important to understand the mechanisms underlying individual migration strategies and population-level patterns in partial migrants. In this study, we found that thermoregulation (body size and winter temperatures) was a key driver of American Kestrel (Falco sparverius) migration decisions. The annual proportion of migrants in the population, however, was not explained by winter weather and may be the result of differential survival. We measured stable hydrogen isotope values (δD) of talon tissues collected from 501 breeding and overwintering birds to distinguish migrant from resident kestrels in a partially migratory population of American Kestrels in southwestern Idaho in 2013–2021. We then evaluated drivers of migration decisions by assessing potential correlates of migration strategies, whether individuals switched migration strategies between years, and whether the proportion of migrants in the population changed over time or was correlated with winter weather. Male kestrels were 1.6 times more likely to migrate than females, and in colder than average winters, smaller birds of both sexes were more likely to migrate than larger birds. Only 27% of 26 recaptured individuals showed evidence of switching their migration strategies on an annual basis. There was no temporal trend in the proportion of migrants in the population, but proportions varied between years. Interestingly, there was no association between winter minimum temperature anomalies and annual migrant proportions in the population, suggesting that differential over-winter survival, or other stochastic processes, may play an important role in population composition. As winters continue to warm, fewer kestrels may migrate and more may remain resident on breeding grounds. However, it is unclear how changes in migration strategies might affect population-level patterns and resilience to climate change.

    

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4. Variation in selective regimes drives intraspecific variation in life‐history traits and migratory behaviour along an elevational gradient

   https://doi.org/10.1111/1365-2656.13134  

   Lundblad, Carl G. ; Conway, Courtney J. ; Ardia, ed., Daniel ( November 2019 , Journal of Animal Ecology)

   Comparative studies, across and within taxa, have made important contributions to our understanding of the evolutionary processes that promote phenotypic diversity. Trait variation along geographic gradients provides a convenient heuristic for understanding what drives and maintains diversity. Intraspecific trait variation along latitudinal gradients is well‐known, but elevational variation in the same traits is rarely documented. Trait variation along continuous elevational gradients, however, provides compelling evidence that individuals within a breeding population may experience different selective pressures.

   Our objectives were to quantify variation in a suite of traits along a continuous elevational gradient, evaluate whether individuals in the population experience different selective pressures along that gradient and quantify variation in migratory tendency along that gradient.

   We examined variation in a suite of 14 life‐history, morphological and behavioural traits, including migratory tendency, of yellow‐eyed juncos along a continuous 1000‐m elevational gradient in the Santa Catalina Mountains of Arizona.

   Many traits we examined varied along the elevational gradient. Nest survival and nestling growth rates increased, while breeding season length, renesting propensity and adult survival declined, with increasing elevation. We documented the migratory phenotype of juncos (partial altitudinal migrants) and show that individual migratory tendency is higher among females than males and increases with breeding elevation.

   Our data support the paradigm that variation in breeding season length is a major selective pressure driving life‐history variation along elevational gradients and that individuals breeding at high elevation pursue strategies that favour offspring quality over offspring quantity. Furthermore, a negative association between adult survival and breeding elevation and a positive association between nest survival and breeding elevation help explain both the downslope and reciprocal upslope seasonal migratory movements that characterize altitudinal migration in many birds. Our results demonstrate how detailed studies of intraspecific variation in suites of traits along environmental gradients can lend new insights into the evolutionary processes that promote diversification and speciation, the causes of migratory behaviour, and how animal populations will likely respond to climate change.

    

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5. Perceived wintering latitude determines timing of song output in a migratory bird

   https://doi.org/10.1002/ece3.5922  

   Brewer, Dustin E ; McGill, Clint A ; Fudickar, Adam M. ( December 2019 , Ecology and evolution)

   Migratory bird populations frequently consist of individuals that overwinter variable distances from the breeding site. Seasonal changes in photoperiod, which varies with latitude, underlie seasonal changes in singing frequency in birds. Therefore, migratory populations that consist of individuals that overwinter at different latitudes with large overwintering ranges could experience within-population variation in seasonal production of song. To test the influence of overwintering latitude on intrapopulation variance in song production in the spring, we subjected two groups of Eastern Song Sparrows (Melospiza melodia melodia) from the same partially migratory breeding population to different photoperiodic schedules associated with a 1,300-km difference in overwintering location. One group remained on the natural photoperiodic schedule of the breeding site (resident group) while the other group experienced a nonbreeding photoperiod that mimicked a southern migration in the fall followed by a northern migration back to the breeding site in the spring (migratory group). We compared song output between the two groups in three different stages (nonbreeding, prebreeding, and breeding). Little singing occurred during nonbreeding stage sample dates (20 November, 6 December) for the resident group, and no singing occurred for the migrant group. During the prebreeding stage (27 January, 7 February), significantly more singing occurred in the resident group than in the migrant group. During the breeding stage (21 March, 4 April), after a simulated migration for the migrants, song output was similar in both groups. These results suggest that within-population variation in wintering latitude may contribute to variation in seasonal changes in singing behavior, which may covary with readiness to breed. Studies utilizing confirmed migrants and residents, rather than merely simulated migrants and residents, are also needed to better understand these processes. 

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