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1. Clinging to the top: natal dispersal tracks climate gradient in a trailing-edge population of a migratory songbird

   https://doi.org/10.1186/s40462-024-00470-0  

   Gaya, Heather E. ; Cooper, Robert J. ; Delancey, Clayton D. ; Hepinstall-Cymerman, Jeffrey ; Kurimo-Beechuk, Elizabeth A. ; Lewis, William B. ; Merker, Samuel A. ; Chandler, Richard B. ( April 2024 , Movement Ecology)

   Trailing-edge populations at the low-latitude, receding edge of a shifting range face high extinction risk from climate change unless they are able to track optimal environmental conditions through dispersal.

   We fit dispersal models to the locations of 3165 individually-marked black-throated blue warblers (Setophaga caerulescens) in the southern Appalachian Mountains in North Carolina, USA from 2002 to 2023. Black-throated blue warbler breeding abundance in this population has remained relatively stable at colder and wetter areas at higher elevations but has declined at warmer and drier areas at lower elevations.

   Median dispersal distance of young warblers was 917 m (range 23–3200 m), and dispersal tended to be directed away from warm and dry locations. In contrast, adults exhibited strong site fidelity between breeding seasons and rarely dispersed more than 100 m (range 10–1300 m). Consequently, adult dispersal kernels were much more compact and symmetric than natal dispersal kernels, suggesting adult dispersal is unlikely a driving force of declines in this population.

   Our findings suggest that directional natal dispersal may mitigate fitness costs for trailing-edge populations by allowing individuals to track changing climate and avoid warming conditions at warm-edge range boundaries.

    

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2. Nest predation and adult mortality relationships with post-natal metabolic rates and growth among songbird species

   https://doi.org/10.1242/jeb.226563  

   Ton, Riccardo ; Martin, Thomas E. ( August 2020 , Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Metabolism is thought to mediate the connection between environmental selection pressures and a broad array of life history tradeoffs, but tests are needed. High juvenile predation correlates with fast growth, which may be achieved via fast juvenile metabolism. Fast offspring metabolism and growth can create physiological costs later in life that should be minimized in species with low adult mortality. Yet, relationships between juvenile metabolism and mortality at offspring versus adult stages are unexplored. We found that post-natal metabolism was positively correlated with adult mortality but not nest predation rates among 43 songbird species on three continents. Nest predation, but not adult mortality, explained additional variation in growth rates beyond metabolism. Our results suggest that metabolism may not be the mechanism underlying the relationships between growth and mortality at different life stages. 

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3. Active facilitation of helper dispersal by parents and siblings in the cooperatively breeding acorn woodpecker

   https://doi.org/10.1007/s00265-024-03505-z  

   Hagemeyer, Natasha_D_G ; Koenig, Walter_D ; Walters, Eric_L ( August 2024 , Behavioral Ecology and Sociobiology)

   Offspring that delay dispersal in cooperatively breeding species have been hypothesized to gain direct fitness benefits via parental facilitation—being passively tolerated on their natal territory by their parents—thereby enjoying enhanced survival and increased probability of acquiring a breeding position in the population. Here we describe active facilitation in the acorn woodpecker (Melanerpes formicivorus) by parents and siblings assisting the dispersal of helpers in their social group. Helpers in this species compete for reproductive opportunities in “power struggles” that take place when all breeders of one sex die or disappear, creating a reproductive vacancy. Individuals compete at power struggles in coalitions of relatives, in which larger coalitions are more likely to be victorious. Based on observations of banded birds, we found that an estimated 26% of individuals competing as part of a winning coalition at a power struggle returned to their home territory at its conclusion, suggesting that they were facilitating the dispersal of kin (generally parents or siblings) that stayed to become breeders on the new territory. In at least one group, sibling facilitation was reciprocated; that is, a bird that was helped at a power struggle by a sibling joined that same sibling as part of a coalition at a subsequent power struggle. Dispersal facilitation is a novel means by which parents can nepotistically enhance the direct fitness of offspring and siblings can enhance each other’s inclusive fitness in this highly social species.

   Parental facilitation—being passively tolerated on the natal territory—may provide significant direct fitness benefits to helpers in cooperatively breeding species. We describe active facilitation of helper dispersal in the acorn woodpecker, where helpers compete in coalitions for reproductive opportunities at “power struggles” following the death or disappearance of all breeders of one sex. About one-fourth of individuals—including both parents and siblings—competing at power struggles were apparently facilitators who assisted related helpers by participating in the power struggle but then returned to their home territory rather than stay to breed on the new territory. In at least one group, dispersal facilitation was reciprocated; that is, a bird that was helped at a power struggle by a relative later joined that same relative as part of a coalition at a subsequent power struggle. Active dispersal facilitation by parents and siblings is an important, previously unrecognized, form of nepotistic behavior in this highly social species.

    

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4. Post‐fledging spatial dispersion and movement behaviour differs between cavity‐nesting and open‐cup nesting passerines

   https://doi.org/10.1111/ibi.13027  

   Şahin Arslan, Necmiye ; Martin, Thomas E. ( November 2021 , Ibis)

   Spatial dispersion and movement behaviour of siblings vary across animal taxa and can affect fitness. The factors underlying why species differ in movement behaviour of young and why some species form family groups while other species have dispersed young have rarely been addressed. We tested a hypothesis that siblings should be more dispersed in space and move less in species that have offspring with poor locomotor abilities to reduce the chance of the loss of the whole brood. In contrast, siblings should aggregate and move more when their mobility and ability to avoid predators is strong. We tested this hypothesis using songbirds because cavity‐ versus open‐cup‐nesting species strongly contrast in nesting strategy. Offspring of species that nest in cavities leave the nest with well‐developed wings and initially are more mobile and incur lower predation rates than fledglings of species that nest in open‐cups. We used radiotelemetry to track fledglings of three cavity‐ and three open‐cup‐nesting songbird species to test whether young differ in spatial dispersion and movement behaviour between nest types. We found that mobile young of cavity‐nesting species moved farther on a daily basis but remained more aggregated as a family than the less mobile young of open‐cup‐nesting species. Movement distances increased with mobility as young aged, especially in open‐cup‐nesting species, but families of open‐cup‐nesting species remained dispersed throughout the first week after fledging. This variation in spatial dispersion and movement behaviour of young can have important implications for parental care strategies and juvenile survival.

    

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5. Limited evidence of biased offspring sex allocation in a cavity-nesting conspecific brood parasite

   https://doi.org/10.1093/beheco/arae024  

   Wells, Caitlin_P ; Lyon, Bruce_E ; Thow, Caroline_M ; Stair, Tez ; Jones, Melissa ; Hinton, Mitch ; Eadie, John_M ; Buston, ed., Peter ( March 2024 , Behavioral Ecology)

   Sex allocation theory predicts that mothers should bias investment in offspring toward the sex that yields higher fitness returns; one such bias may be a skewed offspring sex ratio. Sex allocation is well-studied in birds with cooperative breeding systems, with theory on local resource enhancement and production of helpers at the nest, but little theoretical or empirical work has focused on birds with brood parasitic breeding systems. Wood ducks (Aix sponsa) are a conspecific brood parasite, and rates of parasitism appear to increase with density. Because female wood ducks show high natal philopatry and nest sites are often limiting, local resource competition (LRC) theory predicts that females should overproduce male offspring—the dispersing sex—when competition (density) is high. However, the unique features of conspecific brood parasitism generate alternative predictions from other sex allocation theory, which we develop and test here. We experimentally manipulated nesting density of female wood ducks in 4 populations from 2013 to 2016, and analyzed the resulting sex allocation of >2000 ducklings. In contrast to predictions we did not find overproduction of male offspring by females in high-density populations, females in better condition, or parasitic females; modest support for LRC was found in overproduction of only female parasitic offspring with higher nest box availability. The lack of evidence for sex ratio biases, as expected for LRC and some aspects of brood parasitism, could reflect conflicting selection pressures from nest competition and brood parasitism, or that mechanisms of adaptive sex ratio bias are not possible.

    

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