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.
Developmental responses can help young animals reduce predation risk but can also yield costs to performance and survival in subsequent life stages with major implications for lifetime fitness. Compensatory mechanisms may evolve to offset such costs, but evidence from natural systems is largely lacking. In songbirds, increased nest predation risk should favour reduced provisioning, but also young that fledge (leave their nest) at an earlier age. Both responses can result in fledglings with shorter wings, reduced mobility and decreased survival. Young may compensate for shorter wings developmentally by reallocating resources towards feather development or behaviourally by adjusting flight kinematics or habitat use. However, underfed young may lack the capacity to express these phenotypes due to insufficient resources or an inability to adjust allocation of resources. Using predation risk experiments and 29 years of observational field data, we test whether increased nest predation risk reduces flight performance and survival during the fledgling stage and explore potential mechanisms that might underlie these effects. We show that young from high‐risk nests did not leave the nest earlier on average, but wing growth was slower likely due to observed reductions in parental feeding rates. Wings were shorter in high‐risk nests when fledglings left the nest early. Yet, fledglings from high‐risk nests showed improved flight performance for a given wing length such that flight performance at fledging did not differ between young from high‐risk and low‐risk nests. Young from high‐risk nests may have offset the costs of shorter wings on flight performance by accelerating the emergence of flight feathers from their sheaths to reduce wing porosity, though evidence for this mechanism was mixed. Fledglings from high‐risk nests also selected habitat with denser woody vegetation compared with young from low‐risk nests. Together, these developmental and behavioural responses seem to mitigate the expected effects of increased nest predation risk on fledgling survival. Ultimately, our results show that offspring predation risk can affect parental provisioning and offspring morphology without major implications for performance and survival in subsequent life stages.
A free
- Award ID(s):
- 1656120
- NSF-PAR ID:
- 10456603
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Functional Ecology
- Volume:
- 34
- Issue:
- 10
- ISSN:
- 0269-8463
- Page Range / eLocation ID:
- p. 2147-2157
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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