“Age-dependent modulation of songbird summer feather molt by temporal and functional constraints”

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Yosef Kiat and Nir Sapir

Eurasian crimson-winged finch (Rhodopechys sanguineus), young male after partial post-juvenile molt.
(Credit: Yosef Kiat)

Songbirds molt their feathers on an annual basis because feathers get worn with time due to exposure to UV radiation and other environmental factors. Feather molt is a timely undertaking that is considered among the three most energy-demanding processes in the life cycle of birds, and as such should be made at the most convenient time for the bird. Songbird feather molt usually takes place during fall, just after breeding and before migration, and might be constrained by these activities. To deal with time pressure, passerines may shorten their molt duration by only replacing part of the plumage, through increasing the speed of molt, or by postponing the renewal of some or all the plumage to a later season (i.e., from the summer to the over-wintering period). Yosef Kiat and Nir Sapir from the Hebrew University and the University of Haifa, Israel, used a comparative approach by measuring 12,349 individuals from 134 passerine species in different sites across Israel and two museum bird collections in Israel and the UK to explore how feather molt of juvenile and adult passerines is evolutionarily modulated under time constraints. The results indicate that breeding at northern latitudes and long-distance migration limit the time available for molt and that the consequences of these time constraints were age-dependent. While the duration of adult summer molt decreased, the extent, rather than the duration, of juvenile molt declined under time constraints. The findings suggest that two different adaptations to deal with time pressure have evolved in passerines, and that they are employed depending on bird age. This study highlights the importance of considering time constraints for better understanding the evolution of life history processes and their consequences throughout the annual routine. Read the Article