Migration distance is a fundamental axis of the slow-fast continuum of life history in boreal birds

Author: Winger, Benjamin M. and Teresa M. Pegan

Year: 2021

Publication: Ornithology

Publication Link: https://academic.oup.com/auk/advance-article-abstract/doi/10.1093/ornithology/ukab043/6316518?redirectedFrom=fulltext

Keywords: fecundity, life history, seasonal migration, slow-fast continuum, survival, vital rates

Abstract: Seasonal migration is intrinsically connected to the balance of survival and reproduction, but whether migratory
behavior influences species’ position on the slow-fast continuum of life history is poorly understood. We found that
boreal-breeding birds that migrate long distances exhibit higher annual adult survival and lower annual reproductive
investment relative to co-distributed boreal species that migrate shorter distances to winter closer to their breeding
grounds. Our study uses “vital rates” data on reproductive output and survivorship compiled from the literature for a
species assemblage of 45 species of mostly passerine birds. These species breed sympatrically in North American boreal
forests but migrate to a diversity of environments for the northern winter. After controlling for body size and phylogeny,
migration distance and apparent annual adult survival are positively related across species. Both migration distance
and survival are positively correlated with wintering in environments that are warmer, wetter, and greener. At the same
time, longer migrations are associated with reduced time spent on the breeding grounds, lower clutch sizes, and lower
fecundity (clutch size × maximum number of broods per year). Although seasonal migration is often associated with high
mortality, our results suggest that long-distance migration imposes selection pressures that both confer and demand
high adult survival rates. That is, owing to the reproductive cost of long-distance migration, this strategy can only persist
if balanced by high adult survival. Our study supports the idea that migration evolves to promote survival of species
breeding in seasonal environments. In boreal birds, the evolution of the longest migrations yields the highest survival,
but at an inherent cost to annual fecundity. Our results therefore reveal migratory distance as a fundamental axis of the
slow-fast continuum that predicts, and is inextricable from, the balance of survival and reproduction.

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