Authors: Cory R. Elowea, Derrick J. E. Grooma, Julia Slezacek, and Alexander R. Gersona
Year: 2023
Publication: Proceedings of the National Academy of Sciences
Publication Link: https://doi.org/10.1073/pnas.2216016120
Keywords: migration, blackpoll warbler, yellow-rumped warbler, flight energetics, physiology
Abstract: During migration, long-distance migratory songbirds may fly nonstop for days,
whereas shorter-distance migrants complete flights of 6 to 10 h. Fat is the primary
fuel source, but protein is also assumed to provide a low, consistent amount of
energy for flight. However, little is known about how the use of these fuel sources
differs among bird species and in response to flight duration. Current models predict
that birds can fly until fat stores are exhausted, with little consideration of
protein’s limits on flight range or duration. We captured two related migratory species—
ultra long-distance blackpoll warblers (Setophaga striata) and short-distance
yellow-rumped warblers (Setophaga coronata)—during fall migration and flew them
in a wind tunnel to examine differences in energy expenditure, overall fuel use, and
fuel mixture. We measured fat and fat-free body mass before and after flight using
quantitative magnetic resonance and calculated energy expenditure from body composition
changes and doubly labeled water. Three blackpolls flew voluntarily for up to
28 h—the longest wind tunnel flight to date—and ended flights with substantial fat
reserves but concave flight muscle, indicating that protein loss, rather than fat, may
actually limit flight duration. Interestingly, while blackpolls had significantly lower
mass-specific metabolic power in flight than that of yellow-rumped warblers and fuel
use was remarkably similar in both species, with consistent fat use but exceptionally
high rates of protein loss at the start of flight that declined exponentially over time.
This suggests that protein may be a critical, dynamic, and often overlooked fuel for
long-distance migratory birds.