Reduced lean mass and delays in resuming bird migration

Mariamar Gutierrez Ramirez, Michael S. Griego, Joely G. DeSimone, Cory R. Elowe, Alexander R. Gerson

This is a plain language summary of a Functional Ecology research article which can be found here.

Every spring, billions of birds depart their non-breeding grounds in Central and South America and fly non-stop across the Gulf of Mexico to return to their breeding grounds in North America. After such a physically demanding non-stop flight, birds sometimes land in an emaciated state, with no fat reserves and shrunken muscles and organs. Before they can continue their migration further north, they must recover from crossing the Gulf of Mexico. We suspected that birds that land with very depleted muscles and organs would delay restarting migration and have longer stopovers.

A Swainson’s Thrush (Catharus ustulatus) perched on a branch during stopover. The bird was fitted with a nanotag radio transmitter on its back to track its migration (credit: Cory R. Elowe)

We tested this idea on four species of long-distance migratory birds: the northern waterthrush (Parkesia noveboracensis), Swainson’s thrush (Catharus ustulatus), gray-cheeked thrush (Catharus minimus), and yellow-billed cuckoo (Coccyzus americanus). We measured body composition, took a small blood sample, and fitted each bird with a miniaturized radio-telemetry tag to follow their movements. The telemetry tags allowed us to know exactly when these small birds restarted migratory flight or moved over to a different habitat. Our study indicates that lower lean mass delays migration– but not in all species. Northern waterthrush with less lean mass stayed longer before restarting migration. Swainson’s thrush with higher uric acid levels, meaning they used up a lot of protein during flight, like muscles and organs, also stayed longer before restarting migration. Our results suggest that reductions in lean body mass that occur during flight may have repercussions on how long birds stop before being able to restart migration.

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