Lean mass deposition occurs at a greater rate than fat deposition during pre-breeding stopover in highly depleted songbirds in the northern Gulf of Mexico

Abstract

The Gulf of Mexico represents the largest ecological barrier between breeding and non-breeding grounds for long-distance migratory songbirds in the Nearctic-Neotropical system. Despite the prominence of the Gulf of Mexico, there are still gaps on fundamental physiological aspects of stopover of migrants in this region, including the role and relative importance of fat and lean mass depletion and deposition. We examined the arrival body condition of Nearctic-Neotropical migrants at a coastal stopover site on St. George Island, FL, in the northern Gulf of Mexico during pre-breeding migration in the spring of 2016-2018. We precisely determined lean body and fat masses on individual birds after a trans-Gulf migratory flight via quantitative magnetic resonance (QMR) technology. We hypothesized that birds with non-breeding ranges in South America would arrive with lower fat and lean masses than birds with non-breeding ranges in the Caribbean or Central America. We also hypothesized that songbirds would increase lean mass at a greater rate than fat mass, as they rebuilt muscle and organ masses. We also compared QMR lean and fat measurements to visual measures of fat and muscle scores. A total of 44 Nearctic-Neotropical migratory bird species occur on St. George Island during spring stopover. Non-breeding range did not influence the arrival fat mass or arrival lean mass in 10 focal transient species, meaning those that have no breeding or non-breeding populations on the site. Our results from recaptured individuals indicated that body mass increase during stopover derives from both lean and fat mass accumulation. Our results provide a robust quantitative assessment of songbird arrival body condition on the northern Gulf of Mexico and contribute to the understanding of the physiology of migratory songbirds after a long-distance flight, which will help inform management decisions for stopover sites located around ecological barriers.

Keywords: Body condition; Gulf of Mexico; migratory songbird; quantitative magnetic resonance; stopover.

Figure 1

Map of St. George Island, Apalachicola Bay, FL, in relation to the Gulf of Mexico and Caribbean Sea. Top right map shows the location of the Apalachicola Bay, St. George Island in northern Florida, USA. Bottom right map shows the banding location on St. George Island in the square.

Figure 2

Body composition of 10 focal species during pre-breeding migration on the northern Gulf of Mexico. All birds arrived with body fat averaging <10% of body mass (dotted red line). Total mass represents whole animal mass as measured with digital scale, and includes all body components (bones, lean and fat mass, feathers); lean mass was measured via QMR and represents lean wet mass (muscle and organ tissues); and fat mass was measured via QMR and represents fat content.

Figure 3

Correlation between QMR body composition analysis data for (A) fat mass and fat scores and (B) fat-free and pectoral muscle score of ten focal species of transient Nearctic–Neotropical migrants during spring stopover in St. George Island, FL.

Figure 4

Rate of fat mass and lean mass change relative to minimum stopover duration of individual Neotropical migrants recaptured >1 day after first capture in St. George Island, FL, during pre-breeding migration 2016–2018. Fat and lean mass measured by QMR in the field. The dashed horizontal line indicates no change in mass between first capture and subsequent recapture, so that below the line are birds with negative rate and above the line are birds with positive rate of mass change.