Abstract
Fat contributes most of the energy for migratory flight of birds, whereas lean body tissues (muscles and organs) contribute amino acids and water to maintain metabolic and osmotic homeostasis. During refueling at stopover sites, both fat and lean mass are recovered, but the dynamics of this recovery are poorly understood. We used non-invasive quantitative magnetic resonance (QMR) analysis to measure fat and lean mass of > 3,500 individuals of 25 songbird species during six spring and three autumn migration seasons between 2009 and 2019 at Long Point, ON, Canada. We used allometric scaling analysis and linear mixed-effects modeling of body composition data at both the population level (single capture) and the individual level (recapture). In the population-level analysis, lean mass scaled hypoallometrically with body mass, such that for every 20% increase in body mass, lean mass was predicted to increase by 12.1% in spring and 12.8% in autumn. Fat scaled hyperallometrically with body mass, such that for every 20% increase in body mass, fat mass was predicted to increase by 144% in spring and 136% in autumn. At the individual level, these allometric relationships were more extreme. As a result of this differential allometry, at low body masses, lean and fat mass contributes nearly equally to changes in mass, but at high body mass fat deposition becomes progressively more dominant. Spring migrants deposited relatively more fat than autumn migrants, and in autumn juvenile birds tended to have greater lean mass than adults. Our findings show that lean mass deposition during refueling by songbirds is substantial, and in line with the losses of protein expected in flight. The process of fat and lean mass deposition is characterized by non-linear dynamics which are influenced by the current body composition, season, and, to a lesser extent, age. The patterns suggest that the need for dietary protein to rebuild lean mass will be greater when body mass is low, during autumn migration, and in juvenile birds.
Highlights
Knowledge of the changes in body composition that occur at staging and stopover sites is essential to our understanding of how migratory birds budget fuel, and of the nutrients they require when fueling
Species effects contributed much more than year-season effects (Table 4), and random slopes did not vary with mean body mass, with or without phylogenetic correction (Supplementary Material)
We found that the relative gains of fat increase as birds deposit fuel and become heavier, that migration distance and breeding region had minimal effects, and that there were effects of season and age on body composition dynamics
Summary
Knowledge of the changes in body composition that occur at staging and stopover sites is essential to our understanding of how migratory birds budget fuel, and of the nutrients they require when fueling. Work on fuel storage of migratory birds produced the airplane paradigm where lean body components stayed constant, and fat was used as fuel for flight (Connell et al, 1960; Odum et al, 1964). Trans-Pacific migrant shorebirds show extreme changes in both fat and lean body components as they prepare for and undertake multi-day flights (Piersma and Gill, 1998; Battley et al, 2000). The limited evidence available indicates that even songbirds refueling at stopovers where there is no ecological barrier appear to deposit significant amounts of lean mass along with fat (Karasov and Pinshow, 1998; Seewagen and Guglielmo, 2011; Wojciechowski et al, 2014). Factors that influence lean mass deposition could influence diet selection for protein-rich foods, such as invertebrates, during refueling (Carpenter et al, 1993)
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