Daily routines of body mass gain in birds: 1. An exponential model

Abstract

Small birds show the greatest rates of body mass gain at dawn and before dusk. Some environmental and internal factors may change the intensity and duration of the first period of body mass gain relative to the second one, and thus may change the shape of daily fattening trajectories. A frequent problem for researchers is setting the periods of the day when each of the main environmental factors operates, because its effects usually decay or increase smoothly from dawn to dusk. We solved this problem by fitting the body mass to time of day with a model generated with a differential approach. This model shows the typical bimodal pattern of daily fattening routines in small birds and allows calculation of the inflexion point, that is, the time of day when the trajectory ends the decelerated period of body mass gain and starts the accelerated increase in body weight. A delay or advance in the inflexion point can be related to environmental stress that may otherwise pass unnoticed. We calculated a mean delay of 10% in the inflexion point in a sample of studies with limited food availability, and a mean advance of 6% in a sample of studies with increased predation risk. The change in the shape of daily trajectories of body mass between stressful and nonstressful conditions was inversely correlated with body mass at dawn.