Physiological effects of increased foraging effort in a small passerine

Abstract

Foraging to obtain food, either for self-maintenance or at presumably elevated rates to provision offspring, is thought to be an energetically demanding activity but one that is essential for fitness (higher reproductive success and survival). Nevertheless, the physiological mechanisms that allow some individuals to support higher foraging performance, and the mechanisms underlying costs of high workload, remain poorly understood. We experimentally manipulated foraging behaviour in zebra finches (Taeniopygia guttata) using the technique described by Koetsier and Verhulst (2011). Birds in the “high foraging effort” (HF) group had to obtain food either while flying/hovering or by making repeated hops or jumps from the ground up to the feeder, behaviour typical of the extremely energetically-expensive foraging mode observed in many free-living small passerines. HF birds made significantly more trips to the feeder per 10min whereas control birds spent more time (perched) at the feeder. Despite this marked change in foraging behaviour we documented few short- or long-term effects of “training” (3 days and 90 days of “training” respectively) and some of these effects were sex-specific. There were no effects of treatment on BMR, hematocrit, hemoglobin, or plasma glycerol, triglyceride, glucose levels, and masses of kidney, crop, large intestine, small intestine, gizzard and liver. HF females had higher masses of flight muscle, leg muscle, heart and lung compared to controls. In contrast, HF males had lower heart mass than controls and there were no differences for other organs. When both sexes were pooled, there were no effects of treatment on body composition. Finally, birds in the HF treatment had higher levels of reactive oxygen metabolites (dROMs) and, consequently, although treatment did not affect total antioxidant capacity (OXY), birds in the HF treatment had higher oxidative stress.