Diurnal patterns of mass gain in tropical granivores suggest avoidance of high midday temperatures during foraging, rather than the starvation–predation risk trade-off

Abstract

Diurnal patterns of body mass gain in small temperate birds are of theoretical interest because they are largely a consequence of the starvation vs predation risk trade-off. In tropical birds, diurnal variation in body mass may also be influenced by other constraints such as water availability, digestive bottlenecks and the avoidance of periods of high temperature. We recorded body mass and amount of seed eaten with time of day and temperature in three small granivorous species, feeding on baited balances in a savanna habitat in tropical West Africa. Birds fed in the morning and afternoon but almost never visited balances to feed during the middle of the day. Body mass increased with time of day, but with a clear bimodal pattern so that mass was gained in the early morning and was then lost again by the time birds resumed feeding in the afternoon. Mass was then mostly regained at the end of the afternoon. There was a significant, but biologically small, increase in body mass with increasing temperature. When controlling for the effects of time of day, species, year and day, body mass on average increased to a peak at about 24 °C before declining slightly; daily temperature peaked at 27 °C during the midday period when the birds did not feed, and so mass was lost. Body mass gain between consecutive diurnal hours was greatest when the temperature was about 18 °C, during early morning feeding. Gain in body mass decreased as temperature increased by late morning, and stopped or even slightly decreased above 24 °C when birds stopped feeding. The amount of food eaten per visit to the balance was independent of time of day and temperature, confirming that the birds were regulating their mass adaptively, rather than simply reflecting availability of food. Body mass gain was not restricted to the early morning as expected with low predation risk and high starvation risk, nor was it restricted to just late in the day as expected with high predation risk and low starvation risk. Instead, high temperatures around midday may result in a period where foraging is avoided completely for energetic or water conservation reasons, driving the bimodal diurnal pattern of body mass gain.