The effect of group size on mean food intake rate in birds

A description of the relationship between mean food intake rate (MFIR) and group size is important in assessing the adaptiveness of group living in an imals. Several hypotheses argue that foraging in groups can be beneficial i n terms of MFIR but an overall assessment of the evidence available thus fa r was lacking. I examined this relationship in birds using studies that pro vided a measure of MFIR in groups of different sizes and evaluated the effe ct of study design and food type. In addition, I noted some of the mechanis ms thought to be responsible for changes in MFIR with group size and quanti fied the impact of the addition of individual foragers. The most common pat tern of change in MFIR with group size was a monotonic increase. The differ ent patterns of change in MFIR with group size occurred with similar freque ncies in experimental and observational studies despite the fact that obser vational studies often lacked control for confounding factors. Increases in MFIR were more likely to be associated with seeds as a food type than with fish or invertebrates, and were least likely with carcasses. This finding was related to the ease with which individuals can use aggression to increa se their share of resources that are more concentrated in space. Adjustment s in vigilance levels and aggression, as well as more subtle changes in spe ed of search and handling time, were correlated with changes in MFIR with g roup size. Finally, the addition of three to four individuals appeared to b e needed to double the MFIR of a solitary bird. The results suggest that fo raging in avian groups is often associated with increased MFIRs and that se veral ecological factors, including food dispersion, influence the pattern of change with group size.