Coots Fulica atra reduce their vigilance under increased competition.

Birds frequently interrupt feeding to scan their surrounding environment. Usually an inverse correlation between scan rate and flock size exists. The 'many-eyes' hypothesis suggests that more eyes are able to detect a predator earlier. Due to the 'dilution-effect' animals in larger groups experience 'safety in numbers', while the 'scramble competition' hypothesis suggests that individuals reduce their vigilance in larger groups since they compete for a limited amount of scarce resources. Here, I induced competition in natural coot populations. Coots preferred feeding on grass and were occasionally fed by passers-by at the study sites. However, this resource was scarce and coots experienced additional food as a very limited resource. I sampled coots prior and after an experimental treatment with an additional food supply. Correlations between vigilance rate (number of scans during 1 min of foraging), nearest neighbor distance and flock size existed before the experimental treatment but afterwards the correlation between vigilance and nearest neighbor distance faded. I found a significantly lower vigilance rate and nearest neighbor distance after inducing competition. This study provided experimental evidence for the 'scramble competition' hypothesis namely that individuals in groups lowered their vigilance when faced with competition for a limited amount of food resources given similar flock sizes.