Nested patterns of species distribution and winter flock occurrence of insectivorous birds in a fragmented landscape

Abstract

The goal of this paper is to evaluate whether the probabilities of aggregating in flocks are dependent on the patterns of species distribution across the landscape. I analyzed winter species distribution and flock occurrence (flock formation in a given fragment) at the community and individual species levels of a guild of small insectivorous birds in a fragmented landscape. Species distributed following a nested pattern, which was mainly affected by fragment size. Flock occurrence (single- plus mixed-species flocks) followed a nested pattern, such that the probability of joining flocks was affected by the distribution of species across the landscape. However, when analyzed separately, single-species flock occurrence showed a nested pattern, but mixed-species flock occurrence followed a random pattern. Fragment size and total bird density (as an indicator of the availability of individuals to join flocks) accounted for nestedness. At the individual species level, conspecific and heterospecific densities were the foremost factors increasing flock occurrence probabilities, suggesting that species tended to aggregate when there was an increase in the local availability of individuals. Fragment size and habitat structure (coniferous cover and number of thick trees) also increased flock occurrence, likely because of higher resource availability, which allowed the colonization of more species and individuals. Greater isolation reduced flock occurrence, maybe as a result of lower availability of individuals in distant fragments. Overall, fragmentation conditions can disrupt flocking behaviour of small insectivorous species, which could decrease survival and increase predation risk during winter. Keywords: nestedness, flock occurrence, fragmentation, insectivorous birds, winter flocks.