Heterospecific shorebird flocks increase safety and reduce intraspecific competition

Shorebirds join cohesive flocks for safety. The joining of individuals of
several species in large heterospecific flocks can optimise individual
vigilance and foraging. However, a large number of conspecific individuals
in flocks may deplete food resources and increase intraspecific
competition. In the present study, we argue that Nearctic-Neotropical
migratory shorebird species join large heterospecific flocks in a balanced
number as a way of controlling intraspecific competition. We recorded
monospecific and heterospecific flocks in urban beaches from southeastern
Brazil and annotated the species richness and the number of individuals
per bird species per flock. Besides, we recorded the number of people
around shorebird flocks to examine any relationship with flock size. We
also recorded the foraging rate of the Semipalmated Plover
(<i>Charadrius semipalmatus</i>), a common
migrant species in the region, as a species model to test competition and
vigilance. We hypothesised that this species would exhibit a low foraging
rate in scenarios with high intraspecific competition (i.e., large flocks
with a high number of conspecifics) and high density of people, which may
disturb birds. In the initial results, the density of people did not
influence size of monospecific and heterospecific flocks; heterospecific
flocks were larger than monospecific flocks, and heterospecific flocks
increased in size by the addition of more species rather than more
individuals of the same species. These data reflect a decrease in the
relative abundance of individuals per species. Additionally, the
Semipalmated Plover foraging rate was reduced in monospecific flocks,
flocks with a high relative abundance (and total number) of the specific
individuals and flocks surrounded by a high density of people. All these
results indicate that shorebirds may adjust flock composition and size
according to preferred foraging conditions, i.e., large and balanced
heterospecific flocks with less intraspecific competition and less
vigilance to potential predators in wintering areas.