Covariation between morphological traits and ant-following behavior in Neotropical birds

Some species rely on others to obtain part, or in extreme cases all, of
their food. For example, ant-following birds exploit insects that are
disrupted by swarms of army ants. Such behavioral specialization is often
linked to the evolution of morphological traits that enhance resource
exploitation. In this study, I investigated whether morphological traits
covary with ant-following behavior. I predicted that traits than enhance
competitive ability, like a heavier weight and signals of dominance,
should be more prevalent in species that rely more on ants to obtain food.
Other morphological traits were also expected to diverge reflecting the
unique lifestyle of ant-following birds. I focused on Neotropical families
that include obligate ant-following species, as well as others that follow
ants frequently, occasionally, or not at all—thus providing a gradient of
ant-following behavior. Using a phylogenetic framework that incorporated
primary lifestyle and foraging behavior, I identified several
morphological changes in ant-following species. Compared to non-following
species, obligate species were generally heavier with a significantly
reduced wing area due to shorter wings and secondaries. They also had
shorter tails and narrower, shorter bills. Additionally, bare periorbital
areas, which are thought to act as signals in competitive interactions,
were more common among obligate species than in other groups. Facultative
ant-following species exhibited intermediate phenotypes and species that
rarely follow ant swarms showed no significant morphological differences
from those that never do. Overall, the findings support the hypothesis
that morphological traits covaried with ant-following behavior in
Neotropical birds.