Data from: Assortative flocking in crossbills and implications for ecological speciation

How reproductive isolation is related to divergent natural selection is a
central question in speciation. Here we focus on several ecologically
specialised taxa or “call types” of red crossbills (Loxia curvirostra
complex), one of the few groups of birds providing much evidence for
ecological speciation. Call types differ in bill sizes and feeding
capabilities, and also differ in vocalizations, such that contact calls
provide information on crossbill phenotype. We found that two call types
of red crossbills were more likely to approach playbacks of their own call
type than those of heterotypics, and that their propensity to approach
heterotypics decreased with increasing divergence in bill size. Although
call similarity also decreased with increasing divergence in bill size,
comparisons of responses to familiar versus unfamiliar call types indicate
that the decrease in the propensity to approach heterotypics with
increasing divergence in bill size was a learned response, and not a
byproduct of calls diverging pleiotropically as bill size diverged.
Because crossbills choose mates while in flocks, assortative flocking
could lead indirectly to assortative mating as a byproduct. These patterns
of association therefore provide a mechanism by which increasing divergent
selection can lead to increasing reproductive isolation.