Feeding Behavior, Flock-Size Dynamics, and Variation in Sexual Selection in Crossbills

Abstract

I present field data on feeding and scanning behavior of White-winged Cross- bills (Loxia leucoptera) in relation to flock size. Feeding rates increased and individual levels of vigilance decreased as flock size increased from one to two. Further increases in flock size did not correspond with substantial reductions in individual vigilance, but did correspond with frequent agonistic interactions. As rates of agonistic interactions increased, feeding rates of sub- ordinate age-sex classes (e.g. adult females) declined relative to those of dominant age-sex classes (e.g. adult males). Females (and immatures) may have reduced this effect by avoiding flocks composed mostly of adult males. Assortative flocking occurred in the nonbreeding pe- riod (late November to December), but during breeding periods (September and potentially rest of year) the numbers of each age-sex class tended to be positively correlated. Rates of agonistic interactions were higher when crossbills foraged on conifers whose cones were com- pactly dispersed. Flocks were smaller when rates of agonistic interactions were high and co- nifers had compact cone dispersions. Flock size increased as seed density declined. Larger flocks, with high levels of collective vigilance, were favored because feeding rate and the time spent scanning while seeds were husked also declined. Crossbills (Loxia spp-) that forage on small-crowned conifers are more sexually dichromatic and have other traits indicating stronger sexual selection than crossbills that forage on large-crowned conifers. I suggest that these dif- ferences result from differences in rates of agonistic interactions, which influence female feed- ing rates compared with male feeding rates, and likely have a differential effect on female mortality rates and the population sex ratio. Received 21 June 1996, accepted 17 December 1996.