Cooperative Breeding helps Brown-headed nuthatches (Sitta pusilla) face heterospecific competition

Abstract

My dissertation research tests the previously untested idea that heterospecific competition functions as an important selective force on group size in cooperatively breeding birds, using experimental approaches with the Brown-headed nuthatch (BHNU) (Sitta pusilla). We know from prior work that the western bluebird, which is much larger, excludes nuthatches from nesting sites (Stanback 2011). I tested the following explicit hypotheses: Hypothesis 1: Inter-specific competition selects for increased group size (more individuals helping or co-breeding) in BHNU and may even be sufficient to select for helping. Hypothesis 2: BHNU groups collectively defend against nest competitors, and collectively monitor their nest sites to chase away intruders. I studied the effect of interspecific competition on the fitness consequences of cooperative breeding using an experimental design that involved creating high and low competition sites on golf courses. Half of each course was randomly assigned to the high and low heterospecific competition treatments; high competition territories had a single nest box whereas low competition territories had a pair of identical boxes within 5 m of each other at each box location. If competition acts as a selective force, reproductive success should be lower under high competition. And this is indeed what I found. GLMMs and AICc model selection criteria helped determine the model that best fit the data. The most highly supported model included both competition pressure and group size (ΔAICc >2), strongly supporting hypothesis 1. More BHNU nests failed on high competition sites (N=156, GLMM, course as fixed factor, p=0.043). Larger groups more often fledged young regardless of competition pressure, but small groups were significantly less likely to fledge young in the high than low competition treatment and this higher failure rate was due to competition. I examined the behavior of BHNUs facing competitors using simulated territorial intrusions and nest watches. Larger groups were less likely to have intruders investigate their nest site without their knowledge and retaliation (GLMM, box as fixed factor, n=202, p=0.044). Furthermore, it took large groups less time to chase intruders from their territories. The mechanisms through which helpers increase breeder success in the face of heterospecific competition are twofold, first, they provide more noise and mobbing effort, and secondly they provide more eyes on the nest site, such that competitors are spotted more quickly. Cooperative breeding rescues BHNUs from the omnipresent pressure of competitors and provides a hitherto unconsidered benefit of breeding in groups.