Nonbreeding Foraging Ecology of the Brown-headed Nuthatch (<i>Sitta pusilla</i>)

Abstract

Many animals selectively employ behavioral strategies to balance numerous fitness requirements. Joining mixed-species groups is a common behavioral strategy that may increase foraging efficiency and/or reduce predation threat. I surveyed mixed-species pine-savanna canopy flocks and recorded flocking and foraging behavior of Brown-headed Nuthatches (Sitta pusilla) in both natural conditions and following predator call playback at two publically managed sites in North Carolina, USA, over three years to determine what drives mixed-species flocking behavior in this species. We hypothesized that pine foraging rates and predator vigilance rates would be inversely correlated with nuthatch mixed flock participation, due to the foraging and anti-predation benefits of mixed flocks, respectively. Data were analyzed via group comparison tests and linear model selection. Flocks differed in size and species diversity between sites, and reached maximum participation during cold and reduced daylength periods. I concluded that the Brown-headed Nuthatch should be considered a case-specific nuclear species in mixed flocks, given their prevalence in flocks without traditional nuclear species. Given that nuthatch mixed flock participation is inversely correlated with pine seed foraging rate, Brown-headed Nuthatch mixed flocking behavior appears to be primarily driven by foraging requirements, and flock participation may be costly during high pine seed availability and warm periods when forage is abundant. Though nuthatch flock participation was higher at the more predator abundant site and during periods when predator were more common, I did not find experimental evidence that anti-predator benefits drive nuthatch mixed flock participation; predator vigilance did not differ between conspecific and mixed flocks, and simulated predator presence did not alter nuthatch flock participation. Predator call playback did cause a short-term increase in vigilance behavior, however. Both foraging efficiency and anti-predation benefits may contribute to nuthatch mixed flocking behavior, though evidence for the former is more conclusive.