Tail-flashing as an anti-predator signal in small wintering birds

Abstract

Prey species use pursuit-deterrent signals to discourage an attack, by informing a predator either that the latter has been detected, or that the prey is capable of escaping if attacked. These signals tend to be conspicuous behaviors, such as bobbing, stotting, and predator inspection. Dark-eyed juncos (Junco hyemalis) show prominent tail-flashing in social displays during the breeding season, but they continue to tail-flash in winter. We examined whether such tail-flashing functions as a pursuit-deterrent signal by measuring tail-flashing rates in the presence and absence of a taxidermically mounted hawk predator. Our results showed that juncos tail-flash more in the presence of the predator and at higher rates when in direct view of the predator. This suggests that tail-flashing is directed to the predator as a pursuit-deterrent signal. Additionally, juncos reduced tail-flashing when feeding far from cover (low escape probability), suggesting that tail-flashing likely has an attraction cost. We also found a marked group size effect with solitary juncos tail-flashing more than those in large groups, indicating an additional cost to tail-flashing that need not be paid in larger groups. This additional cost is not related to food intake, since we found no negative association between food intake and tail-flashing. We observed tail-flashing in other sparrow species co-occurring with juncos at the study site, suggesting that tail-flashing as a pursuit-deterrent behavior may be widespread in this taxonomic group of birds. Many animals, and especially birds, use tail movements as a signal in various contexts. Dark-eyed juncos make extensive use of tail-flashing as a breeding signal, but they continue to tail-flash during the winter. Our experiment provides evidence that such tail-flashing serves as a pursuit-deterrent signal, a function previously not examined in these birds. Juncos tail-flashed more in the presence of a visible predator, suggesting that the signal is directed towards the predator. Tail-flashing was maximal when close to cover, indicating that tail-flashing incurs a cost of attracting predator attention. A negative relationship between flock size and tail-flashing suggests an additional cost to this behavior, which we demonstrate is not related to food intake rate. We suspect that tail-flashing functions more widely as a pursuit-deterrent signal in the broad phylogenetic group of emberizid sparrows.