Community science reveals biennial irruptive migration in the White-breasted Nuthatch (Sitta carolinensis)

Abstract

The White-breasted Nuthatch (Sitta carolinensis) is usually considered a resident species, despite occasional reports of local movement. We show that the eastern North American population engages in regular, approximately biennial irruptive migration that extends from the northern edge of the range to as far south as the mid-Atlantic states. Both adults and juveniles take part, and neither sex nor age proportions differ between years with or without irruption. The biennial pattern is detectable in daily counts at migration concentration sites and, more subtly, across the landscape as reflected by a variety of broad-scale community science initiatives. Within northeastern North America, indices of irruption from all data sources were negatively correlated with indices of soft mast production by plant species monitored across southern and central Ontario. Perturbation in the biennial pattern of irruption coincided with change in the pattern of plant masting. Size of the breeding population was unrelated to size of irruption in the same fall, unlike the case for the Red-breasted Nuthatch (S. canadensis). The geographic scale and regular occurrence of irruption in White-breasted Nuthatch have likely been overlooked because migrants are widely dispersed and remain within the breeding range, where immigrants mix unnoticed with local residents. Community science programs provide opportunities for novel insights into seemingly well-known bird species, and further studies may reveal similar patterns in other temperate species thought to move only sporadically.