The southern migration of Nearctic landbirds in autumn is a well-studied phenomenon. Many of these birds exhibit elevated levels of frugivory during migration, and it stands to reason that resultant seed dispersal may be characterized by non-isotropic patterns and a propensity for southern movement. Tree swallows (Tachycineta bicolor) fit this description well; they breed in northern North America and migrate south en masse in autumn to wintering grounds in the southern US and the Caribbean. Populations whose migrations include the Eastern Seaboard travel through habitats containing wax myrtle (Morella cerifera), whose fruits they are known to consume and have unique and innate abilities to assimilate. During cold periods (e.g. < 18.5 C) tree swallows are known to forage primarily on fruits, especially wax myrtle, due to suspected declines in aerial insect prey items. We investigated whether recent (since 2010) georeferenced eBird records of large numbers (> 1000) of tree swallows along the Eastern Seaboard during autumn and winter tracked temperature conditions in a way consistent with tree swallow behavior and migratory movement. We used generalized linear mixed models to relate weekly changes in latitudinal distances of centroids of large numbers of tree swallows with environmental data.
Results/Conclusions
We found evidence for larger southerly shifts in latitude to be connected with lower temperatures. Centroids of large numbers of tree swallow records moved larger distances south during autumn when weekly temperatures were lower. Our findings serve as encouraging first steps in identifying factors associated with migratory movements. The depletion of fruits during cold temperature periods, combined with lower aerial insect abundance, could lead to movement not only of tree swallows further south during autumn migration, but also the seeds of the fruits they consume. We encourage future work that ties together data on individual tree swallow movement patterns (e.g. geolocators) and wax myrtle fruit depletion (e.g. remote sensing) to better understand migratory patterns and long-distance seed dispersal.