Duration of stopover in relation to date of arrival in vagrant Western Kingbirds (Tyrannus verticalis)

Lucinda Christine Zawadzki

Abstract


Western Kingbirds (Tyrannus verticalis) are renowned for their repeated, long-distance dispersal to the East Coast of the United States. While they are known to migrate to the Mexican monsoon region after the breeding season, many individuals are found on the Atlantic coast each fall between Newfoundland and Florida, more than 2000 km east of their "normal" range. This seasonal pattern of occurrence of these vagrants raises many questions about the “intent” of birds engaged in dispersal and migration, and the selective pressures that lead to these behaviors. Using the web-based repository known as eBird, available sighting records of Western Kingbirds along the Eastern Coast of the United States and parts of Canada during the months of August to February were collected. For every bird, the date of arrival was recorded in relation to its duration of stay, and these patterns were analyzed in comparison to normal migratory behavior of the species. It was concluded that the date of arrival of vagrants correlated to their duration of stay on the East Coast, with individuals staying significantly longer if they arrived later in the fall, than if they arrived earlier in the fall. These vagrants were also shown to engage in three distinct patterns of movement that correspond to migratory behaviors within their “normal” range. These results suggest that vagrant Western Kingbirds appear to be undertaking a more complex form of long-distance dispersal, perhaps exploring the East Coast as a possible place for new wintering grounds.


Keywords


Vagrancy; Long-distance Dispersal; Kingbirds

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