Tree Swallow Range Expansion 



109 



I960 



1980 -; 



Fig. 2. The expanding breeding distribution of the tree swallow 

 (Tachycineta bicolor) in the southeastern United States (1894-1991). 



Possible Explanation for Range Expansion — Tree swallows might 

 now be imprinted on bluebird boxes as nesting sites, and this imprint- 

 ing possibly has allowed them to expand their range into areas where 

 the lack of natural cavities would otherwise inhibit nesting. 



Plasticity in food habits may also be important in the range 

 expansion of this swallow. Lee and Franz (1972) reported on a pre- 

 migration staging flock of several thousand tree swallows feeding over 

 a cornfield on the eastern shore of Maryland. Examination of the 

 stomachs of a dozen birds revealed corn flea beetles {Chaetocnema 

 pulicaria). No other food items were found. Food items were counted 

 in one stomach, and 170 corn flea beetles were found. Although I 

 am not aware of other direct observations of tree swallows benefiting 

 from agriculture or agricultural pests, at the minimum the clearing of 

 land for agricultural purposes has created foraging areas not available 

 in the precolonial period. 



Several nesting sites are associated with man-made lakes and 

 large impoundments constructed for waterfowl. Many hardwood trees 

 killed by floodwaters contain cavities made by woodpeckers that provide 

 multiple nest sites where the swallows form small colonies of a dozen 

 or more pairs. Although the beaver was reintroduced by various state 

 wildlife agencies and has made an explosive come-back, this generally 

 precedes the period of range expansion of tree swallows outlined 



