True to their name, tree swallows nest in hollow trees – or in the next best thing, bluebird boxes.

When they arrive in early spring, their first and most important activity is to find a nest hole.  Suitable nest sites are scarce, so tree swallows are aggressive about claiming them and will fight – even kill – another tree swallow of the same sex who dares to claim their nest hole.  At this stage it can get gruesome.  If a site has nestlings and the male dies, the new male may kill the widow’s young.  Females have been known to kill the young of others to make the site become available.  So much drama!

But there are other challengers who want nest holes.  House wrens, house sparrows and northern flickers will destroy tree swallow eggs and nestlings if they can.  Eastern bluebirds also want nest holes but they get along with tree swallows if two boxes are provided near each other, one for each species. 

Aside from nest site competition, tree swallows are very social creatures and tend to nest near each other if enough sites exist.  Once a site is selected the female builds a nest inside it and adorns it with feathers.  Even if she arrived weeks earlier, she waits to lay her eggs in May so the babies will hatch around June 1.  Both parents feed the young, making 10-20 trips per hour to keep those yellow mouths filled! 

Now it’s July and all the tree swallows are about to fledge.  Next month they’ll be on the move in large flocks, headed for Florida and the Gulf of Mexico. 

Watch them while you can.

(photo by Kim Steininger)