Tree Swallow (Tachycineta bicolor) nesting is an intense and perilous business! Human encroachment on forest habitats means fewer natural tree cavities. So these little iridescent superfliers have become dependent on human-supplied nestboxes.

Competition among individuals is fierce. Males perform aerial dogfights in defense of territory, sometimes even toppling from the sky to wrestle on the ground! Nesting females squabble and fight over nestboxes. A desperate female may resort to killing a rival female or her offspring in the nest. More than once I have had to rescue our resident female swallow from murderous rivals. A traumatic experience for all involved! 😱

This evolution toward increased aggression – a direct result of human interference – is not helping Tree Swallows. Studies show that offspring of aggressive females have a lower survival rate.

In this photo two of our resident females argue over a favored nestbox. Fortunately, violence did not erupt! Thomasina won her box affixed above our back yard deck, and Sylph moved into a front yard box.

For now, an uneasy peace! But until both of these gals have successfully raised their young, I remain ever vigilant.

Panasonic Lumix FZ-200

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