Bay Area residents are familiar with the sound of a jay flying into a window. Michael Ellis explains the behavior and more about the family of local jays.
A friend of mine, Bob, called me about a feisty bird, which he called a Blue Jay, that’s been flying against his bedroom window and whacking into it over and over again. He wondered what the bird was doing and how he could stop it. He also watched the jay feed a squished earthworm to another.
I get a variation on this question every year. The bird he saw, while the dominant color was blue, was not a Blue Jay. It is either a scrub jay or a Steller's jay. Blue jays are common in the eastern U.S. but do not normally cross the Rockies into the West. Here in the Bay Area we have two widespread species of jays. The scrub jay (the dominant color is blue and gray) thrives in many suburban yards, chaparral and scrubland. The Steller's jay, with a prominent crest and much bluer, usually frequents forests. The members of the Corvid family, which includes crows, ravens and jays, have little or no difference in size or plumage between the sexes.
That whacking performance Bob witnessed is clearly related to territory control which is in turn a breeding behavior. The jay sees an image of itself in the window and assumes that it is a stranger invading his turf. He therefore “attacks” it to defend his terrain. It is probably the male doing that behavior, though in a monogamous mated pair the females can also be aggressive. The feeding behavior he saw was the fortunate couple sharing food resources therefore reinforcing their pair bond. Mating could follow this. Because you know the same thing often happens in humans. A nice dinner proving financial fitness followed by, you know.
To prevent the bird from injury you must ameliorate the reflection. A paper silhouette of a falcon or an accipiter hawk plastered on the window often does the trick. But eventually the testosterone will wear off and the scrub jay will stop. And hopefully both the bird and the window will still be intact.