Then you have witnessed a behavioral adaptation known as mobbing. Adaptations are physical traits or behaviors that helps an organism survive in its habitat. Mobbing is a behavior that birds use to chase away predators. When mobbing small birds will often gang up on their target. The idea is that although a predator might be able to catch a single small bird diving after it, the predator will be overwhelmed by sheer numbers and leave to avoid further harassment.
Why are small birds acting this way? The answer depends. Sometimes small birds become more aggressive (and territorial) during mating season and they will chase away any bird that flies over their territory. Birds also use mobbing to protect themselves and their young from predators. When young birds are still in the nest adult birds will often fly after any large bird that strays "too close" to the nest. This "too close" distance is arbitrary and a bird may chase a predator hundreds of yards if feeling particularly feisty. Mammals (including people) are also lumped into this danger category - walk too close to a Red-winged Blackbird nest and you are likely to be mobbed.
Small birds don't discriminate. Besides people, mobbing targets include hawks, eagles, owls, herons, and crows. Crows themselves will often mob hawks and owls. Did you ever wonder why owls are camouflaged? You would be too if you had to avoid being mobbed all day when you just want to sleep!
The large poplar trees outside our office are the frequent perch of a pair of Red-tailed Hawks (Buteo jamaicensis). I am often alerted to the presence of the hawks by the sounds of mobbing birds. Last week it was a pair of Blue Jays (Cyanocitta cristata).
The jays took turns diving on the hawk. I don't think they ever made contact, but the hawk kept having to duck to avoid the jays.
After several minutes, the hawk had enough and flew away pursued by the jays.
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