If you watch the birds in your yard closely in spring, you might have the opportunity to observe them engage in courtship behaviors. We all know that birds call or sing to attract mates, but there are also some behaviors you can watch for, some that occur between adult birds only at this time of year.
One of the most common is a male putting on a display to attract a female. These displays are an attempt to show his potential suitor that he's the best male around and the best guy to mate with. Here's a male Purple Finch displaying to a (barely visible) female by raising the feathers on the top of his head and quivering his wings.
Mate feeding, also called allofeeding, is when one bird offers food to another. It's usually done by the male feeding the female. I enjoyed watching this female diva Blue Jay just sit in a tree calling softly (very un-jay-like) while quivering her wings, waiting for a potential mate to come and feed her. A male approached, offered her the food, then quickly moved away to another perch.
When you're fortunate enough to observe these behaviors - wing quivering and one bird feeding another - timing matters. When seen during the early spring, it most likely is between adult birds exhibiting courtship behaviors. But when you see these same behaviors during the summer, or in late spring between early nesting birds, you may not be observing ADULT birds.
In the summer, a wing quivering bird could be a fledgling begging for food from an adult. The behavior would most likely be accompanied by an incessant whining call by the bird. Continue watching and you'll likely see an adult bird nearby and may even see the young one being fed. Most times by mid-summer if you see one bird feeding another, it is likely an adult bird feeding their young.
Either way, they are fun behaviors to observe. Just another reason we love this bit of nature right outside our windows!