It started with a question from a reader: "Do birds mourn?" He described a scene at the family cabin, where a pair of barn swallows had been nesting under the deck. Unfortunately, one of the pair was killed in a collision with a cabin window. The other swallow then seemed to be holding a vigil, sitting halfway between its dead partner and its nest for some time.
The question of whether birds feel emotions — and grief is surely a major emotion — is a contentious area in avian research. Some say it is likely that birds experience feelings, while others hold out for some kind of proof. The problem is that emotions are a tough thing to pin down, even in humans. We're a very expressive species, easily able to read facial expressions to assess how others are feeling, but birds are a pretty expressionless bunch, at least to us.
So, we tend to look at bird behavior to try to gauge what might lie behind it.
Many people seem to have observed what is often called a crow funeral, with crows flying in to gather near a dead crow, usually on a roadside, calling and standing in a semicircle. Is this truly a funeral, or is something else going on?

An adult osprey returns to its nest to find it empty, both young osprey having been snatched by a great horned owl. The osprey stands on the edge of the nest and makes soft calls for a half-hour or more. Is the adult osprey lamenting the loss of its family, or is there something else at work?
And how about the barn swallow seeming to mourn the loss of its partner after a collision with a window?
Fifty years ago, experts would only say that the birds in all three cases were reacting to stress or change. They'd label as anthropomorphism any suggestion that anything more was at work in these scenes.
But since the 1970s, those who study birds, either in the lab or in the field, have found more and more instances where bird behavior often seems similar to our own. Birds have been shown to be having fun, getting angry, expressing pleasure.