Birds are a something of constant hazard for UAV flyers, especially if you fly in mainly countryside, where trees meet the borders of fields, where nests are common. But the large majority do not generally cause problems unless it's nesting season and we fly in proximity to nests.
In general there are 3 types we need to worry about: swallows, crows (though not all types), and raptors. If you are not proximal to their young, any other type of bird will most likely fly away or be uninterested in your craft. Not so with the 'danger 3'. Of those, hawks are most likely to cause serious problems, and can take your machine down with a single strike, or pursue it and attack multiple times until it is removed from the sky. For that reason, it is sensible to leave the area as soon as you see any of these birds in proximity to your craft, and never fly near nesting sites during the breeding season, when they are even more aggressive and relentless in defending their territory. Best bet when under threat from raptors is to get low fast and bring the craft back to you asap. They won't usually pursue the craft in near vicinity of the pilot.
Sometimes of course, even hawks don't attack, but will almost always come over to have a look if you launch in their territory. The trouble is that it's extremely difficult to know what the intention of a circling bird is, and the transition from one that is 'just looking' to one that is angry and diving your craft can be scarily fast, and by then it's probably too late to do anything about it...