Migration is pretty simple. Birds fly south for the winter, and north for the summer. First Robin of spring and all that.
A biologist describes waterfowl movements:
Okay, so maybe it's not as clearcut as all that.
Just to be completely accurate, the biologist was describing a waterfowl bowel movement that just landed on his head. But it sorta applies to any attempt at giving a simple explanation of migration.
[I have a problem. I needed to complete this diary over the weekend because I did not have time during the week, and now I'm doing this from a phone at our cabin. Please be understanding. Much of this may emerge via comments, and there probably won't be any more pictures from me.]
So, why migrate? It's a lot of work - think of the energy demands involved in flying from the Arctic to Australia or South Africa or Argentina. Nonstop flight over the Caribbean when you're a 9 gram warbler... or a Ruby-throated Hummingbird. A thousand miles over open water if you're a Peregrine. Ultramarathoners look like wusses by comparison.
Seriously, why bother?
Food. And sex.
The tropics are great - lots of food, great weather, not much effort required to get by. But the space is limited, especially when there's a sudden population boom with lots of chicks to be fed. So a bird who can get beyond the comfort zone to breed has less competition. Okay, great - go north! But why not just stay north? Because SNOW!
Okay. Good point.
You eat insects - they're just not available, so you head south. You feed on the shore or in lakes. They freeze over and there's no food. You eat chipmunks and ground squirrels - the snow covers them up. You eat the things that eat all those things, and they're flying away to find food. You gotta follow them.
So let's start with ducks, because they do the classic migration that we all think of - north in the spring, south in the fall. They are moving south because the water they rely on is freezing over. For them, the ice isn't just about making food unavailable, it's also a security issue - when they're in the water, it's harder for land-based predators to get to them. Ice deprives them of that safety.
Waterfowl associate in huge flocks on migration and on wintering grounds, making for some spectacular wildlife viewing opportunities. Those flocks are about safety, too - many eyes on the lookout. In flight, the long formations of birds help them conserve energy, with each bird's passage made easier by the bird in front of it, an avian version of "drafting". Many people think the front bird is leading the flock to their destination; in fact, the lead changes constantly, with the bird in front dropping back to take advantage of the energy savings of flying behind others.