In the first post of this series on bird migrations we discussed some of the astounding feats of avian migration. In this post we’ll examine the question of why most birds migrate. After that, we’ll move on to the question of how they migrate.
After my son acquired his undergraduate degree, he moved back to our hometown to pursue his Master’s Degree at the local university. This he did primarily due to financial constraints. He rented an apartment less than one block from his Mother’s house which did surprise me a little. When I asked him why his response was simple, “Dad, mammals have to live close to their food sources.”
The same is true for birds, of course, which is why they migrate.
Imagine that you make your living eating insects. Imagine that you are a wood warbler or a vireo or a thrush or a flycatcher. You love to eat insects. You also know, even though you think without words, that the farther north you go in the summer the more and bigger insects you’ll find. For instance, moving north along the western spine of North America, the insects are smaller and fewer in the southern Rocky Mountains. By the time you get to Montana they are larger, more numerous and much more irritating to the humans you share the planet with than they are back toward Mexico. But even Montana is nothing compared to Alaska where they joke that their state bird is the mosquito. There is even a murder mystery, the name and author of which I disremember right now, where the victim is left outside and naked as a summer Alaskan night comes on and is killed by the mosquitoes.
But if you are a bird, you like mosquitoes; big juicy fat ones in staggering numbers and you’ll find them in the Arctic every summer. You may even find some with nectar clinging to it like this one. Another bonus of the Arctic summer, billions of flowers, full of nectar. And it is beautiful as well and you appreciate beauty.
You’ll also find many more hours of daylight to eat bugs and nectar. If you can get far enough north, you’ll have sunlight almost 24 hours a day. Which is why you’ll go there every June and July. Not only will you have all those calories to eat and all those daylight hours in which to eat them; you’ll also have more energy to reproduce and raise your new offspring. More than 160 species of birds migrate to and from just the Alaska National Wildlife Refuge. You can find the entire list here.
But you certainly won’t want to be there in December when the sun barely makes it above the horizon for a couple of hours each day and the temperature never reaches freezing and all the bugs are long since dead. You have to maintain your body temperature, which is somewhere between 104 to 111 degrees Fahrenheit, depending on which species of bird you belong to. This higher body temperature means that your nerve impulses travel faster and your muscle strength is greater. Unless you hibernate, and only one species is known to do that, you better not be in the Arctic after mid-September.
To maintain your body temperature you need to be somewhere where the sun shines and somewhere you can find lots of bug calories to eat. You aren’t stupid and neither were your ancient ancestors, some of which started living on the planet 150,000,000 years ago. So, you fly south for the winter.