It’s spring in the Rocky Mountains, the air is filled with birdsong and my feathered neighbors are back again. Recently, a pair of American robins arrived and set up housekeeping in the neighbor’s maple tree, just as a pair did last year. There’s a song sparrow in residence again in the lilacs near the creek, belting out its bubbling song. Riding my bike to work takes me past a small field, and sure enough, there’s a western meadowlark back again singing from the same telephone pole and claiming that field for his own.
Why do migratory birds that travel hundreds of thousands of miles to spend the winter come back, not just to the same state or valley, but to the very same copse of trees or patch of shrubs? Biologists call this tendency to return to the same nest territory "site fidelity." Think of it this way: if you had to spend a lot of energy searching for food, and at the same time be on the lookout for big animals that would like to make a meal of you, do you think you’d have better luck in a completely unfamiliar town, or in a neighborhood where you knew every street, store and back alley? How about if at the same time you’re trying to provide food to a mate and growing kids?
There’s another good reason to come back to the same neighborhood, and that is knowing your neighbors. Biologists studying hooded warblers in a woodland in North Carolina discovered that each male warbler defending a territory knew the individual songs of all its neighbors. Although every hooded warbler sounds similar, they also have slight individual variations on their songs. Warblers spend a lot of time fighting and singing to establish a territory. But once they’ve established a boundary with one neighbor, they come to a truce; they agree where the fence should be. If a strange warbler arrives and begins singing, the males will spend more time battling it out with the stranger. So knowing their neighbors’ individual songs saves them lots of energy. “Oh, that’s Joe, I don’t need to worry about him.”
The really amazing thing is that not only did these warblers know their competitors’ songs, they remembered them from one year to the next. Now, there’s about a 50-50 chance of a male hooded warbler surviving from one summer to the next, and young warblers will claim territories of old-timers that didn’t survive. If a male warbler returns to the same territory he claimed last year, that’s a 50-50 chance that each of his neighbors will be someone familiar. If he’s already got the territory boundaries sorted out with his familiar neighbors he only has to worry about fighting things out with any strangers. Knowing his home turf and neighbors means he can spend more time raising a family.
But imagine that little warbler returning from a 1,000-mile journey to Mexico and back, only to find his home territory paved over for a shopping mall or a suburb. What are his chances of survival now?
The second Saturday in May is Migratory Bird Day. Help celebrate the wonder of migratory birds this month. Go for a bird walk, plant native shrubs and trees in your yard, or help preserve a bit of habitat for these bright, long-distance travelers.
"Field Notes" is produced by the Montana Natural History Center.