BEAKS & BILLS
SAN MIGUEL IS not an island, nor is there a sea around it; the reserve has populations of birds that range far and wide, from Alaska to South America. The Groove-billed Ani, for example, is as much at home in Surinam as it is in Guanajuato. The bill is impressively large, suggesting a finger would be lost if the bill clamped down on it. This relative of the cuckoo - a ground-feeder like towhees, quail and many sparrows - hunts grasshoppers and picks ticks from the backs of cows.
If we made a list of all the birds in El Charco that eat grasshoppers, we would find a variety of bill shapes and arouse some compassion for grasshoppers. The Curve-billed Thrasher, Golden-fronted Woodpecker and Loggerhead Shrike all eat them, but not exclusively. So there is no one bill solely fitted for that food. Is there possibly another reason why the beak of the Groove-billed Ani is so large? Anis are very gregarious; they form groups and sit together; some groups are as large as 50 birds; in the reserve one can see ten or more in the same tree. We may entertain the idea that in some cases bills evolve as a result of sexual selection. If the lad with the finest bill gets the best lass, then his genes have an advantage and bills become longer or wider or curved. In summary, there are two ideas about bills and beaks: they are tools fitting a species to a feeding regime and they may play a part in sexual selection.
Some feeding regimes are so specialized the beak fits the diet like a bolt to its nut. The Loggerhead Shrike, for example, has a bill like a raptor, with a down-curved cutting tip on the upper mandible. Shrikes hunt like small hawks, preying on insects and small animals, including small birds. El Charco suits the shrike: there are many thorn trees - huizache and mesquite, for example - and for the shrike, a thorn is a tool. On a thorn the shrike will impale its prey, making it easier to open the mouse or sparrow and eat the flesh. It isn't a specialization that has radiated widely, giving us a family of shrikes as large as that of the sparrows, but then the ratio of carnivores to herbivores, or, in this case, granivores, always favors those with a vegetarian diet.