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March 29, 2005

Mr Answer Man Explores The Wonderful World of Bill Curvature

Fran from Northwest Notes, in the comments on the Black-Winged Stilt: "Intriguing how some of the related (linked) birds' bills curve up, some down, and some are straight. Especially the upward-curved ones (pied avocet); maybe they're always flipping food up out of the mud?"

Though the various avocet species do display a rather striking bill curvature, to my mind the oddest bill shape belongs either to the crossbills, where the upper and lower bills end up pointing in different directions, or to New Zealand's Wrybill, where the beak curves sideways. When it comes to famous and odd, though, the flamingo beats them all hands--or perhaps claws--down, thanks to the late Stephen J. Gould, who wrote on the reasons for the flamingo's configuration in his collection of Natural History essays, The Flamingo's Smile".

In each case, crossbill, Wrybill, flamingo and avocet, the configuration of the bill is a result of the manner in which each species feeds. The crossbill feeds on the seeds inside of evergreen cones, so its bill is used to pry those open. The Wrybill "feeds in a variety of habitats along large rivers but preys mainly on mayfly larvae which cling to the undersides of large rocks. Prey may be taken by a direct peck or by a clockwise motion which utilizes the curvature of the bill. Pierce shows that the sideways curvature of the bill allows the Wrybill to capture prey in situations that would be unexploitable by a species with a straight or upturned bill, and he speculates that the species' feeding habits may have evolved during the Pleistocene, when prolonged cold temperatures would tend to keep insect populations low and inactive."

In the case of the avocets and flamingos, the bill shape illustrates the differing solutions each species has evolved in an attempt to feed most efficiently on a food source they have in common; the brine shrimp, or Sea Monkeys, as they are more popularly known.

Technically, it's not the food source that they have in common, as both flamingos and avocets feed on small creatures other than our beloved Sea Monkeys, but a feeding environment. Both prefer to feed on small organisms found in shallow salty waters, often in areas only one to two inches deep.

The smaller the individual food source the less nutrition there is in each individual specimen of that source, so lots of them must be eaten in order to survive. Members of the animal kingdom have evolved three basic behaviors in response to that iron clad law.

1. The Way of the Sponge: Don't move around a lot, and let the food source come to you. Not a very good choice when it comes to birds

2. The Way of the Whale: Feed on lots of little things all at once. The flamingo has adopted that strategy, sucking water and mud in at the front of its bill and then pumping it out again at the sides. Here, briny plates called lamellae act like tiny filters, trapping shrimp and other small water creatures for the flamingo to eat.

3. The Way of the Japanese Hot Dog Eating Champion: Eat lots of little things one at a time, but do so as fast as possible. This is one of the strategies chosen by the avocet, or six of them, depending on how you count.

Avocets have three visual feeding methods: pecking, plunging, and snatching; and six tactile feeding methods: bill pursuit, filtering, scraping, single scything (bill is held open slightly at the muddy substrate surface and moved from one side to the other), multiple scything, and dabble scything. Scything has been noted as the hallmark method. Avocets have also been observed foraging cooperatively in close groups using the multiple scything method, probably feeding on small fish

Pecking, plunging, and snatching are well and good when the prey is large enough--just ask the herons, but when engaged in the scything feeding behavior, the bill of a particular bird will cover a larger area--and so encounter more food--if it is parallel to the shallow bottom rather than at an angle to it. Thus the up curved bill of the Avocets, and the upside-down bill of the Flamingos, both of which allow those species to feed in shallow waters at the most efficient angle.

Of course, there's always a third way.

Posted by Bigwig at March 29, 2005 02:46 PM | TrackBack
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Thanks, Mr. Answer Man! I had a feeling you might be able to solve the mystery of the curved beaks. Very interesting. And I'm glad I don't have to scythe mud with my face to get a meal.

Posted by: Fran at March 29, 2005 08:22 PM
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