A Flashing Flock of Willets

I spotted this guy at Waukegan Beach this morning:

Willet (Tringa semipalmata), Waukegan Beach, Lake Co, IL 5/2/2014

He's a Willet (Tringa semipalmata), a large species of sandpiper. He was part of a flock of 19, which is the second largest group I've ever spotted in Lake County.

Willets are rather plain critters, with little of the bold rufous or solid black feathers that so many of our shorebirds sport this time of year. But when they fly....

Willet (Tringa semipalmata), Waukegan Beach, Lake Co, IL 5/2/2014

What's up with that black and white wing!

To examine that, here's a bird I've shown before:

Chestnut-collared Longspur (Calcarius ornatus),
Kidder Co, ND  6/2/2013

This is a female Chestnut-collared Longspur (Calcarius ornatus). And this is what she looks like when she flies:

Chestnut-collared Longspur (Calcarius ornatus), Kidder Co, ND  6/2/2013

Here you can see a very common pattern, taken rather to extremes by this species. The white outer tail feathers are something we see in juncos, mockingbirds, many warblers, and quite a few sandpipers. You generally don't see these markings unless the birds fly, just as you don't see the wing stripe in that first shot of the Willet. Given that sudden appearance, it's tempting to think that this display is primarily aimed at notifying other members of the flock that a predator is about. I can't find much on that idea in these species, but Caro, et al. tested this idea in White-tailed Deer (Odocoileus virginianus). Surprisingly, they found very little evidence for the idea, suggesting instead that the white tail may be a signal to the predator that that individual isn't worth chasing. (1)

In birds, much of the research has focused instead on sexual selection and social signaling. Howe noted that Willets use their wing patterns in courtship displays (2), while Fitzpatrick hypothesized that white markings may be used to amplify other signals inherent in tail structure. (3) Ferree investigated whether female juncos might use the amount of white in their partner's tails as a signal to adjust the sex ratio of their offspring. He found instead that the male's tails didn't matter, but that the sex ratio varied with the amount of white in the female's tail. (I wonder if he had considered the impacts of extra-pair paternity on his initial predictions?) (4)

A common pattern here, and elsewhere in this sort of research, seems to be that there is a single purpose behind the feature being examined. Watching those Willets, it's hard to imagine that they ignore each other's markings when a predator flushes one of them. There really isn't any reason why a trait can't be influenced by predation as well as by sexual selection, and trying to tease out the relative contributions of each factor might turn out to be more fruitful than trying to pick just one reason out of a flock of flashing Willets.

(1) Caro, T. M., Lombardo, L., Goldizen, A. W., & Kelly, M. (1995). Tail-flagging and other antipredator signals in white-tailed deer: new data and synthesis.Behavioral Ecology, 6(4), 442-450.

(2) Howe M. A. 1974. Observations on the terrestrial wing displays of breeding willets. Wilson Bull. 86: 286–288.

(3) Fitzpatrick, S. (1998). Birds' tails as signaling devices: markings, shape, length, and feather quality. The American Naturalist, 151(2), 157-173.

(4) Ferree, E. D. (2007). White tail plumage and brood sex ratio in dark-eyed juncos (Junco hyemalis thurberi). Behavioral ecology and sociobiology, 62(1), 109-117.