07 December 2009

Functions of color continued. . .

Black-billed Magpie

2) predator defense via deflection--rump patches, spots in wings or tail. Spots on the tail are especially advantageous because, if they are "caught," damage is limited. Concealed spots may have startle effect.

3) thermoregulation--dark plumage may absorb more radiation than pale ones. Dark makes feathers warm and heat lost more slowly. This may support Gloger's Rule, but it really does not hold water. Light birds are more common in North where the 2 degree advantage in thermoregulation must be outweighed by background matching. Black-billed Magpies face their dark fronts to the sun in the morning and their white sides to the sun in the heat of the afternoon.

4) foraging aides--shorebird eye stripes are "sights" to aim bill with (?). European Kingfishers with covered eyespots inefficient hunters. Fish-eating seabirds ventrally white, so little contrast against the sky (fish see white least well). Most terns are fish-eaters and are white below. Exceptions inlude Black Terns that eat insects, Noddy Terns that are nocturnal and Lava Gulls that are also nocturnal. Spots may startle insects out of hiding and may account for the plumages of mockingbirds and redstarts (look these up in a field guide).

5) mimicry--Basically there are no poisonous birds (although one with toxic feathers and muscles was discovered recently in Borneo--these tasted bad enough that local people don't eat them). Golden-winged Warblers are supposed to mimic chickadees: they often associate together. The advantage for the warbler is that chickadees know area and predators since they are year-round residents. The warbler's black bib lessens interspecific aggression. The advantage for the chickadee is that the warbler can easily open buds that chickadee can then work over. Zone-tailed Hawks and Turkey Vultures converge on flying method and is usually given as an example of aggressive mimicry. Prey is not afraid of the silhouette of a vulture and is surprised when one of the vultures is actually a hawk. Other examples of mimicry usually hypothesize aggressive-dampening effects on competitors.

Lava Gull

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