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<\/p>\nOne of the primary functions of feathers is insulation and they’re very good at preventing heat loss.\u00a0 But this efficiency\u00a0presents a problem – how to keep the egg(s) sufficiently warm during incubation, since feathers act as a barrier to heat transfer from parent to eggs.\u00a0 Typical bird body temperature is about 104 degrees F. and the ideal incubation temperature for their eggs is very close to normal human body temperature – 98.6 degrees F.<\/p>\n
That dilemma has been solved by most bird species\u00a0through the evolution of the\u00a0“brood\u00a0patch” –\u00a0an area\u00a0of belly skin that loses\u00a0its feathers near the end of the egg-laying period.\u00a0 As the feathers are lost, supplementary blood vessels engorge the\u00a0skin of the “patch”\u00a0so that hot blood is brought to the surface.\u00a0 Amazingly, the parent can even shut down blood flow to this region when that bird is off the nest.\u00a0 When the incubating parent returns to the eggs for incubation, he\/she typically goes through settling movements that bring the brood patch into gentle contact with the eggs.<\/p>\n
The patch may be large or small, depending on a variety of factors including how many eggs are incubated.\u00a0 Some species have a single patch in the middle of the belly, others may have one each side or even three.<\/p>\n
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Typically the patch is difficult to observe in wild birds, particularly in flight but I have several images of a female Short-eared Owl where it is visible.\u00a0 This female had two chicks that she was trying to keep warm\u00a0during a typically cold Montana spring.\u00a0\u00a0 The male was bringing food to the nest and she very seldom left it but occasionally, when it warmed up later in the day, she would go out for a little exercise.<\/p>\n
This shot shows the brood patch reasonably well, though the image quality suffers because of low light and a slow shutter speed.<\/p>\n
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One time the female became impatient while waiting for the male to deliver a vole so she went out and took it away from him.\u00a0 Here she is returning to the nest with\u00a0that vole for the chicks.\u00a0\u00a0\u00a0Sharpness is better in this shot but it’s a little difficult to separate the patch from the foot carrying the vole.\u00a0 The patch is slightly more pink and above the foot.<\/p>\n
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An extreme crop allows a slightly better look at the right side of the patch.\u00a0 And hey, I even got a catch light in the eye of the vole! \ud83d\ude42<\/p>\n
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In most species, the feathers covering the belly are lost automatically (through hormonal control)\u00a0just before brooding but ducks and geese must actively\u00a0pluck the feathers, which are then used to line the nest.\u00a0 Pelicans, gannets and boobies have no patch so their eggs are cradled in their webbed feet and warmed\u00a0from below and above.\u00a0 In species where both parents incubate, each sex develops a brood patch but if only the female incubates, as in these owls,\u00a0the male doesn’t develop a patch.<\/p>\n
In precocial\u00a0species (active soon after hatching, ducks and geese for example) the insulating feathers over the patch grow back soon after the eggs have hatched since the chicks leave the nest immediately.\u00a0 But in altricial birds (helpless for an extended time after hatching, most songbirds for example) the regrowth of feathers over the patch is delayed and the patch remains functional\u00a0for\u00a0the full time the chicks are brooded in the nest, as in these Short-eared Owls.<\/p>\n