April is when spring migration kicks into high gear, with more than 50 species of Neotropical songbirds coming into Georgia from winter grounds in the southern tropics to nest here during spring and summer.
Once they arrive, the migrants — warblers, tanagers, flycatchers, vireos, buntings and the like — will waste little time in mating, building nests, laying eggs and raising babies. They will be joining Georgia’s year-round resident birds — cardinals, bluebirds, titmice and others — that already have commenced nesting.
As the songbirds — whether year-round or migratory — prepare to nest, a peculiar patch of bare skin appears on the undersides of most females and on some males. It is an “incubation patch” (or brood patch), a relatively large area of skin totally devoid of feathers and infused with blood vessels.
As its name implies, the incubation patch is vital for keeping eggs heated at a uniform temperature in order for them to hatch. Feathers, of course, act as protective insulators against rain and cold, but they prevent efficient incubation. Birds have solved this dilemma through the incubation patch.
A patch develops shortly before a female lays her first egg or during the few days of egg-laying. Her inner, downy feathers and some of the outer feathers on her belly area suddenly loosen and fall out. An example is a female prairie warbler, a colorful Neotropical migrant that nests in most of Georgia. Her defeathering begins about three days before she lays her first egg and is completed a couple of days after her full clutch is laid.
As the feathers drop off, the bare skin swells as it begins to hold more water and its blood vessels expand, making the patch almost as hot as the bird’s inner body temperature. When an incubating female sits on her eggs, her skin muscles will widen the patch, and she can put the skin directly on her eggs.
Patches also may develop in some male birds — such as brown thrashers, barn swallows and downy woodpeckers — that help their mates incubate eggs.
IN THE SKY: From David Dundee, Tellus Science Museum astronomer: The moon will be new on Monday; on Tuesday evening, look for a thin crescent moon in the west. Mars is low in the southwest at dark and sets in the west a few hours later. Jupiter and Saturn rise in the east just after midnight.
Charles Seabrook can be reached at charles.seabrook@yahoo.com.
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