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February 1999 - Territoriality
The article below is an example of a Behavioural Note, from the May 1999 issue of Talon Tales.
From a human perspective, incubation seems like a monotonous chore. However, it is of course an essential part of a birdís life history, and there is much more to it than meets the eye.
In all birds, normal egg development requires incubation at a constant temperature, which for most species is roughly 37oC (98.6oF). Interestingly, this is 3oC (4.8oF) lower than a birdís body temperature. Birds generally transmit heat to their eggs through their brood patch, a region of the abdomen which becomes temporarily bare during incubation to allow for more direct heat transfer. In some species both sexes develop a brood patch, but in many others only the female does.
Anyone who has spent time watching peregrines incubate will know that they turn and rearrange their eggs frequently. This is something which all birds do, and is essential for the survival of the embryos. It ensures that the eggs are warmed evenly, and that the embryonic membranes donít stick to the shell. Egg-turning may occur as often as every 8 minutes with smaller birds such as warblers, while most larger species do it much less frequently. Peregrines tend to rotate the eggs once or twice an hour, but there is a lot of variation between individuals, as well as over time. In the last few days of incubation, the female in particular often becomes quite restless, and may move the eggs every few minutes.
At an average of only 10 days, the black-and-white warbler has one of the shortest incubation periods among North American birds. Most songbirds sit on their eggs for approximately two weeks, and even the larger species such as crows and ravens donít require more than three weeks. In contrast, raptors incubate much longer. In southern Ontario, peregrine eggs typically hatch after 33 to 35 days of incubation. However, the length of the incubation period varies by region - in other parts of the world it can be as short as 29 days. An incubation period of four to five weeks is quite typical for most North American raptors, although some of the larger species do take a bit longer - the golden eagle, for example, has an incubation period ranging from 43 to 45 days.
Sources: CPF Falcon Watch Centre notes, "The Birderís Handbook" (Ehrlich, Dobkin, and Wheye, 1988), "Guide to Management of Peregrine Falcons at the Eyrie" (The Peregrine Fund, 1996)
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