Talon Tales - behavioural notes

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 bird’s 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 37^oC (98.6^oF). Interestingly, this is 3^oC (4.8^oF) lower than a bird’s 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.

Keeping the temperature constant is very important. Embryos are particularly sensitive to heat, and on warm days may need shielding from the sun rather than incubation. Surprisingly, they are less affected by the cold. Mallard eggs, for example, have been known to crack from freezing, yet still hatch normally. Of course, incubating birds try to compensate for weather variation as much as possible. They can control the amount of heat they transmit to the eggs by both the amount of time they are on them, and how tightly they sit on the eggs. Many birds have receptors in their brood patch which sense the temperature of the eggs, allowing them to regulate their rate of incubation with considerable accuracy. It is for this reason that the attentiveness of the adults often declines in the later phases of incubation, because the amount of heat generated by the embryo increases as it develops.

Once birds have started incubation they remain devoted to the eggs. During a surprise snow storm in Etobicoke on April 10, 1999, Alberta never got off the nest. (Photo by Marcel Gahbauer)

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 don’t 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.

While incubating, peregrines often spend a lot of time ‘housekeeping’. Here, Hamilton’s Dad leans forward off the eggs to rearrange some gravel. (Photo by Marcel Gahbauer)

In most species of birds, the adults share incubation duties, although in many cases the female spends more time on the eggs than the male. Patterns vary considerably from species to species. In some cases the female will incubate through the night and the male will spend most of the day on the nest, while in others they alternate more frequently. Raptors are one of the groups of birds in which the female often does all of the incubation. However, both the peregrine and the osprey are exceptions to this rule. At Mountsberg, we have seen Duncan and Isabell switch (on average) every 45 to 60 minutes throughout the day. With peregrines the sharing of incubation tends not to be as balanced - females do most of it, with the males often only coming in for a couple of hours two or three times a day.

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 don’t 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 Birder’s Handbook" (Ehrlich, Dobkin, and Wheye, 1988), "Guide to Management of Peregrine Falcons at the Eyrie" (The Peregrine Fund, 1996)

The Canadian Peregrine Foundation