Parental Care

After eggs have been laid in the nest, the parents must incubate the eggs. There are several ways in which males and females can brood their young.

One change that forms in females is the brood patch. This is an area of the breast that gets thicker after having laid eggs. The tissues gets thicker and more wrinkled so that it increases the amount in contact with the eggs. It also gets highly vascularized. The incubation patch, thus, is quite literally a heating device.

Boobies incubate their eggs by not sitting, but standing on them. There are a few modifications that occur in the feet of these birds.

The incubation period of the birds is the time from the last egg to the time of first hatching. There a bunch of birds that begin incubating eggs, though, before they lay the last egg. Thus, there are some times when this definition does not always fit.

The Galliform birds (megapodes), manage incubation pits with rotting compost. They put sand over the eggs to prevent them from overheating. These birds are the ultimate in precocial birds (a young megapode hatches and has no parental care whatsoever. It hatches from the pit and is ready to go.).

The size of a brood patch can vary with the clutch size. For example, a number of galls, have clutch sizes of three eggs. Thus their brood patch has a nook for each egg.

Physiologists have spent a lot of time looking for the temperature of the eggs during incubation. One of the aspects of incubation is trying to keep the temperature of the egg fairly constant over the whole surface of the eggs.

Altricial species (who are born looking mostly like maggots) have a greater application of heat over time. Precocial species (which generate endothermy when they are still within the egg), do not increase their incubation over time.

The weight of the egg decreases over time from the time it is born to when it hatches. Usually there is about 14% loss over time.

Is incubation costly?:
To look at this you need to look at the metabolic rate of the individual. There is no evidence that incubation is more costly inside the nest than it is outside the nest.

Incubation constancy:
There is great variation between species in the number of times that a birds will stay on the eggs.

Sessions. Attentiveness:
When the air temperature is high, there is less heat loss from the eggs, so they spend longer off the eggs. As the air gets cooler, they spend more time on the eggs. Thus, incubation of the eggs is clearly ‘air-dependent’.

Incubation based on introduced species:
In a cold place the starlings, bring their ancestral patterns of incubation to it, but the crested myna in cold places does not do so well. Most of the reason for this may be because they have traditionally different incubation patterns. The crested myna in Vancouver simply doesn’t have the capacity to adapt at incubation.

Incubation periods of hole-nesting birds as compared to open nesting birds is slightly longer in hole nesting birds. Most notably because predation is lower in hole-nesting birds.

Classic study: Copper eggs
A dove was given metal eggs with an inlet into it to alter heat. When the egg is warm, the bird thermoregulates as if it is warm. When its cold, it’s the opposite. Thus the bird is treating the egg like it's part of the core.

Other classic studies: Tinbergen looking at behaviorally how a bird recognizes its nest.

By Rob Nelson

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