Incubation is a trade-off

Incubation represents a trade-off for parent birds between spending enough time warming the clutch and obtaining enough nutrients for self-maintenance. Too much time off the eggs might cause them not to hatch whereas too little time feeding could cause malnutrition. 

Some food supplementation studies have shown that providing extra food for birds can reduce incubation period duration. This could be as a result of improved foraging conditions for the incubating bird, thereby enabling nutrients to be gathered more efficiently and more time to be spent on the nest, or by altering the perceived peak in food availability for nestlings. 

Males often help females to gain sufficient nutrients during incubation by courtship feeding their mates.

Keeping the eggs warm

Before incubating, birds develop a highly vascularised brood patch on their breast and belly. The numerous blood vessels in this patch keep the area warm, and allow heat from the body to warm the eggs. To enable efficient heat transfer from the brood patch to the eggs, the incubating bird loses its feathers on the breast and belly, either by dropping them or pulling them out. These feathers are often repurposed as a nest lining.

Controlling incubation temperature is important. The ideal temperature for most birds to incubate their eggs is around 38°C, and the incubating bird raises or lowers its body over the eggs to adjust the warmth. The adult will also turn the eggs and move them around to facilitate development. 

If the eggs are overheating, birds will spend some time off the nest to enable them to cool. In warmer climates, birds sometimes use their bodies as a parasol for the eggs or wet their breast feathers before returning to drip water onto the eggs.

What happens inside the egg during incubation?

Incubation enables small clusters of cells that are positioned on the surface of the yolk at the time of laying to divide, differentiate and eventually develop into a fully-formed chick. 

Calcium for the formation of the bones is obtained from the eggshell, which weakens the eggshell structure, helping to facilitate hatching. 

The yolk acts as a nutrient reserve during this chick development and forms the basis of a small food reserve for the bird just after hatching. This reserve might be particularly important if foraging conditions for parent birds are poor when the chick emerges from the egg.

Timing is everything

The duration of the incubation period influences hatching date which, in turn, can have a profound influence chick and fledgling survival.

In some species, such as Blue and Great Tits, early hatching almost always increases chick and fledgling survival rates. To advance hatching, birds might incubate more intensively, although this is a trade-off with time for self-maintenance (see above). Alternatively, birds could start incubation earlier in the laying sequence.

If incubation starts at or near the end of the laying sequence a clutch will hatch relatively synchronously. If incubation starts earlier in the laying sequence, however, the clutch will hatch asynchronously. In some species (e.g. Jackdaw, Tawny Owl) asynchronous hatching is deliberate – a strategy whereby the last to hatch, and smallest chick will be reared if conditions are good but will soon die if conditions are poor (and may even be eaten by its hungry siblings).

In most garden-nesting species clutches hatch synchronously with adults attempting to raise all young because, on average, this is the most productive strategy.

Who does the work?

In most frequently seen garden birds, such as Great Tit, Blackbird, Dunnock, Robin, females incubate their clutches alone. In a few garden species, however, such as House Sparrow, Woodpigeon, Collared Dove and Starling, incubation is shared between the sexes.