Bluebirds lay one egg a day, until they have a complete clutch. (They may not be around the nest much inbetween laying.) Incubation begins when the last egg is laid, so all the eggs will hatch around the same time.
The number of eggs a female bird will lay before her clutch is complete depends on several factors:
Some species (called determinate egg layers) will not lay extra eggs in response to egg removal (simulating removal of eggs by a predator), or do not stop laying eggs if eggs are added to their nest. (See studies, below)
Others (indeterminate egg layers) will keep laying if eggs are removed, or may respond by stopping laying, or may lay fewer eggs if extra eggs are added to their clutch. The timing of egg removal or addition may affect the response.
Food Supply and Diet: It takes energy to manufacture eggs. Clutch sizes may be larger if more food, or higher quality food is available. During a drought, clutch size (and hatching success) may be lower.
Occasionally a monitor finds a box with 7 eggs, but it is likely that not all 7 will fledge, probably because of lack of food - supplementing with mealworms or suet might help.
Age and Experience: An older female may have a larger clutch. Older females may be better at foraging, and in better condition, so that might be why they lay more eggs.
Health: A healthy bird in good condition may lay more eggs than a bird in poor condition.
Time of Year: Towards the end of the nesting season, clutches may be smaller (i.e., early broods may have an egg or two more). If, for example, a Tree Swallow has a failed nesting and has to start over, they may lay a smaller clutch the second time. Domestic ducks often stop laying during the winter, when temperatures are cold and day length is shorter.
Location: Clutch sizes are typically larger at higher latitudes. For Eastern bluebirds, clutch sizes farther south are small early in the season, increase in mid season, and become smaller again late in the season. (Dhondt 2000)
Availability of a mate: The eggs need to be fertilized. In many species, courtship induces secretion of follicles hormone by the pituitary gland. If the mate is lost, egg laying may cease (or the female may abandon the nest). See widows/widowers.
Egg dumping: This sometimes happen when a female lays her egg(s) in the nest of another bird, sometimes creating very large clutches, e.g., chickadees (also called conspecific brood parasitism). There have been rare reports of two female bluebirds laying their eggs and rearing their young in the one nest.
In studies of bluebirds, the floor size of a nestbox does not appear to influence the number of eggs laid per clutch (e.g. Purcell et al 1997).
Predation: If nests are predated over and over, the female may continue to lay a new egg every day. If the clutch was already complete when the eggs were removed, there may be a delay until egg production starts up again. It has been suggested that the process of incubation may trigger endocrines that cease egg production. As laying eggs uses a lot of energy, it is imperative to discover and control the cause of repeated predation. See Predator ID and Solutions.
Studies of egg removal and addition: (NOTE: It is illegal to remove eggs from a native bird's nest without a permit, under the Migratory Bird Treaty Act.)
L.J Cole (1913) found that if eggs, presumably from the same female house wren, were removed daily, the bird laid an unusually large number of eggs. This bird laid 13 eggs, rested 4 days, laid 7, rested 4 days, laid 5, rested 5 days, laid 5, and stopped. In a subsequent experiment the addition of 7 eggs on the day that the first egg was laid did not prevent the laying of 7 eggs.
Cowbirds (which parasitize the nests of other birds) usually lay about six eggs (one each day) in different nests, wait a few days, and then start again. They may lay more than 11-20 eggs per season. A captive female was recording laying 77 eggs, 67 of those in a continuous sequence.
Fostering: Some people want to foster abandoned eggs into the nest of another bird. This is generally NOT a good idea - More....
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