NOTE: Unless birds are banded, it is difficult to know whether the next nest in a box was from the same pair (or a different male/female.) However, some observant backyard bluebirders can tell bluebirds apart (by behavior, size, coloration or other features.) Also, sometimes, for whatever reason, birds move to a different location for subsequent broods.
Availability of a mate: Both a male and female are needed to lay fertile eggs. The male bluebird helps guard the nest and feed and care for the young. If a mate is lost during breeding season, that may be the end of breeding activity for the year, unless the surviving adult finds another mate. See widows/widowers for information on what happens when a mate is lost after eggs are laid or nestlings have hatched.
Location: In the northern parts of their breeding range, bluebirds may have just one or perhaps two clutches per breeding season. (In northeastern CT, I have only had three broods.) In the southern reaches, they may have as many as four, but two or three is more common. In warmer climates, breeding may start earlier (allowing more time for additional broods.)
Time of Year: The breeding cycle takes time, from nest site selection to nest building to egg laying to incubation to hatching to fledging to when young become independent. (See All About/Biology pages for time frames for various species.) Also, many bluebirds migrate after breeding season ends (in the fall.) Adults and fledglings need to have completed molting (feather replacement) in order to be able to fly long distances.
Food Supply, Diet and Health : It takes energy to court and for the female to manufacture eggs. It also takes energy and lots of food to feed growing nestlings and hungry fledglings. (Bluebirds continue to feed their young for about 28 days after they leave the nest). The availability of more food, or higher quality food, increases the likelihood that the parents will start another brood. A healthy bird in good condition may be more likely to start another brood.
Age and Experience: An older, more experienced female (or a pair that bonded in the previous season or early in that year) might be more likely to start nesting early, and have multiple broods. Older females may be better at foraging, and in better condition, so they might have more broods. Note: Adults bluebirds do not start breeding until the year after they are born. Bluebirds may only survive to breed for a couple of seasons - three or four if they are lucky.
Helpers: With bluebirds, the young from previous broods may help feed subsequent broods. This can make it easier to support multiple broods. More.
Birdality: Individual birds have different behaviors, and some may be more family oriented or "driven" to produce more young than others. This may be a result of genetics, learned behavior, or something else we can't quite explain. Some females have been known to produce four broods every year, in the same box. On the same trail, another female may only produce one brood per year. More.
Species: There are three species of bluebirds: Eastern, Mountain and Western. This webpage focuses on the number of broods bluebirds have. Eastern Bluebirds may have 3 or 4 broods, Western may only have 2 or maybe 3. Other species of cavity nesters are different. For example, Tree Swallows and Chickadees typically have only one brood per breeding season, regardless of the factors above, unless a nest attempt fails. See differences between bluebirds and other small cavity nesters.
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