The Western Bluebird Mating Habits
The western bluebirds are monogamous passerine birds and keep long-term bonds. However, they are sometimes known to engage in extra-pair copulations (mating outside their monogamous relationship). In some cases of extra-pair copulation, the female is more likely to mate with an older male intruder.
Bluebirds are also considered cooperative breeders or helpers because they often help raise young birds which are not their own. This situation is, in fact, beneficial in reducing the chance of extra pair copulations. Females usually drive away male intruders when their male mate is not in their nest. They use display signals to fend off other males or attack the breast of a male. Attempts by many males to engage in extra pair copulation are not successful as the females do no agree to it.
Mates already in a relationship normally follow their female mates closely during the receptive period or the time when females agree to copulate. This behavior decreases the likelihood of extra pair copulations. If a female accepts a male, she will then check out the nest and add more materials.
Females are very receptive to their mates about ten days prior to laying eggs until the last day of laying. If a female does not agree, she will leave the perch and the male chases her in a looping flight. Other refusal actions include attacking the front part of the male and flattening her body against a branch.
In a research that observed their mating behavior for five years, it was noted that the birds copulated frequently although the frequency of mating did not increase within the hour after the female laid eggs. It is believed that fertilization occurs a little later after copulation. The study observed that male western bluebirds mate with the females regularly 60 days before laying eggs until the start of the incubation period. There was an 80 percent increase in the female acceptance of within-pair mating ten days before egg-laying and continued throughout the laying period.
Both male and female western bluebirds can start reproducing upon reaching one year of age. During courtship, males stay on perches then sing and flutter in front of females with their wings half open and tail fanning. To get the female’s attention, a male will fly or move from one place to another within the nest site, flutters his wings and chirps. The male may also provide food to the female during this time.
Breeding happens from May to July during which the female lays one to two clutches of eggs. Each clutch typically has five eggs. The incubation period, meanwhile, takes place from 12 to 18 days.