By Terry W. Johnson
When we watch the nesting activities of the eastern bluebird, it appears that these beautiful birds live a pretty uncomplicated life style. However, researchers are finding that the private lives of these popular backyard birds often seem to be taken from the script of a daytime soap opera.
Bluebirds were long considered to be one of the 91 percent of all bird species that are monogamous. Such is not always the case, however. In one study, for example, 15 percent of adult females and 5 percent of adult male bluebirds were found caring for at least one nestling that was not their own. Another study conducted in South Carolina revealed that 5 percent of the bluebird pairs followed had multiple mates during the nesting season.
The chances of female bluebirds mating with more than one male were higher in locations with high population densities. By the same token, male bluebirds are more likely to attend to the needs of nestlings fathered by another male when their mates spend more time away from their nesting territory during the mating season.
Male bluebirds appear to have no problem caring for young sired by other males and don't show their own young any preferential treatment. Additionally, they tend to feed offspring more often than their mates.
Female bluebirds were found to feed male and female young equally. However are you ready for this? Many male bluebirds actually discriminate against male offspring! That's right, male bluebirds practice sex discrimination.
Here is what happens. During the first few days after bluebird hatchlings break out of the egg they are fed entirely by their mothers. During this time, male bluebirds supply their mates with small, soft insects such as caterpillars. The females, in turn, feed the protein-rich insects to the hatchlings.
Later, both parents forage for the steady stream of caterpillars, beetles, grasshoppers, crickets and other insects needed to satisfy the seemingly insatiable hunger of their rapidly growing young. It is during this phase in the development of the young that males begin a pattern of discrimination.
When the males return to their nests with food, they feed female nestlings twice as often as males. Consequently, during the 16 to 20 days the young remain in the nests, females receive considerably more food than their male nest mates. This gives them a better chance of surviving the rigors of the world once they leave the nest.
This discrimination extends well beyond the time the young fledge because both parents feed the offspring for up to a month after they fledge.
Nobody knows for sure why male bluebirds discriminate against male nestlings. Some biologists suggest this behavior reduces chances of male bluebirds having to compete with one of their male offspring for mates, nesting sites and food. Female fledglings may disperse up to 12 miles or more from the box where they are hatched. However, males often establish breeding territories within three miles or less of the cavity where they were raised.
But some experts question this theory, pointing out that female bluebirds seem to shy away from young, inexperienced males. They appear to prefer more mature males experienced in providing for a nest full of demanding young. Older males also often do a better job of defending their nesting territory.
The family life of the bluebird is just one example of the fascinating true-life dramas taking place in the natural world. If you want to enjoy truly compelling drama, turn off the television, step outside your backdoor and carefully watch and listen. Who knows? You may find you don't need that new big screen television after all.
Terry Johnson is a former Nongame program manager with the Georgia Wildlife Resources Division, a noted backyard wildlife writer and expert, and executive director of TERN, the friends group for Wildlife Resources' Nongame Conservation Section.