Bluebird juveniles or adults may help other breeding bluebirds feed nestlings, remove fecal sacs, defend the nest, etc. See video. This is called helping by conspecifics. Helping between different species (called heterospecifics) has also been observed.
Joint nesting between conspecifics can also occur - e.g., two female Prothonotary Warblers nested simultaneously (with a combined clutch of 9 eggs) in the same cavity - Loucks 1894 BNA) This may be costly to the nestlings, as they are more likely to starve when there are more mouths to feed.
For example:
Swallows: A “cooperative” relationship between Violet-green Swallows (VGSW) and Western Bluebirds was observed in Oregon, in which swallows helped defend the nest and care for young bluebirds. Three pairs of adult VGSWs visited Western Bluebird nests after foraging (presumably to feed nestlings), removing fecal sacs and defending nest sites against predators. (Eltzroth and Robinson 1984) In two cases, the VGSWs had failed nests nearby, and two of the pairs took over the cavities after the WEBL nestlings they had helped fledged.
Chickadees: Two color-banded adult male Carolina Chickadees and 1 female tended same brood of nestlings and fledged young in Carolina/Black-capped hybrid zone population in SE Pennsylvania, possibly following death of mate of one of the males (RLC and K. L. Cornell unpubl.). Species status of individuals involved uncertain, although all 3 adults involved had Carolina mtDNA haplotypes (Cornell 2001).
Woodpeckers: A Hairy Woodpecker fed a begging fledgling Downy Woodpecker (Davis 1973) At 4 of 13 nests in Ohio, Dodenhoff (2001) found a female helper that assisted with feeding nestlings. No helper was the offspring of the pair she helped.
House Sparrows: In Detroit metro area, a female House Sparrow (HOSP) was raising 3 bluebird young. The sparrows chased away the bluebirds and took over the nestbox with bluebird eggs in it. (Kurt email)
House Finches: "Two instances of Western Robins and House Finches using the same nests have come to our attention during the past three years. In May, 1934, we were informed that House Finches were feeding young robins in a nest on a front porch in east Denver, Colorado. On investigation we found four half-grown robins, two newly hatched finches and four finch eggs. There were two female finches apparently with the same mate, and the three finches and the two adult robins fed the young regularly. Unfortunately, however, the large robins smothered their small nest mates. We did not determine whether the four remaining eggs hatched. All three adult House Finches fed the young robins in the nest, and after the young had left the nest. On May 15, 1936, in a similar instance, the nest was on the back porch of Bailey's home, 2540 Colorado Blvd., Denver. The young robins were nearly ready to leave the nest, and there was no evidence that the pair of House Finches had laid eggs. However, both adult finches and robins fed the young regularly. The male finch was particularly solicitous and would alight on a wire a few feet from the nest and sing whenever one of the other birds brought food. The young robins left the nest May 20, and the finches were the only ones noted feeding them from that time one, although the adult robins were about and no doubt shared the responsibility. " (from from Life Histories of Familiar North American Birds, reported by A.M. Bailey and R.J. Neidrach in 1936 in Denver)
