I see three kinds of black birds out of my SoggyBottom office window at the feeder and on the pasture fence: Red-winged blackbirds; Brown-headed cowbirds; and Starlings. I also see Brewer blackbirds but they are fewer in number.
There is no mystery about the development and coming of age of Red-winged blackbirds and Starlings. The eggs of these birds are laid in their biological parent’s nest where they are incubated, hatched, and cared-for by the parents. Their nest mates are their biological siblings; the whole brood fledges more-or-less together and is soon integrated into the larger species-specific flock. As nestlings and fledglings, these young birds are part of the flock from the beginning. They hear the voices of their parents and other close-by flock members, they eat the appropriate food for the species, and they begin doing species-specific things from the get go.
It is different for Brown-headed cowbirds. The Brown-headed cowbird is one of a few bird species that lay their eggs in other species’ nests and rely on the nest host to incubate, hatch, and raise the parasitic nestlings until they fledge and leave. Species that lay their eggs in other bird’s nests are called “brood parasites” and the Brown-headed cowbird is the most notorious brood parasite in North America. As an outcome of this practice, cowbird chicks have only the voice, habits, and food of their adoptive parents to pattern their behavior on. That behavior is as varied as that of the fly catcher (feeding by sallying-forth from a perch to catch a flying-insect and singing a song that has been characterized as:“QUICK-THREE-BEER”) to that of the warbler (feeding with caterpillar-catching foraging behavior and singing a variable-pitched song characterized as “sweet sweet sweet sweet peachy peachy”). Thus, there is no consistent parental behavioral pattern that is transmitted to cowbird nestlings by the various nest hosts before the nestling leaves the nest. The nestling cowbird’s brood mates are not of its kind and it often outcompetes its fellow nestlings and leaves the nest as a solo fledgling with no sibling influence.
Yet by mid-June, soon after the cowbird nestlings have fledged, I see small groups of juvenile cowbirds riding the backs of our sheep and cows. These juvenile birds have already become cowbirds in the short time that has elapsed since they fledged. My question is "With adoptive parents that are unfamiliar with cowbird food, song, and social ways, how do Brown-headed cowbirds learn to become Brown-headed cowbirds?"
There are many examples of brood parasitism among birds but the consequences of this is greatly different among species. For birds like song birds whose hatchlings are altricial (i.e., hatched as naked helpless chicks requiring feeding, protection, and incubation during development in a nest) brood parasitism has a negative impact on the young of the hosts. In addition to cowbirds, cuckoos are another species with altricial young that practices brood parasitism of this type. On the other hand, waterfowl hatchlings are precocial (i.e., hatchlings covered in down and capable of feeding and leaving the nest immediately upon hatching without parental help) and brood parasitism among waterfowl, which is common, has little effect on the young of the host. It hardly makes any difference to a hen mallard and her brood if a Gadwall slips into her nest and lays an egg or two. The resulting composite brood of ducklings doesn’t do anything any different than if the whole brood was mallards, or gadwalls for that matter. For some ducks, Redhead ducks being a prime example, brood parasitism is a prominent part of the duck’s reproductive strategy which includes raising broods of its own hatchlings in its own nest as well as brood parasitism where other hens incubate and raise the redhead progeny. Some degree of brood parasitism is common among all ducks but the consequences on development of the hatchlings are slight.
These thoughts of adoptive parents bring to mind recent news articles about Red-tailed hawk chicks that have been abducted from their nests by Bald eagles and then transported to the eagle’s nest, adopted, and raised as if they were a nestling eagle. Many birders express concern about the success of these adopted hawks to learn the ways of hunting and the food preferences of Red-tailed hawks.
The abduction paradigm of the eagle is a total outlier. In fact, the Bald eagle is a notorious nest predator; the primary victims being mostly-grown nestlings of osprey, red-tailed hawks, and herons - herons are known to abandon entire rookeries to escape Bald eagle nest predation. Sometimes, the eagle gets confused when it attacks a redtail nest and captures a chick. By the time it gets the chick to its own nest, the cries and actions of the abducted chick cause the eagle to no longer think of the chick as a food item but to think of it as one of its own. Thus, it feeds and cares for it along with its much larger eagle chicks. The redtail chick has to be super aggressive to survive in competition with its bigger eagle siblings but, apparently, some are sufficiently aggressive and do survive.
For redtails raised in their own nest, there is an extended time between leaving the nest as a fledgling and transitioning to independent living as a juvenile. Newly fledged redtails must continue to be fed by the parents as they are trained to hunt and taught the ways of the species. This is a difficult time even if they are raised by hawk parents and mortality is high. But this period must be doubly difficult for redtail chicks raised by Bald eagle parents who cannot teach the new fledgling the ways of the redtail - eagles tend to have a smoother, easier time in the transition from nest to independent living than do redtails and, consequently, adult eagles do not provide much parental care once the chick has fledged despite that juvenile eagles tend to hang around their parents for a long while.
So, a redtail that has survived after being adopted by bald eagles is a special bird indeed. However, one wonders if it ever develops a preference for rabbits over fish, for quail over ducks, for open fields over shore line and open water? Does the eagle-raised hawk make the distant-sounding, scream of an adult hawk as in western movies or does it tend to emit the short, high-pitched chirps of the adult eagle? These questions beg the age-old argument of nature vs. nurture. Whether aspects of bird behavior are hardwired or learned has fascinated ornithologists for years. But here we have the added complexity that one way or another, nature does the nurturing. Maybe there is no dichotomy here at all.
We have at least one local nesting pair of Bald eagles and one local nesting pair of Red-tailed hawks here at SoggyBottom Farm. I see juveniles from each nest from time to time. Juveniles appear and then disappear over the course of the year but the adults are year-to-year constants. I assume that each of the local nests successfully fledges chicks and these fledglings go on to become juveniles and then adults just like they normally do elsewhere. Unlike other places where I have lived, where the animosity between eagles and redtails were often on full display, the birds at SoggyBottom seem to share the skies with no obvious conflict. (Don’t assume that the smaller redtail is at a disadvantage against an eagle; I once watched a pair of redtails attack, injure, and drive an adult eagle to the ground.) However, I see such a small part of the lives of the SoggyBottom birds that I cannot generalize about their special relationship and about how they interact around their respective nests. Until I locate these nest sites and make extended observations, I will have to rely on the information I glean from news reports to inform my knowledge about the role of these birds in the parenting of the other species offspring.
My two examples of adoptive parents represent two extremes, the cowbird example of brood parasitism is well known and negatively impacts the nesting success of the host parents; the Bald eagle/Red-tailed hawk example of abduction is a rarity and probably has little impact of the success of the eagle to raise its nestlings. However, even after all these considerations, we still don’t know how the cowbird learns to be a cowbird. But we can be assured that nature has a clever way for nurturing that transformation else the species wouldn’t exist.