“My, they grow up so quickly!” How many times have you heard a parent or grandparent exclaim with disbelief the perceived blink-and-they’re-all-grown-up phenomenon? As real as this may feel to recent empty nesters, our sense of time passing swiftly pales compared to what avian parents experience.
In the songbird world, early summer is a blur of rapid changes, as tiny babies hatch, naked and blind, then steadily grow to adult size in several weeks. Unlike their waterfowl and shorebird cousins, whose precocial young emerge from the egg fully feathered and ready to explore beyond the nest almost immediately, songbird babies are altricial. This means that they enter the world completely helpless at first and require the around-the-clock care of parents to provide all the nourishment and protection they need. Over just a two- to three-week period, they develop fully functioning senses, grow wings and tail feathers, and achieve the musculature and coordination to become self-sufficient organisms that can move about, feed, and defend themselves. Presumably this rapid development is a blessing for the parents, who run themselves ragged gathering insect food to fill hungry mouths non-stop
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Mid-June through early July is generally the best time of year to observe young birds venturing away from the nest for the first time; taking the time to closely observe this fledging process can be entertaining and rewarding. Like our children, young birds are curious, boundary-pushing, and adventuresome--often with comical consequences. Remember your most cringy junior high photo with the bowl-cut, leisure suit, or maybe even (yep, I had ‘em) orthodontic “headgear”? Juvenile birds also go through an awkward adolescent phase, complete with ‘bad hair’ days, a bright gape-induced perma-scowl expression, and ludicrously short tail feathers that seem laughingly out of proportion. Immature ‘teenaged’ birds, which are full-sized and seemingly able to fend for themselves, still beg to be fed, often in an obnoxiously demanding way. At first, parents oblige, although gradually they begin the weaning process and encourage independence; the back-and-forth that ensues is not unlike familial bickering over who gets the car keys on a Saturday night.
Don’t be surprised when you’re watching avian families this summer if you encounter what appears to be a diminutive adult bird tending to an oversized youngster which surely cannot be its own offspring. Increasingly, young Brown-headed Cowbirds are being raised by smaller ‘foster parents’ right in our suburban neighborhoods, often at the expense of the parents’ own nestlings. Cowbirds are most common along habitat edges, and, as we continue to fragment large parcels of forest into smaller, disjointed woodlots, we create an abundance of prime habitat for this unique native species; its population is on the rise, as a result.
A “brood parasite,” the Cowbird doesn’t follow the typical breeding rituals of setting up a territory, building a nest, laying a clutch of eggs therein, then devoting several weeks to raising the youngsters. (The same is true of the European Cuckoo and a handful of other species in several parts of the world.) In all, this cycle requires roughly a six-week commitment of undivided attention and energy. Cowbirds, instead, lay their eggs in other birds’ nests—one here, another there, up to 30 in all—midway through the host’s own egg-laying period. Cowbirds tend to choose the nests of birds slightly smaller than themselves, and Cowbird eggs have a shorter incubation period than most songbirds’. These characteristics take advantage of the strong maternal instincts that female songbirds possess. Knowing the chance that all of her eggs survive to adulthood is slim, a mother bird tends to feed the largest, loudest, and presumably strongest youngsters to guarantee their success while risking sacrificing the runt(s) of a clutch. As young Cowbirds tend to share the nest with smaller nestlings of their host species, they end up receiving more than their share of food and attention from unsuspecting parents, who merely believe they are raising an unusually thriving behemoth of their own.
In anthropomorphic terms, this process of leaving other species to raise their young makes Cowbirds seem like the most vicious of bullies and truly heartless parents. However, that’s a gross oversimplification. Cowbirds (and other species of brood parasites) have developed this unusual reproductive strategy not out of spite or callousness, but because it incurs survival advantages that have been favored by natural selection. There are several likely theories about the specific advantages; each may have played some role in the evolution of brood parasitism.
One theory pertains to the feeding ecology of Cowbirds being ill-matched to an extended nesting period tied to a single location. Historically, the Brown-headed Cowbird was a bird of the Great Plains. Primarily an insectivore, the Cowbird feasted upon the flies and other bugs kicked up by and on the hides (and dung) of large ungulates, especially buffalo. Herds of bison tend to be nomadic and often move great distances when feeding conditions deteriorate; if that were to happen in the middle of an avian nesting cycle, parent birds would lose access to their primary food source at the time when they need it most. Parasitizing other birds’ nests allows for the transient lifestyle needed to follow herds when they roam.
Another selection pressure has to do with energy allocation. Parenting baby birds—and all that it entails (nest-building, territorial defense, transfer of body heat, and near-incessant gathering of food)—requires an enormous amount of energy. This greatly limits the number of eggs a female can lay and, thereafter, the number of nestlings which can be raised successfully. Cowbirds take a big chance depositing eggs and then leaving them with no further investment in their development or survival. Some host species (most notably, Yellow Warblers) do recognize the imposter egg and remove it, build nest material over it, or otherwise avoid incubating it. Undoubtedly, many Cowbird eggs never hatch. And, yet, the species thrives, largely because the energy saved by circumventing most of the nesting process permits Cowbirds to produce eggs at a much higher rate—generally one per day for close to a month. From a parenting perspective, Cowbirds choose quantity over quality.
Some biologists have hypothesized that the Cowbird’s strategy may have evolved during a time of very high nest predation by non-avian predators. If so, it actually may represent a clever response (not unlike overly cautious families who split up on different flights when they travel) to the potential threat of a complete loss all at once.
Regardless of the evolutionary causes, brood parasitism by Brown-headed Cowbirds is increasingly easy to observe in our parks and gardens, primarily because we continue to create ideal edge habitat for this species. The impact of Cowbirds on other avian species becomes magnified as additional factors (feral cats, window strikes, severe weather events, etc.) are driving their populations down. One can only wonder if the success of the Cowbird was perhaps inspiration for the idiom, “Don’t put your eggs in one basket!”
