Waiting for this moment: blackbirds announce spring
Spring is coming; the signs are all around! Robins have emerged from the forest and are plucking worms from lawns and fields. Back in the woods, the year’s first wildflowers, like harbinger-of-spring and spring beauties, are flowering among the leaf litter. But nothing declares spring as reliably and unmistakably as the territorial song of the male red-winged blackbird. Conk-la-reeee!
Historically, red-winged blackbirds (Agelaius phoeniceus) were birds of wetlands, at least for the breeding season, as they prefer to nest in cattail marshes. However, with the widespread loss of wetlands to agriculture and development during the past century, blackbirds expanded their nesting habits to include grasslands and other upland habitat. Modern agricultural practices have diminished Ohio’s population of red-wings, but still they nest in every Ohio county and remain among the state’s most numerous birds.
While sitting perched with wings closed, male red-winged blackbirds appear sleek and uniformly steely black. Females, in contrast, are streaky brown and white, easily mistaken for a large sparrow. The breeding season is when males show off their flashy yellow-bordered crimson wing patches (which, though called epaulets, actually are located at the wrist). To establish his territory and attract mates, a male sits on an exposed perch, throws back his head, flares his wings to expose his colors, and sings with beak opened wide. It’s hard to imagine a female blackbird, or a human observer for that matter, tiring of that spectacle!
Establishing a territory is just the beginning of the red-wing’s breeding season. The subsequent male/female relations that lead to a next generation of blackbirds have been the focus of substantial research. The great majority of bird species practice social monogamy, meaning that one male associates with one female for one or more breeding seasons. That makes sense, since nestlings are vulnerable and both males and females can feed them to promote speedy growth and maturation. (The opposite is true in mammals, in which only the female feeds the pre-weaning young and very few species are monogamous.) Red-winged blackbirds are an exception to that general avian rule. Instead, red-wings practice social polygyny, in which a single male on his territory associates with a number of females—maybe even a dozen or more. Each of those females builds a nest and raises young, with males helping mostly to defend the nests from predators.
The reasons for polygyny in red-wings probably relate to their habit of nesting in wetlands. Wetlands are a limited resource, and territories within a wetland vary in quality, perhaps especially in the availability of good nest locations. So, a female seeks a male with a high-quality territory, even if that male already has a mate. To make their selection, females may not actually evaluate the territory but instead evaluate the males themselves. The vigor of a male’s displays, including the vibrancy of his epaulets, may indicate the male’s health and, therefore, his ability to defend a good territory and father vigorous chicks.
Although polygyny was observed in red-winged blackbirds a long time ago, DNA testing in more recent decades has revealed additional complexities of the mating system. In particular, it turns out that a significant number of red-wing chicks—sometimes as many as 40%—result from “extra-pair copulations”—in other words, females mating with males other than their main social partner.
If females have successfully paired up with high-quality males, why would they also mate outside of that pairing? Research suggests several possible benefits of doing so. First, additional matings may increase the number of chicks that a female can raise. Extra-pair matings can improve the chances that all of a female’s eggs are fertilized, especially if the main male has numerous partners. In addition, the “outside” males may allow the female access to their territories, potentially increasing her foraging area and food supply. Beyond those benefits, multiple paternities produce more genetically diverse offspring, which may be better suited to meeting whatever challenges come in the seasons ahead.
Later in the year, after breeding is completed, blackbirds congregate in large flocks, sometimes with grackles and starlings and potentially numbering into the hundreds of thousands. In those collectives, blackbirds can become quite a scourge on neighborhoods and, especially, agricultural crops, as they descend on grain fields to satisfy their hunger. A lot of effort and ingenuity has been invested in developing methods to deter those flocks. But that’s a problem for another time of year.
As Wallace Stevens famously captured in his poem, there are many ways of looking at a blackbird. “I don’t know which I prefer, the beauty of inflections or the beauty of innuendoes, the blackbird whistling or just after.” Right now, territorial male blackbirds are brightening the wetlands with their flashes and trills. But what comes after—that’s springtime. And what’s not to love about that?
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.