January 15, 2015
We love this guest student blog post by Andrew Dreelin — keep an eye out for cooperative breeders at your feeders!
One of the most fascinating aspects of birds is their wide variety of mating systems. We are most excited by the return of spring not just because birds are dressed up in their colorful regalia, but also because it heralds the onset of the breeding season, (which means that you get to participate in NestWatch and watch BirdCams)! We love birds that pair for life, like swans and cranes, and are fascinated by polygynous birds like manakins with their colorful, bizarre displays. But perhaps more interestingly, some birds maintain family groups just like us! Even better, they can be found in your backyard!
Behavioral ecologists call this phenomenon cooperative breeding. The premise is essentially that a mated pair of birds will have one or several “helpers” at the nest. These helpers most often assist the breeding pair by defending the nest or feeding the young. The Brown-headed Nuthatch is one such bird that exhibits this fascinating social mating system. A resident inhabitant of Southeastern pine forests, they can often be found at feeders in the proper habitat (I’ve even found them nesting in a porch umbrella in my backyard). In a study from 2007, ornithologists studied nesting sites across Florida by capturing (in the nest or with mist-nets) and color-banding Brown-headed Nuthatches so they could identify individuals. They found that cooperative breeding occurred in roughly 20% of the population, and that the helpers were second-year birds that are closely related to the pair they’re helping. This unusual mating system holds a lot of implications for how we think about evolution. If the evolutionary goal for an individual is to produce as many offspring as possible, then why would a bird (or any other organism) spend its time helping another bird achieve reproductive success?
Although much remains to be learned about cooperative breeding, scientists have some idea as to why this behavior occurs. While some birds, like Southern Ground Hornbills in Africa, are obligate cooperative breeders (i.e. they need helpers at the nest in order to fledge young successfully), it is far more common for birds to be facultative cooperative breeders, meaning that they can breed successfully without helpers present (indeed, this is the strategy that the Brown-headed Nuthatch follows). The fact that cooperative breeding can be optional suggests that negative environmental conditions, such as low food abundance or few females in a population, may make it more beneficial for young birds to forgo a breeding attempt and help their parents instead. While they don’t get a chance to produce offspring, these young birds may receive indirect benefits, such as reduced energy expenditure and higher survival. Maybe the practice that they gain as helpers allows them to be more successful when they breed on their own?
Interestingly, the Brown-headed Nuthatch isn’t the only cooperative breeder found in southeastern pine forests. The Red-cockaded Woodpeckeer, a federally endangered species, is a cooperative breeder that nests in cavities of old-growth Longleaf Pine that have had their bark softened by heart rot fungus disease. This raises another question: is there something about this habitat that encourages bird species to adopt cooperative strategy as a life history strategy? There are many more questions that remain to be answered about cooperative breeding, as well as the ecological and evolutionary forces that give rise to it. That’s why it’s so interesting!
If you don’t live in the southeast, there are other cooperative breeders in North America. The Pygmy Nuthatch, a western relative of the Brown-headed Nuthatch, is also a cooperative breeder! Together these two species cover a wide swath of the United States, so many of you reading this can attract a cooperative breeder to your feeder! That’s not to say that there aren’t other cooperative breeders in North America. The Acorn Woodpecker and the Florida Scrub-Jay are other great examples! But regardless of whether you live near a cooperative breeder or not, pay attention to the breeding habits of your local birds the next time spring rolls around. You just might notice evolution in action!
Cox, J. A., & Slater, G. L. (2007). Cooperative breeding in the Brown-headed Nuthatch. The Wilson Journal of Ornithology, 119(1), 1-8.