If you missed the previous Mating Monday posts you can check them out here and here.
Cooperative breeding occurs when a mating pair has other birds of the same species helping raise young from one nest. Many time the “helpers-at-the nest” are previous offspring of the parents, but sometimes non-related individuals provide care. This system is widespread throughout the animal kingdom and practiced in approximately 9% of bird species.
“Helpers” play multiple roles. They assist parents with defending the nest and feeding the offspring. A study of Brown-headed Nuthatches (Cusick et al., 2018) found that helpers also removed fecal sacs, but had no part in nest excavation and had no influence on maternal egg production. Cusick et al. concluded that the offspring raised in cooperative groups had more access to food and therefore weighed more. The cooperative nests were also more likely to fledge at least one offspring. Cooperative breeding seems to usually have positive effects for the breeders. But why would an individual forego their opportunity to breed in order to help a nest in which they are not passing on their own genetic material?
There are many factors that influence why cooperative breeding groups exist. One reason may be that environmental constraints limit the opportunities for young birds to breed. Many species that use this system live in regions with low temperature variations and warm winters. This allows the species to become sedentary and establish year-round breeding territories. There may be limited territories due to high-quality habitats being saturated with mature breeders. Low mortality, small clutch sizes, and longevity of offspring contributes to a more stable population that has lower turnover rates, so more birds are available as helpers. Skewed sex-ratios or a shortage of partners (usually females) may also encourage cooperative groups. There are more cooperative breeding groups in altricial species (incapable of moving around on its own shortly after hatching) than precocial (capable of moving on its own shortly after hatching).
Just because cooperative breeding is practiced within a species does not mean that all individuals of the species participate in the system. For example, studies have found that 20-30% of Brown-headed Nuthatches use cooperative breeding as a strategy, with many of the helpers being second-year birds. A study of Long-tailed Tits by Meade & Hatchwell found that helpers had higher overwinter survival rates than non-helpers. Whether or not an individual tit decided to be a helper was determined by the availability of a relative with an active nest. They found that tits that decided to help a relative had a 61% survival rate and the survival rate of those who did not have any relatives with an active nest was 52%. However, male birds that chose not to help a relative with an active nest had a survival rate of 24%. The authors found individuals among the helpers were in good physical condition, the birds without relatives to help were a mix of good/poor condition, and the birds that chose to not help relatives were in poor condition. Meade & Hatchwell also suggested that helping did not have significantly positive effects on reproductive success in the future.
In addition to higher survival rates, helpers may receive other benefits. Helpers may improve their chances of inheriting a mate, territory, or nest. They also gain experience in foraging and may form alliances. Strengthening the genetic makeup of a species through the survival of offspring may be an indirect fitness benefit to the helpers and breeders.
There are many North American bird species that use cooperative breeding. These include Florida Scrub-Jays, Acorn Woodpeckers, Red-cockaded Woodpeckers, Grove-billed Anis, Pygmy Nuthatches, and Gray-breasted Jays.
References
- Arnold, K. E. & Owens, I. P. F. (1999). Cooperative breeding in birds: the role of ecology. Behavioral Ecology, 10(5), 465-471. https://doi.org/10.1093/beheco/10.5.465
- Cusick, J. A., de Villa, M., DuVal, E. H., & Cox, J. A. (2018). How do helpers help? Helper contributions throughout the nesting cycle in the cooperatively breeding brown-headed nuthatch. Behavioral Ecology and Sociobiology, 72(43). https://doi.org/10.1007/s00265-018-2470-1
- DickDaniels. (2013, April 2). Brown-headed Nuthatch [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Brown-headed_Nuthatch_RWD13b.jpg
- Dreelin, A. (2015). Cooperative Breeding? In My Backyard?. Project Feederwatch. https://feederwatch.org/blog/cooperative-breeding-backyard/
- Ehrlich, P. R., Dobkin, D. S., & Wheye, D. (1988). Cooperative Breeding. Stanford.edu. https://web.stanford.edu/group/stanfordbirds/text/essays/Cooperative_Breeding.html#
- elfruler. (2018). How And Why Birds Cooperatively Breed. elfruler.com. https://www.elfruler.com/?page_id=908
- Lours. A. (2022, January 15). Long-tailed tit Gennevilliers 04 [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Long-tailed_tit_Gennevilliers_04.jpg
- Meade, J. & Hatchwell, B. J. (2010). No direct fitness benefit of helping in a cooperative breeder despite higher survival of helpers. Behavioral Ecology, 21(6), 1186-1194. https://doi.org/10.1093/beheco/arq137
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