Cooperative breeding is a common phenomenon in birds where nonbreeding individuals assist other birds with reproduction, potentially delaying or even foregoing their own breeding to engage in these behaviors (Koenig & Dickinson 2004). About 3 percent (approximately 300 species) of bird species worldwide are cooperative breeders.
There are two types of cooperative behaviors: those in which mature nonbreeders (“helpers-at-the-nest” or “auxiliaries”) help protect and rear the young, but are not parents of any of them, and those where there is some degree of shared parentage.
Cooperative helpers are often closely related to the breeding individuals; they may be previous offspring of one or both breeders (half or full siblings of the brood) or siblings of one of the breeders (uncle/aunt of the brood), therefore receiving indirect fitness benefits from this behavior. Cooperative breeders could also exhibit shared maternity, shared paternity, or both.
In North America, some well-known cooperative breeders include the Florida Scrub Jay (Aphelocoma coerulescens) and the Acorn Woodpecker (Melanerpes formicivorus). In the order Charadriiformes, where waders are placed, biparental care is the typical breeding strategy, with cooperative breeding only a facultative behavior, whereby pairs and cooperative groups coexist and dispute territories.
The Hawaiian Black-necked Stilt (Himantopus mexicanus knudseni) is an endangered subspecies which breeds on Hawaii archipelago. They are pair breeders; both female and male incubate the eggs. The stilts normally lay four eggs. However, cooperative breeding behavior by juvenile and nonbreeding adult Hawaiian Stilts has recently been discovered on the islands of O‘ahu and Moloka‘i during the 2012–2020 nesting seasons (Dibben-Young et al. 2021).
They describe two different cooperative breeding behaviors: (a) egg incubation by multiple adults; (b) helpers-at-the-nest, whereby juveniles delay dispersal and reproduction to assist parents and siblings with reproduction.
On 25 May 2019 in Waiawa wetland, they found a Hawaiian Black-necked Stilt nest with three eggs. A Bushnell trophy camera was used to monitor this nest, and using consecutive photographs, two males and one female were monitored while they took turns to incubating the eggs. The breeding pair was potentially assisted by a male offspring from a previous year. Though the eggs never hatched, in the end, they could confirm the adults took turns incubating the eggs over the course of 30 days.
On 25 April 2020, a nest with five eggs was found within Marine Corps Base Hawaii—Kaneohe Bay on O‘ahu. Again via remote camera monitoring, consecutive photographs confirmed that two females were observed taking turns incubating the nest, together with one male.
On 25 May 2017, a nest containing eight eggs was discovered at the Koheo wetland in Moloka’i. One adult male and two adult females were observed taking turns incubating. They sampled three of the hatched chicks for future determination of parental lineage.
Moreover, during the 2012-2015 breeding seasons at Moloka’i, marked nonbreeding juveniles were observed helping parents and siblings defend nesting territories. Because these individuals were marked with leg bands as chicks, relatedness among individuals in this population was known. These were the first confirmed records of helpers-at-the-nest for this endangered subspecies. Cooperative breeding has rarely been documented in the order Charadriiformes (Lees et al. 2013).
In the present study (Dibben-Young et al. 2021), the authors present the first published observations of cooperative breeding behaviors (incubation by multiple adults and helpers- at- the- nest) in multiple populations of Hawaiian Stilts on two islands more than 100 km apart.
Clutch failure has been demonstrated as one of the greatest threats to Hawaiian Stilts (Harmon et al. 2021, cited by Dibben-Young et al. 2021). So, determining the prevalence of cooperative breeding across populations, as well as the impact of cooperative breeding on reproductive success, would be of crucial importance for the management of this endangered, endemic subspecies.
References
Dibben-Young, A., K.C. Harmon, A. Lunow-Luke, J.L. Idle, D.L. Christensen, and M.R. Price (2021). Cooperative breeding behaviors in the Hawaiian Stilt (Himantopus mexicanus knudseni). Ecology and Evolution 11(10): 5010-5016.
Harmon, K.C., K.B. Winter, N., Kurashima, C.H. Fletcher, H.H. Kane, and M.R. Price (2021). The role of indigenous practices in expanding waterbird habitat in the face of rising seas. Anthropocene, in press.
Koenig, W. D., & J.L. Dickinson (2004). Ecology and Evolution of Cooperative Breeding in Birds. Cambridge University Press.
Lees, D., M.A. Wetson, C. Sherman, G. Maguire, P. Dann, A. Cardilini, and L. Tan (2013). Occurrences of cooperative breeding in the Masked Lapwing Vanellus miles. Victorian Naturalist 130: 84– 85.
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