The Hawaiian Stilt is a threatened subspecies of the Black-necked Stilt, federally listed as Threatened but classified as Endangered at the state level, and considered conservation-reliant. Fewer than 1,800 individuals remain across the Hawaiian Islands, with fewer than 200 on Molokaʻi itself. The birds are typically monogamous, nesting in sparse low vegetation or on mudflats and laying clutches of two to four eggs. Yet in recent years, researchers documented behaviors that did not fit the textbook picture: multiple adults incubating a single nest, helpers-at-the-nest assisting breeding pairs, one remarkable nest containing eight eggs tended by one male and two females, and juvenile siblings from earlier broods repeatedly returning to help their parents defend nests and chicks against unrelated birds across multiple breeding seasons.

These observations raised an obvious evolutionary puzzle. Cooperative breeding, in which individuals forgo their own reproduction to help others raise young, is predicted by Hamilton’s rule to evolve when the inclusive fitness benefits of helping outweigh the costs, which usually requires that helpers be related to the beneficiaries. But cooperative behavior is rare in the shorebird order Charadriiformes, where parental care is typically confined to breeding pairs. The alternative explanations carry very different implications: conspecific brood parasitism, in which females lay eggs in the nests of other pairs, can be a competitive or opportunistic tactic, while extra-pair copulations complicate paternity. Distinguishing among these possibilities requires genetic evidence, which is exactly what the research team set out to collect.

The study leaned on an extraordinary long-term dataset. Over the past decade, more than 250 Hawaiian Stilts on Molokaʻi were marked with unique combinations of colored leg bands by a resident observer working under all necessary permits, and behavioral surveys were conducted at least weekly year-round across wetlands on the island’s south shore. Most birds concentrate at the Kaunakakai Wastewater Reclamation Facility, where water levels are maintained at stilt-friendly depths and predators are minimized by year-round trapping and a chain-link fence, because invasive mangrove encroachment has severely reduced the island’s remaining nesting habitat. Observers recorded parental care, pair interactions, courtship displays, mate guarding, and social associations, noting the identities of every marked individual involved.

From this population, the team sampled 91 banded birds, representing roughly 64 percent of the most recent Molokaʻi population estimate, collecting blood and feather samples for DNA extraction. Using an ezRAD sequencing approach, they generated approximately 913 million raw reads and, after stringent quality filtering, retained 6,837 biallelic single nucleotide polymorphisms from 80 individuals. The analysis pipeline included SNP-based parentage inference with the R package apparent, pairwise genomic relatedness estimates using Yang’s A_jk estimator, and a Bayesian phylogenetic tree built in BEAST to visualize how closely each bird sat to every other on the island’s family tree. Because parentage software performs poorly in small, inbred populations with overlapping generations, the researchers manually reclassified assignments using observed hatch years, a crucial step that recovered biologically plausible relationships from otherwise ambiguous output.

The results were striking. Just over half of the relationships inferred from behavior, 51 percent, were supported by the genetic data, while 17 percent were rejected and the remainder were indeterminate. Of 40 sibling pairs hatched in the same year, only six were rejected. But the rejections proved as informative as the confirmations. In one case, a behaviorally observed mother and father were both genetically rejected as parents of a particular offspring, a pattern consistent with conspecific brood parasitism: the genetic mother likely laid her eggs in another pair’s nest. Five cases of nestmates hatched in the same year that were expected to be siblings were also rejected, providing further evidence of either brood parasitism or extra-pair paternity, though more data would be needed to separate the two.

Only seven relationships across the 80 sampled individuals achieved consensus status, meaning they were supported simultaneously by behavioral observation, parentage analysis, genomic relatedness, and phylogenetic position. Five were likely full sibling pairs, one was a true parent-offspring relationship, and one was a mate pair so closely related that the birds may have been siblings. That last finding carries real conservation weight. The population showed a heterozygote deficit, with a mean inbreeding coefficient of 0.23, indicating that observed heterozygosity fell significantly below expectations. If inbreeding intensifies in a population this small and isolated, it could reduce fitness, heighten disease susceptibility, and erode the genetic variation the species will need to adapt to habitat loss driven by sea-level rise.

The study’s authors emphasize that both cooperative and noncooperative behaviors coexist within the same populations. Cooperative chick defense has been observed on a remarkable scale, with ten adults joining to mob a potential predator at one nest, and at least one case of five adults surrounding a crèche of chicks. Yet egg destruction by conspecifics has also been documented in densely packed wetlands, and brood parasitism may impose costs on host parents if competition for food intensifies after hatching or if the physical constraints of brooding more than four eggs reduce survival. Density-dependent pressure on limited nesting space, the researchers suggest, may push individuals toward tactics that harm their neighbors, while habitat restoration that creates additional nesting areas could relieve those constraints and favor cooperation.

The research also highlights the methodological value of triangulating multiple lines of evidence. No single analysis, whether behavioral observation, parentage assignment, relatedness estimation, or phylogenetic reconstruction, could have resolved the breeding system on its own, and each approach corrected or contextualized the others. The team notes that stilts do move among islands despite strong fidelity to individual wetlands, and that gene flow between islands may help maintain genetic diversity in these small populations. Other endangered stilts, such as New Zealand’s Black Stilt, have weathered dramatic bottlenecks while retaining genetic diversity, offering a measure of hope for the Aeʻo’s recovery.

For a subspecies squeezed between rising seas, invasive predators, and shrinking wetlands, understanding who breeds with whom is not an academic luxury. As restoration efforts expand nesting habitat across the Hawaiian Islands, ongoing monitoring of genetic relatedness and reproductive strategies will be essential to distinguish cooperative behaviors that build population resilience from competitive tactics that undermine nesting success. What began as curious observations of extra adults at stilt nests has matured into a genomic portrait of a species improvising multiple reproductive strategies under pressure, a reminder that even well-studied conservation icons can harbor behavioral secrets that only DNA can unlock.

Subject of Research: Breeding behavior, kinship, and conservation genetics of the endangered Hawaiian Stilt

Article Title: Breeding Behaviors and Kinship in the Endangered Hawaiian Stilt (Ae‘o; Himantopus mexicanus knudseni)

Article References: Price, M. R., Harmon, K. C., Toonen, R. J., Freel, E. B., Emms, M. A., Dudoit, A., Loockerman, C., & Dibben‐Young, A. (2026). Breeding Behaviors and Kinship in the Endangered Hawaiian Stilt (Ae‘o; Himantopus mexicanus knudseni ). Ecology and Evolution, 16(10), Article e74297. https://doi.org/10.1002/ece3.74297

Image Credits: AI Generated

DOI: 10.1002/ece3.74297

Keywords: Hawaiian Stilt, cooperative breeding, conspecific brood parasitism, extra-pair paternity, inbreeding, conservation genetics, Molokaʻi, parentage analysis, genomic relatedness, endangered species, wetland birds, Charadriiformes