Girl power: Hawk parents invest more in female offspring

Few among us appreciate being called cheap, yet this term has a useful application in bird world. A new study published in the Journal of Raptor Research, “Factors Influencing Nestling Sex Ratios of Suburban and Rural Red-Shouldered Hawks, 2004-2016,” finds that male Red-shouldered Hawks (Buteo lineatus) are the cheaper of the sexes, meaning hawk parents prioritize producing female offspring when conditions are optimal. This means that higher quality territories could result in more females, offering biologists clues about habitat health, which is important intel as the species adapts to urban environments with increasing frequency. 

Red-shouldered Hawks are medium-sized generalist raptors inhabiting forested, suburban, and urban habitats. Females are roughly 25% heavier than males and can be sexed in the nest after three weeks of age based on measurements of their legs and feet. Like other raptors, Red-shouldered Hawks can adjust the sex ratios of their offspring prior to hatch which can lead to investment in the more “valuable” sex during times of plenty. Raptor biologists theorize that since females can begin breeding earlier than males, it could be advantageous for parents to invest in female chicks because they will begin contributing genes to the population sooner than males. In other words, females get a head start. Additionally, producing the larger, more energetically demanding females may simply be too difficult when resources are scarce.  

Lead author Cheryl Dykstra has been studying Red-shouldered Hawks in Ohio for 30 years. Her team has amassed a robust dataset on various realms of this species’ ecology, and they have worked with multiple stakeholders to gain research access to hawk populations in rural, suburban, and urban environments. Her team set out to determine whether there is a relationship between higher quality territories, meaning they contain more prey, and the proportion of female versus male nestlings. They found that higher quality territories indeed yielded more female nestlings, confirming previous research on other raptor species such as the Eleonora’s Falcon (Falco eleonorae), and Lesser Kestrels (Falco naumanni), among others. They also found a correlation between brood size, hatch date, and sex ratio in the nest: Solitary young are more likely to be female than male, and the probability of an egg hatching into a male increased for eggs laid later in the season regardless of habitat quality. 

The urban Red-shouldered Hawks featured in this study encounter different threats than their forest-dwelling counterparts, an important consideration when developing species-wide protection plans. In the future, the team hopes to investigate, for example, whether urban Red-shouldered Hawks are exposed to rodenticide since they occasionally feed on rats. “We also think it is likely that urban birds experience more collisions with vehicles than rural birds, with effects on mortality and turnover rates,” say Dykstra. Her team wants to continue involving the public in activities like banding nestlings to help increase public awareness of the importance of the hawks in their area. “The relationship between humans and urban-adapting birds is critical for conservation,” she says, “and few people can resist the appeal of an awkward, half-feathered nestling raptor.” Top predators are important members of any ecosystem, yet their contributions often fly under the radar. Projects like this not only answer pertinent questions about the species’ biology but also help increase collective appreciation of raptors. 

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Paper:

Dykstra, Cheryl et al. (2026). Factors Influencing Nestling Sex Ratios of Suburban and Rural Red-Shouldered Hawks, 2004-2016. Journal of Raptor Research, 60(2):1-11. 
DOI: 10.3356/jrr2569

Notes to Editor:

1. The Journal of Raptor Research (JRR) is an international scientific journal dedicated entirely to the dissemination of information about birds of prey. Established in 1967, JRR has published peer-reviewed research on raptor ecology, behavior, life history, conservation, and techniques. JRR is available quarterly to members in electronic and paper format.

2. The Raptor Research Foundation (RRF) is the world’s largest professional society for raptor researchers and conservationists. Founded in 1966 as a non-profit organization, our primary goal is the accumulation and dissemination of scientific information about raptors. The Foundation organizes annual scientific conferences and provides competitive grants & awards for student researchers & conservationists. The Foundation also provides support & networking opportunities for students & early career raptor researchers.