Red-shouldered hawks have nested in American suburbs for many years.
Across parts of Ohio, they appear so consistently on library roofs and in garden trees that residents scarcely notice them. They have become part of the background.
Yet this everyday presence masks an unexpected trend. Hawk parents occupying the most productive hunting areas are quietly raising more daughters than sons.
To identify the circumstances behind this pattern, researchers monitored Ohio hawks for 30 years.
Tipping the balance towards daughters
The species in question is the red-shouldered hawk, a medium-sized bird of prey found in woodland, suburbs and, increasingly, urban areas.
As with many raptors, red-shouldered hawks are able to affect the sex ratio of their young before the eggs hatch.
Where food is abundant, parents tend to favour the more costly sex.
Scientists do not yet know how females establish that bias before laying, although the result becomes evident after the chicks emerge.
Female chicks are the more expensive offspring. A daughter grows roughly a quarter heavier than a son, and reaching that size requires more food.
At about three weeks of age, a chick’s sex can be identified from its body features, particularly its legs and feet.
Three decades of tracking red-shouldered hawk nests
Dr. Cheryl Dykstra has studied red-shouldered hawks throughout Ohio for 30 years through her research company, Raptor Environmental.
Her team has ringed chicks and monitored nests in rural woodland, peaceful suburbs and the centres of busy cities.
Dr. Dykstra and her co-authors, among them biologists from Ohio University, examined a straightforward question.
They sought to find out whether richer territories, with greater supplies of prey, produced more daughters than less productive territories.
Sex ratios vary between nests and from one year to the next, meaning that only records extending beyond a decade can distinguish a genuine trend from chance variation.
Why daughters are more costly to rear
For red-shouldered hawks, daughters may be a valuable investment when circumstances are favourable.
Female hawks frequently start breeding sooner than males, enabling them to pass on their parents’ genes earlier.
Males, on the other hand, can spend years waiting to secure a territory before they are able to reproduce.
However, daughters carry a greater cost. Since female chicks become larger than males, they need more food as they develop.
Parents are therefore more likely to produce daughters where prey and other resources are plentiful.
This trade-off fits a long-established biological theory. Parents in strong condition benefit most from investing in the more demanding sex, whereas those under pressure are better off with the less costly one.
The richest territories produce more females
The pattern remained when the researchers compared territory quality with the sex of the chicks.
Hawks occupying the most prey-rich ground reared more daughters than those on poorer land. This had not previously been demonstrated in red-shouldered hawks.
The finding mirrors results in other bird species. A study of a different raptor recorded the same bias, with the richest territories producing the greatest number of female young.
Comparable trends have been reported in some falcons and kestrels overseas.
The lengthy Ohio dataset provides a clear connection in a hawk species that is now expanding into towns and cities.
Since the best-fed parents produce the most daughters, the proportion of female chicks can act as an indicator of habitat quality.
Brood size and timing also matter
Territory quality was not the sole influence on the sex of hawk chicks.
Nests containing only one chick were more likely to hold a daughter, indicating that when parents rear a single offspring, it is often the larger, more demanding sex.
The date at which eggs were laid also mattered. Eggs laid later in the breeding season were more likely to yield males, whatever the local prey supply.
This suggests that, as the season advances, parents may favour sons because they take less energy to rear, making them a safer choice even in habitats that are otherwise of high quality.
Food supply and brood size also repeatedly emerge as factors affecting the sex of young raptors.
Hawks in the city
Urban hawks operate under different conditions from their woodland relatives and encounter distinct risks. A nest beside a busy road is surrounded by hazards.
One concern is rodenticide. Urban hawks occasionally catch rats, which are the same animals people poison, and may therefore ingest toxins indirectly.
Dr. Dykstra’s team hopes to establish how extensively urban hawks are exposed.
City birds are also likely to collide with vehicles more frequently than rural birds. The team believes these losses increase mortality and reduce the length of time a bird retains its territory.
Understanding these risks is part of safeguarding a species that is moving into neighbourhoods shared with people.
A hidden signal of habitat quality
The trend revealed by decades of nest monitoring was strikingly clear. Red-shouldered hawks in territories rich in prey produced more daughters than birds in poorer habitats.
As female chicks need more resources to rear, their presence seems to indicate the environmental quality surrounding them.
As red-shouldered hawks continue to spread into cities, the balance between sons and daughters could offer clues about habitat quality.
The same measure could help conservation teams identify which urban green spaces genuinely sustain hawks and which merely appear suitable.
“Few people can resist the appeal of an awkward, half-feathered nestling raptor,” said Dr. Dykstra.
Public support may do as much to protect these birds as any dataset.
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