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Food supply determines reproduction among Brazilian eagles

In four bird of prey species, the factor that most influenced reproductivity was the availability of prey, not environmental changes

Crested eagle adult and chick: this species reproduces throughout the year in a pattern similar to that of the ornate hawk-eagle

Willian Menq

When veterinarian Ricardo José Garcia Pereira of the University of São Paulo’s School of Veterinary Medicine and Animal Science (FMVZ-USP) provided consultancy on the reproduction of birds at the São Paulo Zoo, he noted that some eagles were laying eggs at different times of the year. He and biologist Marcel Henrique Blank, studying for a postdoctorate under Pereira’s tutelage, decided to investigate the ecological factors influencing the mating period of four species of Brazilian eagles living in Central and South American forests, as described in an article published in the March edition of the journal Scientific Reports.

“Contrary to northern-hemisphere birds, whose young generally hatch in the springtime, the four species presented three different patterns,” says Pereira. The results indicate that harpy eagles (Harpia harpyja), some of the biggest eagles in the world, reproduce all year round, but primarily on shorter days; black hawk-eagles (Spizaetus tyrannus), on longer days; ornate hawk-eagles (Spizaetus ornatus), and the crested eagle (Morphnus guianensis) procreate year-round. Despite some of these four species being known as hawks, they are technically all eagles due to their larger size.

“The factor that best explained the reproductive period of each species was the diversity of prey consumed by each,” says Lucas Ferreira do Nascimento, one of the article’s authors and a PhD student at the Institute of Biosciences of the University of São Paulo (IB-USP). Reproduction among harpies probably coincides with that of sloths and monkeys, when they are able to catch their favorite prey. Eagles that hunt a variety of small mammals, birds, and reptiles reproduce throughout the year. “Among the four species there was never a clear relationship between the reproductive period and the length of the day, temperature, or levels of rainfall,” notes Nascimento.

Pereira’s team collated data on 414 newborn clutches, including the location and date on which the eggs were laid, obtained from 25 zoos across the globe. In addition, they analyzed photographs from the Brazilian bird site Wikiaves, a collaborative platform with contributions from ornithologists and birdwatchers, by which they inferred the egg-laying date, considering the estimated age of the chicks and average incubation time of eggs from the species.

The researchers consulted databases to determine, during the time of each clutch, the temperature, rainfall, and length of the day, known as the photoperiod, which is longer in the summer and shorter in the winter. They then conducted a bibliographical survey of each species’s prey and their numbers throughout the year. According to Pereira, this was the first correlation between the reproduction of neotropical predators and environmental factors.

Even with the constant availability of food in captivity, the data presented in the article indicate that the harpy and ornate hawk-eagles maintained their preference for the same procreation period as that seen in the natural environment. Pereira talks of the possibility that among the ancestors of these eagles, animals wired to reproduce during a certain period managed to obtain more food for their young. This behavior is thought to have been genetically adopted.

The study also finds that the evolution proximity does not indicate similar reproductive periods. Despite being from the same genus (Spizaetus), black-hawk and ornate hawk-eagles have different mating patterns. Ornate-hawk and crested eagles, not as closely related, present a similar pattern. Pereira emphasizes that there is even more risk in comparing reproduction among northern-hemisphere birds, which have been studied in more depth, to those from tropical areas, as the two groups are distinct in terms of evolution

“As the seasons of the year do not fluctuate so much here, there is no reason for such a restricted reproductive season for most species,” explains the researcher. He goes on to say that reproduction among tropical birds may also be seasonal, but the influence of rainfall or food availability is greater, often equating to or exceeding the effect of the day’s length. “We demonstrate that it is not always possible to simply extrapolate knowledge of one bird group to others. Each group needs to be studied.”

“This is an important study, as the reproductive behaviour of these animals is not well known,” points out Willian Menq, author of the book Aves de rapina do Brasil (Predatory birds of Brazil) and presenter on the YouTube channel Planeta Aves (Bird Planet). According to Menq, these are rare birds because each pair of eagles occupies an extensive swath of forest to find prey and sustain their young, significantly hampering field studies. In a study of harpies, a maximum of five nests were found in each 100 km2, an area slightly bigger than the city of Vitória in Espírito Santo State. “Drawing on indirect data, the authors managed to get around this problem,” says Menq, not a researcher linked to any University, but one of the most significant specialists in Brazilian eagles according to ornithologist Luís Fábio Silveira, vice director of São Paulo University’s Museum of Zoology (MZ-USP).

Eagle reproduction in captivity is a challenge because they pair up, normally monogamously, after a complex recognition ritual, which may be successful or not. The eagles normally produce one chick every two years, making their multiplication slower. The harpy, for example, broods its eggs for 56 days and the chicks remain in the nest for 8 to 12 months.

Biologist Fernanda Junqueira Vaz, head of São Paulo Zoo’s Bird Sector, who worked with Pereira during her master’s, says that during the approximation process she kept a pair of ornate hawk-eagles separate by a visual barrier so that the male and female could not see each other. There was only vocal contact for six months.

“The risk of aggression is considerable, normally to the detriment of the males, which are smaller,” reports Junqueira. The approximation was successful and the pair has raised 12 chicks since their first encounter in 2005. One pair of harpies, however, produced only infertile eggs, and the other pair did not bond.

“Knowledge of the reproductive period for eagles may help,” says Junqueira. There are induction techniques, such as those used in the raising of domestic birds, with artificial lighting to simulate increased day lengths characteristic of the spring. To return eagles hatched in captivity to the natural environment, there is still the challenge of finding pristine forests, the only environment in which they survive, and where they make their nests in the highest trees.

Scientific articles
BLANK, M. H. et al. Ecological drivers of breeding periodicity in four forest neotropical eagles. Scientific Reports. vol. 13, 4385. mar. 16, 2023.
MIRANDA, E. B. P. Long-term concentration of tropical forest nutrient hotspots is generated by a central-place apex predator. Scientific Reports. vol. 13, 4385. mar. 17, 2023.