In marked contrast to the common image of a pair of birds incubating eggs and looking after their young, a guira cuckoo’s nest (Guira guira) may have the young of as many as seven mothers. This is far from being a peaceful coexistence, with spells of explicit violence against the young, but in compensation means a greater defense capacity against predators, as the work of the team of biologist Regina Macedo, from the University of Brasília (UnB), has shown for almost three decades. And there’s more: in the most recent study, published in 2011 in The Auk, Marcos Lima shows that polygamy reigns in these societies and that the males of a group are related.
“This is the first time that genetic characterization has been undertaken in any species,” explains the researcher, who carried out some of her work in the laboratory of biologist, Jeffrey Graves, at the University of St Andrews, Scotland, during her master’s degree at UnB. The results show that 72% of the nests are characterized by an egalitarian polygamy: both males and females have more than one sexual partner.
The relationship between the males revealed by Lima’s work (which is always closer than would be expected in the population studied) suggests that while the females emigrate as soon as they are big enough, the males remain in the nest where they were born or in a neighboring territory, where they live with their brothers and other close relatives. “Even if a male does not produce offspring he benefits from the evolutionary point of view by contributing to the success of the group,” explains Lima.
The researchers are also looking at the results with a certain degree of caution because it is impossible to obtain a complete sample. The zone of study selected by Regina is on the outskirts of Brasília, in a residential area known as Park Way, where in addition to houses, gardens and plantations there are also some areas of cerrado vegetation. Not satisfied with the size of the region’s low and twisted trees, the guira cuckoos prefer to make their nests in araucarias, which are not native to the area. As they are tall, it was necessary to climb from 5 to 14 meters to install nets, examine the eggs or collect blood samples. The group from the UnB also used traps, the bait of which were recordings of the male song and a tame ani, which sometimes acted as an invader. As a result, many of the birds captured were males that had come to defend the territory. According to Lima, “the ideal would have been to have a whole group to check the relationships and the paternity of the young.”
Even so, the genetic advantages of group life detected by the study has helped to understand how a social system evolved in which some of the eggs are thrown out of the nest and some offspring are killed after they hatch, which generally results in 5 or 6 young surviving, even though as many as 17 eggs were laid. Regina does not risk indicating suspects for the attacks on the eggs. “I’ve only witnessed rejection twice,” she says; she managed to collect these eggs by placing nets around the tree and then collecting samples of genetic material. In another species of ani, it is believed that females, which have not yet laid eggs, rid themselves of the eggs of their rivals, but the biologist from the UnB does not agree that such is the case with the guira cuckoos. In a previous study she showed that the females do not lay eggs in the same order between one brood and another, eliminating the possibility that the dominant female would lay her eggs last, when they would have less chance of being eliminated.
Even after they break out of the eggshell, the young are not safe. Adults, which may be males or females, often throw one of them from the nest and peck them to death, even though other members of the group may cry in alarm from a neighboring branch. The attacks begin with the smallest, as shown in the article from 2004 in Animal Behaviour, and may be repeated until all the young are killed. Regina still has no explanation for this behavior, but believes that the guilty ones are those that did not manage to reproduce at that opportunity. “Eliminating the young would allow the group to make a new attempt at reproduction more quickly,” she explains. She has already acquired blood samples from adults that killed young as well as the young they killed. When the genetic analyses have been done it is hoped to throw some light on the enigma. Lima adds another possibility: the reduction in the brood to a number that the adults are able to sustain more easily as they take turns at being guardians and food providers. “They develop very quickly and in 12 days begin to fly,” she says.
Since her doctorate in search of unveiling the secrets of the guira cuckoo, Regina explains that she knew very little about the behavior of this bird with its remarkable appearance, with its raised crest and almost 40 centimeters in length which lives in practically the whole of South America, above all in open areas with human habitation, where they look for food in groups on the ground. “Because they are common, they arouse no interest.” The more the researcher learns, however, the more fascinating she finds the social and reproductive system of this species. “Studying the interface between the cooperation and competition that occurs in any society may contribute a lot to understanding the evolution of this type of system,” she explains.
LIMA, M.R. et al. Group composition, mating system, and relatedness in the communally breeding guira cuckoo (Guira guira) in Central Brazil. The Auk. v. 128. n. 3, p. 475-86. 2011.
MACEDO, R.H.F. Significance of social parameters on differential nutrient investment in guira cuckoo, Guira guira, eggs. Animal Behaviour. v. 68, p. 485-94. 2004.