‘Birds of a feather flock together’ is an old adage to explain some well-observed aspects of social behaviour among humans and animals. In its most literal sense, however, the idiom may not be entirely true. While most organisms do socialise within their own species, examples of inter-species interactions are not entirely uncommon either.
Social behaviour among animals has been studied for a long time. Most of the scientific interest has been focused on intra-species social interactions, and scientists have a fairly sophisticated understanding of group behaviours. However, relatively less is known about the socialising of certain animals including birds and mammals with individuals of other species.
Research by Hari Sridhar and Vishwesha Guttal of the Centre for Ecological Sciences at the Indian Institute of Science in Bengaluru offers new insights on inter-species social behaviour among animals.
Their paper in Philosophical Transactions of the Royal Society B, published by the Royal Society, is divided into two parts. In the first part, Sridhar and Guttal have reviewed the existing scientific literature on inter-species socialising, and shown that some common beliefs about such behaviour need to be modified. In the second part, they provide a conceptual framework to explain the probable reasons for certain organisms preferring to mix and live among individuals of different species.
“For a long time, people have treated mixed-species sociality as being different from same-species interactions. In our study, we challenge this common thinking. The general idea has been that in same-species social interactions, all individuals get similar benefits. In contrast, it is thought that the different species in mixed-species groups get different benefits, ones that cannot be obtained by grouping with their own species. For example, if one species is very good at spotting predators in the sky, like eagles, and another one at doing this on the ground, then these socialise together to benefit from each others’ skills,” the researchers told The Indian Express.
Through the literature review, the scientists tried to ascertain whether the motivation for mixed-species socialising was always to benefit from such complementary skills.
“What we found was that most cases of mixed-species sociality were very similar to single-species groups. The benefits that individuals were getting in mixed-species interactions were not different from what they would have got from their own species. It is very interesting, because it then raises the question as to how the organisms then choose between same-species and mixed-species groupings,” Sridhar said.
The researchers have tried to answer this question in the second part of the paper. Sridhar carried out extensive field studies in the Western Ghats, where he observed the behaviour of a variety of birds in mixed-species flocks. These included some of the most common insect-eating species found in tropical forests like racket-tailed drongos, babblers, warblers, woodpeckers and trogons.
The researchers identified some of the plausible motivations for these birds to become part of mixed-species flocks. One, of course, is the relevance of such socialising. The participating individual must look forward to some concrete benefit, like protection from predators, something that can happen only if the two species share predators.
The quality of benefit is another possible factor. “You could have two potential partners with whom you share predators. The one that is better at spotting and evading the predator is likely to attract individuals from other species into their flocks,” Sridhar said.
Birds probably also take into account the cost of competition while deciding whether to join a flock of different species. “For instance, members of the same species have similar food habits, there is an increased competition for the same food. If another species has a different food habit, but shares a predator, it might be beneficial for some individuals of the first species to join them,” Sridhar said.
While joining another group, birds possibly also consider whether they would be able to coordinate their activities with the flock. “If you need to fly together, for example, you need to be similar in your flight behaviour and skills,” Sridhar said.
A combination of these, and probably more, reasons decide whether these birds restrict themselves to flocks of their own species or join other groups.
While the research is certain to improve the understanding of mixed-species social behaviour of animals, Sridhar said this kind of knowledge could have other kinds of implications as well — in conservation, for example. “Supposing a certain species gets extinct, or changes behaviour or habitat, because of climate change or some other reason, there could be a cascading effect on other species. It would help if we know about such interactions of these species,” he said.
The research: Reviewing inter-species socialising, and finding the reasons that motivate organisms to choose between same-species and mixed-species groupings
Researchers: Hari Sridhar and Vishwesha Guttal Centre for Ecological Sciences, Indian Institute of Science, Bangalore