Insect-eating birds need friends in the forest

If you were given a pot of conservation money to spend on species of your choosing, who would you choose? Hari Sridhar would pick six common, unremarkable birds of the forests of Anshi in the Western Ghats. In this article he tells you why.

I am happy now. In fact, I am in one of those rare moments when I would not rather be somewhere else, or doing something else. I am sitting in the dining area of the forest department-run tourist camp in Anshi National park. After two plates of poha and three cups of tea, I am ready and looking forward to the fieldwork that lies ahead of me. It is a bright day and the birds are talking; a definite relief from yesterday’s gloomy silence. The sounds I hear hold the promise of drama and excitement. This is more than abstract anticipation, for, from where I sit, I can hear the very birds that I will see shortly, on the trail that runs behind our camp. First a fulvetta calls, then fulvetta and drongo, then a monarch joins in… a flock is forming. Nagesh and I set off immediately.

Five minutes later, we are at the point on the trail nearest to the flock. I hear the birds clearly from here – fulvetta, drongo, monarch, as well as warbler and minivet – to the right of the trail, across the stream, about 100 metres away. I step off the trail and start making my way through the forest – dense and wet forest, what you might call ‘jungle’, but not difficult to walk through. Nagesh stays behind, as he usually does. Not for him this mindless bird-chasing. He settles down, on a concrete-made-to-look-like-wood park bench, and pulls his phone out of his shirt pocket. I reach the stream and pause, deciding how to cross. The water is only ankle-deep but I do not want to get my shoes wet. I try jumping from rock to rock but, as always, I miss a step and my left shoe is filled with water. As I climb out of the stream and up the other bank, I hear the birds just ahead of me. Fulvetta, drongo, monarch, warbler, minivet, nuthatch. I am at the edge of the flock, peering in. Binoculars uncapped, dictaphone switched on, I am ready to report on the action, like a commentator at the start of a cricket match. At first I see nothing. Then slowly, bit-by-bit, the flock reveals itself.

A drongo—the one with long tail streamers—sitting still, on a branch at eye level, and looking upwards. At the end of the drongo’s gaze, on the same tree, is a flameback woodpecker. Clinging precariously to the trunk, he scans his surroundings with a slow sweep of his head. I know he is a ‘he’ because of his red crown. Now, he flies to another tree, and a second later, the drongo follows him. Higher up, on the same tree, a nuthatch zigzags all over the trunk—up and down, left and right, front and back—probing the bark for tiny in sects. I do not know why, but from where I stand, the scurrying nuthatch looks more beetle than bird. My thoughts are distracted by a small green bird flying across my field of vision, from left to right, between me and the nuthatch. Through my binoculars, I recognise it as a warbler. As I lower the binoculars from my eyes, I notice many more flying in and settling on the trees around… ten… fifteen… at least fifteen warblers. As soon as they land they get busy, checking every leaf, above and below, for insects. Among the pale green warblers, is a bright blue bird—a monarch—watching the warblers with keen interest, following their every move, occasionally flying out to snap up an insect, in mid-air. It seems like the frenetic activity of the warbler army, is, somehow, making flying insects available to the monarch. A pair of bulbuls flies into the flock, from across the stream. They sit, on a low-hanging liana, shoulders touching, and clean themselves. They are wet, and in their wetness they look more green than yellow. One of them calls intermittently, a loud and harsh call, with no discernible rhythm, a bunch of notes randomly thrown together. In comparison, the calls of the minivets that I hear from the upper reaches, sound pleasant and happy, like the laughter of children on a playground. I see the minivets now, orange males and yellow females, flying between trees, like confetti in the sky. A sharp monosyllabic “kraak” pulls my gaze down. The maker of the sound, the drongo, is now clinging to a tree trunk, and watching the woodpecker, a few centimetres below, extricate a grub from under the bark. When the woodpecker finally pulls the grub out, the drongo tries to snatch it, but is unsuccessful. Just then, a flash of white draws my attention. A paradise flycatcher has joined the flock. Like the monarch, he too is hanging around the warblers and chasing insects in the air. His handsomeness when perched is only matched by the clumsiness of his flight. A description that applies equally to another bird I just notice: a trogon. He is sitting motionless on a branch about four metres high. The calls of the minivets make me look up again. A different drongo – the slender one with a deeply forked tail – is flying behind the minivets, doing to the minivets what the other drongo was doing to the woodpecker.

I have been watching this flock for ten minutes now. But there is still one bird, which I know is in the flock, but that I have not seen yet. I know this because I have been hearing it all along. The fulvettas’ loud, rhythmic calls have been a constant presence, like a background score to the flock’s visual theatre. But, try as I may, I am unable to spot the fulvettas. Even now, I can hear their sounds, one directly above me, one to my left, one roughly 30 metres ahead. In fact, it seems like the whole flock is contained and moving within imaginary lines that connect the calling fulvettas. I wonder if the fulvettas’ calls serve as the flock’s rallying point. Are the fulvettas inadvertent pied pipers, leading all the other birds?

I will have my answer soon. The flock is at the edge of the stream, about to cross. I stand in the middle of the stream and wait, no longer caring about my shoes becoming wet. My hunch seems right. The first to cross are three fulvettas. Then the drongo with the long streamers, then two more fulvettas, four warblers, monarch, seven more warblers, bulbuls, trogon, more warblers, paradise flycatcher, minivets, the other drongo, nuthatch, and finally the woodpecker. Eleven species, more than 30 individuals. The flock makes its way into the forest on the other side of the stream, moving too fast for me to follow. Binoculars capped, dictaphone switched off, I too head to the other side, to find Nagesh and continue along the trail.

A mixed-species flock is extraordinary in two different ways. The first is aesthetic. My attempt at description does little justice to the visual and acoustic spectacle that a flock provides. Watching a flock is like watching a movie trailer: snatches of action coming at you at a pace too quick to process. Visuals, sounds and movement flooding your senses, demanding your attention all at once. And just when you are getting used to the sensory overload, the trailer ends; the flock has passed. It is as if all the drama of the forest is encapsulated in a brief moment of time.

A flock is also remarkable in an ecological sense. We, ecologists, tend to view interactions in the animal kingdom through a lens of nastiness. We expect animals of different kinds – of different species—to chase, to fight, to kill or to eat each other; at best, to ignore each other. Niceties have no place in our construct of the animal world. A mixed-species flock flies in the face of this conventional view because friendship and cooperation lie at the flock’s heart, of a kind that we only expect among kith and kin. Do not get me wrong. I am not talking about sacrifice—about one species losing out for the benefit of another. A flock is a win-win situation, one in which all parties involved stand to gain. In the flock I saw that morning on the trail behind the camp, it was clear that some birds were getting food from the other birds—drongos stealing from the woodpecker and the minivets; monarch and paradise flycatcher snapping up flying insects that the warblers made available. But what about the other birds—nuthatch, fulvetta, warbler, bulbul, minivet, trogon, woodpecker—what were they gaining? The answer is not clear but it probably has to do with safety and protection. In the flock, in the company of other species, these birds are safer than if they were on their own—maybe because there are more eyes to spot an approaching danger; maybe because they are each less likely to be singled out by a predator; maybe they can all gang up and chase the predator away. It is also known—and I have seen it myself – that birds like the fulvetta and drongos are especially quick to spot approaching danger and cry out warnings.

That is probably why the other birds followed the fulvetta; probably why the woodpecker and the minivet tolerated the sustained harassment by the drongos—a small payment for the safety they get in return.

So far, I have focused on a single example. One flock, one moment in time, one point in space. A collection of 30-odd individuals of 11 species whose fates were closely intertwined. But even as I was watching that one flock that day, if, somehow, I had been able to zoom out, and see the entire forest like a soaring raptor would, I would have seen and heard hundreds of flocks all over Anshi. Flocks that differed in composition—different sizes, different individuals, different species—yet, identical in purpose—a way to food and safety for the actors involved. In fact, if I had looked really carefully from my vantage point high above, I would have noticed that almost every insect-eating bird of the forest was in one flock or the other.

If you are wondering where I am going with this, or what this has to do with conservation, here is my answer. To protect a species, any species, to ensure that it stays with us in the future, there are a few steps that we must take. We must ensure that the place it lives in is safe, that this place has enough food, that its enemies—both those that might eat it and those that might fight it—are not too many. There are other steps too, but these are the important ones. But, if the species that we want to protect is an insect-eating bird, there is an additional precaution required: we must protect its friends too. From what I just described, it is clear that insect-eating birds are not islands in the forest. They are dependent on each other for food and safety, linked to each other through invisible bonds of cooperation. These bonds form the building blocks of a network—a friendship network— that connect all the insect-eating bird species in the forest. But not all bonds in the network are equal. Just like us humans, each insect-eating bird too, picks and chooses its friends carefully, driven by more than one consideration: is the potential friend’s behaviour compatible with mine? What kind of help will the friend provide? Will the friend be available and willing to flock with me when I need to? To protect an insect-eating bird species therefore, we need to identify and protect its chosen friends.

We can look at this issue in a different way. Instead of asking who a particular species’ chosen friends are, we can ask how often a particular species is the chosen friend of others. Think of it like a popularity chart. We can rank each species according to its popularity in the friendship network, according to how often and how many other species want to flock with it. How does this help? If ever we have the unfortunate situation where we had to prioritise species to protect, then we could work our way down from the most popular to the least popular in our list. Because, by protecting the popular ones, ‘keystones’ in ecological parlance, we also help the many others that depend upon them for food or protection.

That day, after the flock crossed the stream, Nagesh and I continued along the trail behind the camp and encountered two more flocks.

The first one, on a hilltop, had only two species—a lone white-bellied blue flycatcher and 5 dark-fronted babblers—all keeping close to the ground, flycatcher following the babblers. In contrast, the next flock was the biggest I had ever seen, including as many as 55 individuals of 23 species. Over the next four months, I walked all over the forests of Anshi, trying to observe as many flocks as possible, to observe from the ground what I might have seen that day if I had been able to soar like a raptor. During this time, I encountered 250 flocks and recorded all that each of them contained. Back in Bangalore, we used this information to construct the network of “friendships” among the insect-eating birds of Anshi. I will not go into the details of how we did this. If you would like to know more, you can find it in the paper mentioned under ‘Further reading’. What I want to tell you is who the popular ones, the superstars of the network, were.

The answer could not be clearer. If you arrange the 36 species in the Anshi friendship network from most popular to least popular—and indicate each species’ level of popularity by the height of a vertical bar, you will see six tall skyscrapers followed by 30 little stumps that hardly get off the ground. In other words, six species were much much more popular than all the others.

Who are these superstars? In no particular order, since they were all equally popular, the six species were: brown-cheeked fulvetta, scarlet minivet, yellow-browed bulbul, black-naped monarch, western crowned warbler and greater racket-tailed drongo. To confirm our findings, we went back to Anshi the next year and repeated the entire exercise. Walked all the trails again, recorded all the flocks, built the network, and found out who the popular ones were. The answer was identical. Therefore, there is no doubt about who the key players are—the go-to birds for food and safety in the Anshi friendship network. That part was easy. What is puzzling is why these six species, and not any others? The puzzle deepens when you consider these six are an odd assortment with little in common. Is it because their behaviours are compatible with those of many other birds? Is it because they are particularly good at helping and at providing benefits to other birds? Or is it because they are easily available and willing to flock? We do not know, and as with most such questions with multiple possibilities, the answer is probably a little of everything. What we do know is that that these six species are important and play crucial roles in determining the fates of numerous other species.
The good news is that all six species are doing well. They are abundant in the forest and show no signs of imminent decline. There is little risk of any of them going extinct in the near future. The bad news is, also, that all six species are doing well, because it means that they will attract no conservation attention. The enterprise of conservation is interested in the rare and the threatened, not in the safe and the common. While this might be a good strategy generally, it requires a rethink in this case, because the fates of many an uncommon species rests on the future of these common birds. Therefore, if I was given a pot of money to spend on the species of my choosing, I would choose these six birds. I would use the money to find out what makes these birds tick. And once I found that out, I would also make sure that what keeps them going, keeps going too. Because, if these six species, which hold the reins of the friendship network in Anshi, go down, they will take everyone else down with them. A loss, both ecological and aesthetic.

Further reading:
Sridhar H, F Jordán & K Shanker. 2013. Species importance in a heterospecific foraging association network. Oikos 122:1325-1334

I thank Ferenc Jordán, Pavithra Sankaran, Joyshree Chanam and TR Shankar Raman for their comments and suggestions on this article.

This article is from issue


2012 Dec