When Birds of Different Feathers Flock

By wildgirl -

Birds with shared interests – safety, food, similar nesting spaces – often roost in and traverse the forest together. These mixed-species flocks are a boon for the birder, according to Faraaz Abdool. All photos by Faraaz Abdool

 

In tropical rainforests, we imagine that the trees must be positively dripping with birds. Massive, bromeliad-laden boughs thirty metres in the canopy should be cradling all manner of avian diversity. Our imagination conjures toucans skipping over tanagers and honeycreepers in pursuit of the finest berry, as hummingbirds linger on the periphery, eager to sip from the nectar of delicate epiphyte blooms. Sometimes, or often, we put ourselves in a place like this and find a deafening silence. In forests that span hundreds, even thousands of square kilometres, one can easily spend a couple hours scanning without result at the risk of neck strain. Hope rises again upon the arrival of a mixed-species flock.

The self-explanatory “mixed-species flock” hides nothing; it is exactly as is implied - a group of birds comprising several species. This is not a group of birds that has been summoned from disparate directions in the forest, but a cohesive flock that operates together.

An insectivorous mixed-species flock in central Trinidad consisted of a White-winged Becard, a pair of Black-crested Antshrikes, an Ochre-lored Flatbill, and this Rufous-browed Peppershrike. Photo by Faraaz Abdool

 

A careful distinction must be made here, as some birders and birdwatching guides claim to be able to “pull birds out of the forest” via the use of audio recordings. This tends to involve the playback of alarm or mobbing calls - the sound of birds responding to an intrusive threat. Sometimes the vocalisation of a common predator may be used, broadcast far and wide on a bluetooth speaker. This behaviour whips birds away from their usual responsibilities such as feeding, caring for their young, or simply staying concealed. Apart from being regarded as unethical by contemporary birders in the wider world, the unhinged use of audio lures can also negatively impact the bird population by exposing them to threats - for example leaving nests unattended, or dismantling bonds between mated pairs of adult birds.

The mixed-species flock courses through the forest to its own beat, without regard for where birders’ eyes are. They are intent on finding food and staying alive. Within their structure, they cooperate and compete. Nevertheless, they move as one creature. They disrupt the quiet with their often frenetic activity, even if for a few minutes at a time. They may trapline food sources, and they can band together for specific targets. There are frugivorous (fruit-eating) flocks as well as insectivorous flocks. Migratory flocks are slightly different to the mixed-species flocks discussed here; these amalgamations of birds can involve songbirds or raptors - all on the same journey. Mixed-species flocks are generally food-driven, non-migratory, and would be extremely localised. This unexpected cornucopia brings a lottery of common and uncommon sights for the patient birder. 

A Red-throated Ant-Tanager and a couple Bicolored Antbirds look on as army ants course through the forest in Costa Rica. Photo by Faraaz Abdool


We don’t know what goes into the formation of a mixed-species flock at this point. Clearly, birds that sound very different from one another to our untrained ears are expert interspecific communicators. How else would they know to leave their routine post to attend this traveling circus of misfits? Regardless of the formation protocol, the mixed-species flock carries with it a few key benefits which would be impossible for single birds to boast about.

The Dilution Effect outlines that within a group dynamic, any individual stands a much lower probability of being attacked - the chance of being eaten is diluted. This reduced risk means that the flock can remain feeding for longer, tolerating the presence of predators in closer proximity than if they were present as singles or pairs.

The Many-eyes Effect is a term for what happens when individuals who all have a common threat get together - it is in the interest of everyone, both on an individual and collective level, to keep an eye out for predators. If everyone keeps watch, many eyes are involved. This increased vigilance coupled with rapid communication - most often via specialised alarm calls - allows a group of animals to deal with an impending threat much more efficiently. Mixed-species flocks often include a “whistleblower” - one particularly wary species who is very quick to sound the alarm. This holds true across all animals, with many documented cases around the world, not only within forested areas. A mixed-species flock of shorebirds feeding at a tidal mudflat would be put on high alert by the alarm calls of Black-necked Stilts, for example. These tall, elegant black-and-white birds are usually the first to notice a stranger presence and they waste no time in alerting everyone. Once the stilts begin their staccato alarm, all others stop probing around in the mud and lift their heads, fully alert.

Birds like the Speckled Tanager will often band together with similarly sized Bay-headed Tanagers as they forage in the forests of Trinidad's Northern Range. Photo by Faraaz Abdool

Social Learning takes place as the group interacts with one another and the environment. New members would be learning the location of preferred fruiting trees, for example, facilitating the efficient transfer of knowledge across generations and species of birds. This aids in foraging efficiency, as the collective wisdom of the flock persists year after year. It may even make individuals more resilient to changing conditions as they carry the experiences of their predecessors.

While mixed-species flocks have been observed all around the world, it is in the mega-diverse tropical regions where they are most often encountered. Tropical conditions allow for several species to have a common food source, or a common predator. Some flocks consist of birds that occupy specific roles, or birds that take certain physical positions. A mixed-species flock that follows a swarm of army ants through the Neotropical jungle, for example, has a common goal of feeding on the small animals displaced by the rampaging ants. All birds cannot be after the same prey, however. Thankfully, there is a niche for everyone. Antbirds and antshrikes hop alongside the river of ants, while ant-tanagers and antwrens hang out in thin vegetation just above the ground. Woodcreepers lurk vertically, motionless on the trunks of massive trees adjacent to the ant swarm while flycatchers follow the proceedings from the branches above. For the thousands of insects, spiders, frogs, lizards, and small mammals that scurry, hop, and flutter away from the pitter-patter of millions of feet, there is little hope of escape when the mixed-species flock is in attendance.

Thick, humid, impenetrable jungles are near-hopeless birding areas unless one comes across a mixed-species flock. They require patience and silent attention to the environment - knowledge of the food sources dramatically increases one’s chances of witnessing this mesmerizing flurry of cooperative activity. Of course, there’s always plain luck involved, but our chances remain at zero if we stay indoors.

The Plain-brown Woodcreeper is a regular attendee of army ant swarms on both Trinidad and Tobago. Photo by Faraaz Abdool

 

References

Goodale, E., Ding, P., Liu, X. et al. The structure of mixed-species bird flocks, and their response to anthropogenic disturbance, with special reference to East Asia. Avian Res 6, 14 (2015).

Tracey L. Hammer, Pierre Bize, Benoit Gineste, Jean-Patrice Robin, René Groscolas, Vincent A. Viblanc, Disentangling the “many-eyes”, “dilution effect”, “selfish herd”, and “distracted prey” hypotheses in shaping alert and flight initiation distance in a colonial seabird, Behavioural Processes, Volume 210, 2023

Wenyi Zhou, Ask a Birder: Why Do Birds Form Mixed-Species Flocks?, 23 April 2025, 10,000 Birds, https://www.10000birds.com/ask-a-birder-why-do-birds-form-mixed-species-flocks.htm

 


 


 


 


 

 

 

 


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