Most animals group together due to the advantages of group vigilance to detect potential dangers and increase foraging efficiency. While birdwatching, sometimes you may find a monospecific flock, meaning that it only consists of individuals that belong to one species with the same ecological requirements. Other times, you may also find flocks with members from multiple species, which are called heterospecific flocks.
Characteristics
Mixed-species flocks are heterospecific flocks. However, mixed-species flocks are different from feeding aggregation. In simple terms, rather than being attracted to the same type of food source, each species in a mixed-species flock targets sightly different prey items, and they would also benefit from other constituent species with different behavioral roles that contribute to the cohesion of the flock (Mangini et al., 2023).
Several explanations for why mixed-species flocks exist are mostly based on two main hypotheses:
Improved feeding efficiency
Reduced risks of predation
The two hypotheses are supported by a 2009 study published in Animal Behavior, where it is found that most birds in mixed-species flocks are insectivores, meaning that they only eat arthropods such as spiders, ants, etc. Most birds in mixed-species flocks are also small birds, so participating in flocks could protect themselves from larger predators (Sridhar, Beauchamp and Shanker, 2009). The study also discovered that birds in mixed-species flocks tend to spend less time staying vigilant and more time foraging compared to monospecific flocks (Sridhar, Beauchamp and Shanker, 2009). However, what is interesting is that the increase in foraging rates is spread unevenly in some mixed-species flocks due to the structure of roles within the flocks.
Roles within a mixed-species flock
It is important to note that the identification and characterization of each role within the flock is inconsistent across different literature findings as the social composition of these flocks remains uncertain (Mangini et al., 2023). However, there are mainly two distinct roles in a mixed-species flock: the “nuclear” species, or the “leader” that forms and dictates the flock, whereas the rest are “followers”. According to some studies, there are even other, more detailed classifications of roles based on behavioral activities, such as:
Attendants/associates: Birds that only join the mixed-species flock when it has entered their territory and leave when the flock exits their territory.
Sentinels: Birds that alert the flock to the dangers and predators nearby.
It is found that the increase in foraging rates within a mixed-species flock is only seen in the followers but not the leaders, as the leaders tend to spend more time detecting predators than foraging. Although it may appear that the leaders may not benefit much from sharing a flock with other species, they are not necessarily at a disadvantage. In fact, some nuclear species have developed interesting behaviors in order to “steal” food from other members of the flock. A study in 2017 found that a tropical bird, the Greater Racket-tailed Drongo (Dicrurus paradieseus), sometimes makes false alarms in a mixed-species flock to get the followers to drop their food and seizes them (Suzuki and Kutsukake, 2017).
Conclusion
The structure of mixed-species flocks is complex, which is why it is still a challenge for scientists to classify and describe them consistently to date. Nevertheless, it is still fascinating to observe and study the sophisticated social structure of these amazing avian friends!
Works Cited
Fischer, R.A. (2014). Figure 2. The rufa Red Knot (Calidris canutus rufa) was listed as... [online] ResearchGate. Available at: https://www.researchgate.net/figure/The-rufa-Red-Knot-Calidris-canutus-rufa-was-listed-as-threatened-under-the-ESA-in-2014_fig2_334041484 [Accessed 15 Aug. 2024].
Madhavan, N. (2024). Greater Racket-Tailed Drongo - Facts, Diet, Habitat & Pictures on Animalia.bio. [online] Animalia.bio. Available at: https://animalia.bio/greater-racket-tailed-drongo [Accessed 15 Aug. 2024].
Pallardy, R. (2018). Smarter Together: Mixed species foraging flocks. [online] Earth.com. Available at: https://www.earth.com/news/mixed-species-foraging-flocks/ [Accessed 15 Aug. 2024].
Mangini, G., Rutt, C.L., Sridhar, H., Buitron, G., Muñoz, J., Robinson, S.K., Montaño‐Centellas, F., Zarco, A., María Elisa Fanjul, Fernández-Arellano, G., Xing, S. and Ettore Camerlenghi (2023). A classification scheme for mixed-species bird flocks. Philosophical Transactions of the Royal Society B, 378(1878). doi:https://doi.org/10.1098/rstb.2022.0100.
Sridhar, H., Beauchamp, G. and Shanker, K. (2009). Why do birds participate in mixed-species foraging flocks? A large-scale synthesis. Animal Behaviour, [online] 78(2), pp.337–347. doi:https://doi.org/10.1016/j.anbehav.2009.05.008.
Suzuki, T.N. and Kutsukake, N. (2017). Foraging intention affects whether willow tits call to attract members of mixed-species flocks. Royal Society Open Science, 4(6), p.170222. doi:https://doi.org/10.1098/rsos.170222.
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