Testing this hypothesis with some clever experiments.
Closely related species tend to look slightly different in areas where they co-occur (i.e. sympatry) compared to areas where they do not (i.e. allopatry). This pattern of greater divergence in sympatric than in allopatric species calls out for an explanation. One possibility relates to aggressive interactions. During the breeding season, for example, males of some bird species vigorously defend their territories. To avoid spending too much energy, they should direct their attacks to members of their own species. If they cannot recognize males from another species, they might engage in costly aggressive interactions. Hence, selection should favor males that are able to discriminate between species, leading to divergence in the traits that these males rely on (such as plumage patterns or songs).
In a recent study, Haley Kenyon and Paul Martin put this scenario to the test. They focused on three species of Poecile chickadee that mainly differ in their eyebrow stripe. Specifically, the researchers checked whether the Black-capped Chickadee (P. atricapillus) is able to recognize the Mountain Chickadee (P. gambeli) and the Mexican Chickadee (P. sclateri). If the aggression hypothesis is correct, you would expect that less attacks are aimed at birds with more divergent plumage patterns. That would be the Mountain Chickadee in this case.
The researchers placed 3D-printed models of the three species in the field. They monitored the behavior of allopatric Black-capped Chickadee in response to three combinations of bird-models:
- Black-capped Chickadee + Mountain Chickadee
- Black-capped Chickadee + Mexican Chickadee
- Black-capped Chickadee + control (i.e. Northern Cardinal)
Because Mountain Chickadees have a more pronounced eye-stripe, you would expect that Black-capped Chickadees are able to recognize them. Hence, they will mainly attack the Black-capped Chickadee model (which they see as a conspecific). Mexican Chickadees, however, look quite similar to Black-capped Chickadees. The birds might thus not discriminate between these two species and attack both models equally. And finally, the Northern Cardinal (Cardinalis cardinalis) looks so vastly different, that the chickadees will ignore it.
The results of this clever experiment did not support the aggression hypothesis. The researchers reported that “territorial Black-capped Chickadee males were equally likely to attack Mountain Chickadee and Mexican Chickadee models when they were paired with Black-capped Chickadee models.” Aggressive interactions do not seem to account for the divergent plumage patterns.
The physicist Richard Feynman once said that “If it disagrees with experiment, it’s wrong. In that simple statement is the key to science.” That statement might be true, but it is too harsh for biological field experiments with many confounding factors. In this case, the unexpected outcome of the aggression experiments could provide insights into the behavior of the chickadees, and could inform future experiments.
The researchers offer two possible explanations why the Black-capped Chickadees were not able to discriminate between the 3D-printed species. First, the Black-capped Chickadees in the experiments were allopatric. Perhaps they need to co-occur with the other species in order to learn the difference between conspecifics and heterospecifics. Second, the experimental models only differed in plumage patterns. It is possible that the Black-capped Chickadees use additional signals (such as songs) to recognize other species.
Hybridization or Habitat?
However, it could still be that the aggression hypothesis is indeed wrong. It would be worthwhile to explore other explanations for the plumage divergence. One alternative mechanism is selection against hybridization (i.e. reinforcement). Females might be able to discriminate between males from different species, thereby avoiding maladaptive hybridization. And indeed, the levels of hybridization are very low in areas where Black-capped Chickadees and Mountain Chickadees co-occur.
Another alternative explanation relates to habitat use. Species that reside in different habitats could be subjected to contrasting selective pressures. Specifically, selection for optimal signal transmission might favor different plumage patterns in different habitats. Black-capped Chickadees and Mountain Chickadees do seem to inhabit different habitats (deciduous vs. conifer forests).
Two exciting explanations that require further investigation. Time to plan for the next experiments.
Grava, A., Grava, T., Didier, R., Lait, L. A., Dosso, J., Koran, E., Burg, T. M. & Otter, K. A. (2012). Interspecific dominance relationships and hybridization between black-capped and mountain chickadees. Behavioral Ecology, 23(3), 566-572.
Hill, B. G., & Lein, M. R. (1989). Territory overlap and habitat use of sympatric chickadees. The Auk, 106(2), 259-268.
Kenyon, H. L., & Martin, P. R. (2021). Experimental tests of selection against heterospecific aggression as a driver of avian colour pattern divergence. Journal of Evolutionary Biology, 34(7), 1110-1124.
Featured image: Black-capped Chickadee (Poecile atricapillus) © USFWS | Wikimedia Commons