There is some interesting new research you will want to know about concerning Reed Warblers and Cuckoos.
In the common European Cuckoo, Cuculus canorus, females come in two morphs: Gray or rufus. It is thought that the gray morph mimics a bird eating hawk. In this way, the cuckolding Cuckoo can convince its cuckoldee, the Reed Warbler, to back off when the Cuckoo comes around, allowing the Cuckoo to toss out one of the Warbler’s eggs and replace it with one of its own, to be raised by the hapless Warbler parents.
European Cuckoo. Photo by jamalhaider.
However, Reed Warblers are social learners. They observe each other and learn about food sources, predators, etc. And, thus, they can learn that the Cuckoo is not really a bird-eating raptor. And thus, the second female morph has emerged in these Cuckoos. She has an advantage once the intended host birds are on to the other morph.
I’ve written a brief overview of the evolutionary principle of Mimicry here. The famous bird experts, Thorogood and Davies, carried out an experiment which…
reveal that social learning is specific to the cuckoo morph that neighbors mob. Therefore, while neighbors alert hosts to local cuckoo activity, frequency-dependent social information selects for a cuckoo plumage polymorphism to thwart host detection. Our results suggest that selection for mimicry and polymorphisms comes not only from personal experience but also from social learning.
Johanna Mappes, Leena Lindström, in a commentary on Thorogood and Davies paper, write:
Thorogood and Davies’s findings show that, 150 years after its discovery, mimicry still contributes fundamental insights into ecological and evolutionary biology. It remains unclear why rufous cuckoos are not increasing in numbers in the UK, despite their rarity advantage. Perhaps the rufous cuckoo is not as good a predator imitator as the gray cuckoo, and thus cannot fool the hosts as well. Or perhaps the rufous females have some physiological or reproductive disadvantages compared to the gray females. The answer may lie in locations, like Hungary … where rufous cuckoos are more numerous than in the UK.
Another fascinating question is why individual reed warblers vary so much in their responses [as demonstrated in the study] and risk-taking toward the hawk and the parasite. And how many individuals must learn to discriminate between the cuckoo and the hawk and share this information with others to maintain the plumage polymorphism in the cuckoo?
Source:
Cuckoos Combat Socially Transmitted Defenses of Reed Warbler Hosts with a Plumage Polymorphism, by Rose Thorogood and Nicholas B. Davies. Science 3 August 2012: 578–580
Cool insights on mimicry in the avian realm. I wonder if it is a similar case with Asian hawk-cuckoo species? On another mimicry note, the immature plumages of several raptor species appear to mimic other raptors that tend to be larger or maybe potential predators of the bird doing the mimicking (assuming that is what is going on). In the neotropics, this trend is apparent in Gray-headed Kite looking like Black and white Hawk Eagle, Hook-billed Kite and Bicolored Hawk looking like Collared Forest-Falcon, and Gray-bellied Hawk resembling Ornate Hawk Eagle.
Well, I think the Asian cuckoo must be doing the same thing. Interesting point about immature raptors. I recently learned that cub Cheetahs could be mimicking honey badgers. I won’t say where I learned that, though.
Wouldn’t social learning in Reed Warblers favour the development of more than two morphs of the Common Cuckoo?
Furthermore, there doesn’t seem to be a correlation between the relative abundance of the brown morph in female Cuckoos throughout the cuckoo’s range and the distribution / selection of their hosts, assuming social learning in reed warblers is the same in all their populations. As you state in your post: why aren’t brown females increasing in the UK (and elsewhere in Europe where they are rare) when the hosts are evenly distributed throughout Europe and the UK? Female brown cuckoos are quite common in NE Germany and very, very scarce in SW Germany. However, the population in NE Germany is healthier than in SW Germany, which does not suggest the brown females have much difficulties chasing off host adults to successfully place an egg in their nest.
Interesting, interesting stuff.
Exactly, good questions. They are not perfectly evenly distributed, and maybe the variation is explained as a changing strategy, but as the authors of the study point out, they just dont’ know yet.
Why there are not more than two morphs is an interesting question in and of itself and I suspect this has something to do with the developmental process that give you the morphology to begin with.