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?
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