Unique Egg Signatures Help Drongos Reject Imposters in Their Nest and Avoid Getting Tricked by African Cuckoo

In the animal kingdom, some species are known for demonstrating brood parasitism or laying eggs in other nests to force them to raise their young. An example of a brood parasite in birds' families is the African cuckoo (Cuculus gularis), which lays its eggs in the fork-tailed drongo (Dicrurus adsimilis) nest.

(Photo: Wikimedia Commons/ Xenia Ivanoff-Erb)

Spotting the Difference

Fork-tailed drongo is an aggressive type of sub-Saharan African bird that thrives in savanna woodlands, bush areas, and forests. During their nesting season from August to January, drongo lays eggs with a huge diversity of colors and patterns. Unfortunately, these visual markings are forged by the African cuckoo.

African cuckoos lay their eggs in other birds' nests to avoid taking care of their offspring themselves. By imitating the colors and patterns in drongo eggs, the African cuckoos try to trick the drongos into thinking that the foreign egg is one of their own. Through natural selection, the eggs of African cuckoos have evolved to resemble drongo eggs in an event known as high-fidelity mimicry.

However, it was predicted that drongos could identify and reject the eggs of African cuckoos from their nest on almost every occasion. Drongos are thought to know their own egg 'signatures,' which makes them unique.

This behavior has been the focus of a research study conducted by researchers from the University of Cambridge and the University of Cape Town. By collaborating with a community in Zambia, the scientists explored the effectiveness of egg signatures in defending their nest against highly accurate mimicry.

In four years, fieldwork was conducted in the Choma district of Zambia. The team then created a model which predicted the frequency of fork-tailed drongos rejecting the eggs of African cuckoos. The result of the experiment reveals that the predicted rejection rate is 93.7%.

According to lead researcher Jess Lund, their additional simulations show that this attribute is likely due to the evolution of signatures in drongo's eggs. Although the African cuckoos have also evolved their forgery skills, the individual cuckoos do not aim for individual drongo nests that can match their eggs. In other words, every time a cuckoo egg is laid, there is a very low likelihood that it will match the egg signature of drongos.

What is Brood Parasitism?

Brood parasitism refers to the behavioral pattern of some animals where the parents leave their young with another animal who will act as their foster parent. The brood parasite manipulates the host animal using egg mimicry in this evolutionary strategy. This phenomenon also usually appears among birds, insects, and fish species.

In birds, at least 300 avian species engage in brood parasitism, the most famous being the cuckoos and cowbirds. A female bird lays an egg in another bird's nest; then it flies away, never to see her offspring again. On the other hand, the host bird is fooled into raising the foreign bird as their own.

Just like other breeding systems, brood parasitism has its benefits. This strategy was found to be ideal under some conditions, such as having unpredictable environments. It is also the best chance for some animal species to pass their genes down to the next generations.