When you think about parasites, usually one of the "mind-controlling" types comes to mind, or maybe the chestbursters from the Alien franchise. But in the animal kingdom, particularly among birds, there's another kind: the brood parasite.
Image: Cuckoo finch chick sitting in the nest of its foster parents. Courtesy of Claire Spottiswoode.
Cuckoo finches reproduce by stealing something very unusual from other birds: parenting skills. The cuckoos lay their eggs in the nests of other bird species, hoping their young will blend in and be fed. Even though other birds have evolved to detect minute differences between eggs, and will eject foreign eggs from their nests, somehow cuckoo finches get by.
Brood parasites are organisms that trick other animals — sometimes individuals of other species, sometimes members of their own species — into rearing their young. For birds, this means laying their eggs in another bird's nest while it's away. But the exact evolutionary strategy used by avian brood parasites, and the countermeasures adopted by host parents, vary greatly.
In some cases, a brood parasite's eggs will look nothing like the host's eggs, and this is OK because the host cannot tell the difference. But in other cases, especially those involving cuckoo birds, the parasite's eggs and the host's eggs have similar patterns or colorations, and this results in some very cool evolutionary arms races.
A particularly interesting case involves the cuckoo finch (Anomalospiza imberbis) and one its many hosts, the African tawny-flanked prinia (Prinia subflava). University of Exeter ecologist Martin Stevens told io9, "There's an extremely high level of variation from one prinia's eggs to another." Some birds' eggs resemble the surface of Mars, while other eggs may be green with brown spots.
Some of the different types of patterns found in cuckoo finch eggs, which mimic African tawny-flanked prinia eggs. Courtesy of Claire Spottiswoode.
Importantly, the cuckoo has evolved to produce eggs that look similar to the prinia's eggs. But rather than seek out prinia nests with eggs that look like its eggs, the cuckoo leaves it all up to chance, laying eggs in seemingly random prinia nests. In response to this mimicry, the prinia, like many other host species, has developed mechanisms to tease out imposter eggs.
Scientists have concluded that these defensive mechanisms likely fall into one of two categories: discordancy or template-based. The discordancy mechanism involves rejecting eggs that are in the minority, whereas the template mechanism involves rejecting eggs that don't match up to a parent's internal template of what its eggs should look like.
So Stevens and his colleagues wondered: Which defensive mechanism(s) does the African tawny-flanked prinia use, and does the cuckoo finch still have a way to trick its host?
The team began by swapping eggs in the nests of prinia parents. "By doing this you can play the part of the parasite and simulate parasitism," Stevens explained. They varied the ratio of host eggs to foreign eggs — if the prinias used the discordancy mechanism, they should reject their own eggs if they're in the minority. Turns out, the prinias primarily used their internal template to make decisions, but also seem to take into consideration the relative proportions of eggs (in a few cases the birds rejected their own eggs when they were outnumbered).
The researchers then sought to determine which physical features the prinias paid attention to most when trying to separate their eggs from imposters. They combined their new rejection data with data from a previous study on the birds, and looked at the relative importance of different egg pattern and color characteristics — marking size, dispersion, contrast, diversity and proportion coverage — when it comes to egg discrimination.
"You can measure the color of the egg and you can quantify what the color looks like in terms of the bird's visual system," Stevens explained. "You can then use digital image analysis and computer programs to quantify various features of the marks."
They found the difference in color is really important, as well as the difference in marking sizes and how dispersed the markings are across the eggs. They also discovered that as the proportion of foreign eggs increased in a nest, the host parent required great color differences to tell the eggs apart. Using this information, the researchers created a model that predicts whether a host parent will reject an egg or not.
The researchers took 999 randomly chosen pairs of 300 prinia and 84 cuckoo finch eggs in their study population, plugged them into the model and simulated egg rejection using different host-to-parasite egg ratios. They saw that the cuckoo finch eggs are less likely to be ejected when they comprise a greater proportion of the nest, especially if the mimicry is good.
Left: Two prinia eggs and one cuckoo egg (good mimicry). Right: One cuckoo egg and two prinia eggs (poor mimicry). Courtesy of Stevens et al./Nature Communications.
For example, when the eggs are very similar and the host eggs outnumber the foreign eggs three to one, the foreign eggs have a 64 percent chance to be rejected by the host parent. But this probability drops down to just 9 percent if there are twice as many foreign eggs as there are host eggs.
So what's going here?
Well, the host knows what its eggs should look like, but it sees a greater number of eggs that don't fit in with its internal template than those that do, causing it to doubt itself. That is, the bird's two defense mechanisms — template and discordancy — don't align. "They're getting conflicting information because the eggs in the majority don't match their template," Stevens said. The birds become confused and unsure which eggs are really theirs, leading them to keep all eggs in the nest.
What's more, cuckoos in the wild will often lay an egg or two in a prinia's nest, and then return some time later to lay more eggs. They seem to purposely try to outnumber the prinia's eggs with their own, suggesting this behavior is an adaptive strategy, Stevens said.
The researchers are now interested in seeing if the prinias have a counter-strategy to balance the scales in this evolutionary arms race. "It would also be really good to know whether this strategy is found in other brood parasites, too," Stevens said. "It seems like a pretty good idea, so it wouldn't be too surprising if other species used it."
Stevens and his colleagues detailed their work in the journal Nature Communications.