It was a beautiful spring afternoon at Amico Island. I was on a quest to find warblers, and I was excited when I spotted a life bird in the meadow.
It was a male Common Yellowthroat. There was also another bird with it. Common Yellowthroats are tiny warblers that are yellow and olive with a black mask over the eyes. The other bird though…was strange. It was almost double the size and brown. The big bird was crying for food while the little Yellowthroat tirelessly tried to satisfy it with whatever food it could find. Then it hit me: the Common Yellowthroat was feeding a Brown-headed Cowbird chick! Brown-headed Cowbirds are brood parasites. This male Yellowthroat had no idea that he was feeding a chick that was not even the same species as him. How did this happen?
There are about 100 species of birds that are considered brood parasites. Parasitic birds don’t want the responsibility of raising chicks; they pass that on to other unsuspecting bird species. These birds are sneaky: they will target females of other species and lay their eggs in that nest instead of creating one of their own. Why would they do this? Parenting consumes a large amount of time, resources, and energy that can be saved by having another bird do all the work. So then how do brood parasites pull off such trickery?
Brood parasites may use different technique in order to invade a nest. Sometimes a bird will patiently wait for the host to leave the nest so they can sneak in. Certain species will use a “disguise” by mimicking other harmless species to fool the host.. For example, in this study found in Proceedings of the Royal Society B, biologists learned that Cuckoo Finches from Africa will attempt to fool Tawny-flanked Prinias by pretending to be the harmless Southern Red Bishops (Check out the studies by Claire Spottiswoode and other scientists here). Other will pretend to look like predators and scare the poor hosts right off the nest.
If a brood parasite successfully makes it to a host nest without being detected, it will lay eggs that look similar in size and color to the correct specie’s eggs. The parasite’s eggs will be thicker but usually have a shorter hatch time than the host species so that the parasite will hatch first. Many birds cannot tell the difference between their eggs and the intruder eggs and raise whatever is in their nest. If the parasitic chick does end up hatching it will usually murder its siblings so it can get all the food. For example, Common Cuckoos have been known to push their sibling’s eggs right out of the nest to avoid them from ever hatching. African Honeyguide chicks will stab their siblings to death with their sharp hooked bills.
All of this sounds pretty terrible, right? However, it’s not always victory for the brood parasite. Many host birds are discovering these tricks and fighting back. Some host species will lay eggs that have a unique pattern, or “signature” so if they see an egg that doesn’t match they will dispose or destroy it. For example, Gray Catbirds will puncture any parasitic eggs they discover. Superb Fairy-wrens from Australia will teach their chicks a specific sound while they are still in the egg. Only Fairy Wrens are able to learn it, so if the chick is not including the “password” while calling for food, the host will know who the intruder is. Other species form adult lookout groups to catch and attack brood parasites while certain species will design their nest so that the invader gets trapped.
Brood parasites and their hosts are a fascinating example of avian evolution. While parasitic birds like the Brown-headed Cowbirds and Cuckoos are continuing to try and trick other species, the hosts are developing new strategies to defend themselves and their broods.
Have you ever experienced a brood parasite at work? Let us know if you have in the comments.