Biology is awesome. Just look at this photograph!
On top is a reed warbler (probably the Eurasian reed warbler, Acrocephalus scirpaceus) dutifully bringing food to its nest for its offspring. The hungry recipient is obviously not a reed warbler. It is a common cuckoo chick (Cuculus canorus).
As you can see, common cuckoo chicks are rather large compared to the reed warbler. They have to be, because cuckoos are brood parasites. Cuckoos don’t build nests to rear their young. They don’t raise their young at all. Instead, after mating, cuckoo females fly from nest to nest of selected bird species (a reed warbler, for example), laying a single egg during a brief window when unsuspecting parents leave the nest for a few seconds.
In that ten-second window (if she’s lucky to have that long), the cuckoo female must work quickly. She’ll tip one of the reed warbler’s eggs out of the nest, lay one of her own in its place, and leave before the warblers return. Sometimes, the warblers will spot the switch. Even though cuckoos have evolved to mimic the egg patterns of the species they parasitize, it’s not always a perfect match. So, keen and experienced reed warbler parents may spot the imposter and heave the egg out.
If the parents don’t, their adoptive offspring will do some heaving of its own. If the cuckoo chick hatches before its reed warbler siblings, the chick will use its relatively large frame to shove the unhatched eggs out of the nest. If the reed warbler chicks have already hatched, no matter. The cuckoo chick is still usually larger and stronger, and will simply maneuver its adoptive brothers and sisters toward the edge, and tip them off when mom and dad are away.
Now the only resident in their adoptive nest, the cuckoo chick works hard to fool its adoptive parents. Though there’s only one gaping maw begging for food, the chick’s cries attempt to imitate the calls of multiple hungry mouths. In the case of the cuckoo in this image, it has obviously been successful in getting the reed warbler parent to overlook glaring differences in size, shape, and color.
Reed warblers aren’t the only victims of cuckoo parasitism. Cuckoos have a wide native range across Europe, parts of Asia, and northern Africa. Different cuckoo populations appear to focus on parasitizing a unique subset of possible species, and evolutionary forces may gradually select for cuckoos that can effectively mimic the egg color and chick calls of only a handful of species.
This biologist also wonders how cuckoos effectively compete with one another for nesting space. If a female cuckoo, during her ten-second dragoon of another’s nest, recognizes the egg of another cuckoo, what does she do? Does she preferentially remove the cuckoo competition for her own chick? Does she relocate entirely to another nest? I also wonder about how cuckoos choose their victims. Does always choose species that are smaller, with eggs that develop slower than her own? Or, is it more important to choose a species with a general call, that is easier for the cuckoo chicks to mimic when fooling their adoptive parents into bringing them food?
Questions, questions, questions. No doubt, ecologists and evolutionary biologists have pondered these ideas already, and conducted studies on cuckoo behavior and the factors cuckoo females weigh when selecting unsuspecting victims. I’m sure they’ve also studied the abilities of cuckoo victims to recognize cuckoo eggs. I didn’t post this picture to talk specifically about brood parasitism and cuckoos. I posted it because, for me, it’s “a hook.” An image that, simply by its subject matter, so fascinates me and captures my imagination that I can’t help but sit back in my chair and conclude: biology is awesome.
I may post other hooks, about biology or some of the other scientific subjects I’ve found interesting (astronomy) or studied (geology). But for now, just enjoy the image and the ideas it conveys. This is life, and life is pretty cool.
Image credit: Per Harald Olsen, April 2007.