Rowan Creek Journal June 2010

Cuckoos and tent caterpillars

While many of us are cursing the tent caterpillars that have invaded our yards and neighborhoods, we could take this opportunity to get to know one of the more enigmatic families of birds in North America. Cuckoos are the only known group of birds to regularly feed on and, in fact, seek out tent caterpillars.

There are two species of cuckoos we’re likely to see in Wisconsin, black-billed (Coccyzus erythropthalmus) and yellow-billed (Coccyzus americanus). As would be expected, the names are based on the difference in the color of the bill, specifically the lower mandible. Both would be considered attractive with a warm brown back, white undersides, and white spots on a black tail. While they both share the same basic color scheme, the spots on the tail of the yellow-billed are larger and more distinct, and it has more rufous visible in the wings during flight. Neither, however, is well known as they tend to be secretive and forage quietly in the dense foliage of trees and bushes.

Their primary food, as noted, is tent caterpillars. Most birds can’t digest the hair found on tent caterpillars. Cuckoos have evolved to a mechanism to store the hair and then regurgitate it later. Not only do they eat tent caterpillars, they eat large numbers of them. There are reports of individuals eating as many as 100 in a single sitting. Their digestive systems have to be specifically adapted to handle hair and to handle that much at one time. Survival, for a species, is all about exploiting a niche that other species do not. Eating hairy insects allows cuckoos to follow an abundant resource with no competition, except for other cuckoos. They also eat cicadas; again taking advantage of an irregularly abundant resource.

Both species are nomadic, meaning they don’t have a particular location they return to after returning from their wintering grounds. They wander until they find an outbreak of tent caterpillars since outbreaks are localized. Is it reasonable to expect that next year’s outbreak will be near this year’s? That would provide the cuckoos a place to start their search. I don’t know the answer to that question. It is believed that most long-distance migrants have their migration patterns hard-wired in their brains. But what if that bird, or even species, does not return to the same place every year? How do they learn migration routes? Let’s take this line of questioning one step further: how can a species be nomadic, yet still maintain a population? If a bird, say an oriole, successfully raises a clutch of young, those young can return to that site and reasonably expect to find suitable conditions to breed and raise their own young. Eventually that habitat could become saturated and not be able to support more of the same species. The newcomers would then be forced to seek out other favorable habitat. However, other individuals from that species would be doing the same and they would likely encounter each other in good habitat. Cuckoos, on the other hand, specialize in a prey base that is geographically inconsistent and not tied to a particular habitat type. So not only do individuals have to find where the tent caterpillar outbreak is happening, but they need others to find it, too. And enough birds need to find each other every year, or on some regular basis that they can sustain the population. Final question: what would happen to our cuckoos if tent caterpillars were eradicated? As prey-specific as cuckoos are, they could likely face extinction.

Another unique trait of cuckoos is asynchronous hatching of eggs. Most species lay one egg a day, but don’t start incubating until the last egg in a clutch is laid. That’s so all the eggs develop on a similar schedule. If the eggs hatched on different schedules, the older, and therefore larger ones, would out-compete the younger smaller ones for food, and the smaller would have little chance of surviving long enough to fledge. Not so with our cuckoos. Since prey is usually abundant, even the smallest young in the nest will get its share. And that makes sense since they follow outbreaks.

All of this adds up to a very different survival strategy than most other birds. Instead of focusing on one habitat type that is likely to be reliable every year or changing very slowly year to year and allowing populations to adjust over time, cuckoos have adopted a nomadic life style that adapts to a geographically changing resource, but one that is almost always abundant and for which they’ll have no competition.