For the Birds Radio Program: Army Worms
Is there anything good about army worms? There is if you love cuckoos.
Something about June makes my heart swell. As Rodgers and Hammerstein wrote, “June is bustin’ out all over, all over the meadow and the hill.” The problem this year is that as quick as those buds can bust out into leaves all over bushes and trees, forest tent caterpillars devour them.
I was recording birds in the Nicolet National Forest in early June and every time I tried to pick up a bird high in the trees, I heard a sound like a gentle but steady rain. It took me several minutes to realize that what I was hearing was the sound of these army worms chewing and pooping. This is the third year in a row for most of the Northland deciduous trees to face being eaten alive by them. Fortunately, by now parasitic flies in the genus Sarcophaga are reaching their own population highs. These flies will lay their eggs on the webbing where the caterpillars pupate, and their larvae will eat the pupae before they can emerge as adults. That will cause the forest tent caterpillar population to plummet, and we’ll have a grace period of several years before they return.
Most birds apparently find the hairy skin on these caterpillars unpalatable, but I’ve watched jays, crows, and orioles eating the squished innards on roads. The caterpillars often denude branches of their leaves after birds have built their nests, exposing eggs and nestlings to the burning rays of the sun and to the eyes of predators. I’ve also watched as dozens of them crawled over tiny, naked redstart nestlings. The sight of them crawling over these tiny birds’ eyes seemed downright ugly. Overall their presence probably reduces nesting success for many species during the time their population is high.
But one bird’s population increases dramatically with army worms. The Black-billed Cuckoo feeds voraciously on them, and feeds them to its nestlings. Right now cuckoo numbers are quite big. These slender, jay-sized birds can be found in any north country yard, but are quiet and inconspicuous, so unless they bonk into a window the only people who discover them are those paying attention. Black-billed cuckoos have a pleasing “cu cu cu” call that they make repeatedly for many minutes at a time, but it’s soft enough and given at such a frequency, that it stays off most people’s radar screens. I don’t know how many times I’ve pointed out a singing cuckoo’s calls to someone who was quite close but hadn’t noticed.
Thank goodness that the expression “you are what you eat” isn’t true in the case of .Black-billed cuckoos, which are surprisingly beautiful and graceful. Their underside is snow white, their back a rich, soft brown, their tail long and graduated with white tail spots, and their eyes rimmed with a conspicuous red eye ring. Although their favorite food is hairy caterpillars, they do eat other insects and spiders, small vertebrates like frogs and toads, and fruits. They are true long distance migrants, flying beyond Central America all the way down to the northern parts of South America. They arrive a bit later than most tropical migrants, probably because their food isn’t really available until the trees have completely leafed out.
Our American cuckoos are related to the European cuckoos,, which are nest parasites. There are records of American cuckoos laying their eggs in the nests of several other species, but this is very unusual. Black-billed cuckoos are normally wonderful parents–well, if you can consider any bird a good parent that feeds its babies army worms.