Barred owl.  Photo by Doug Hitchcox

Owls are one of the earliest groups of birds to start nesting in Maine, and we are beginning to hear their young out and about now. Diane from York wrote in this week asking about owls they’ve been hearing this summer, noting that “sometimes hooting back and forth lasts 10 minutes, but one night it was half an hour with a few breaks, maybe to catch their breath.” These vocalizations could be anything from adults conversing, an adult encouraging a juvenile to fledge, a fledgling still begging for food, or (as barred owls are known to do) celebrating having taken a large prey item.

Diane was specifically wondering about the long calling behavior, and while it is hard to know without hearing or seeing, I thought a different part of her question was more interesting. While maybe the comment was tongue-in-cheek, the idea that the owls need “to catch their breath” is a fun one to discuss, since a bird’s respiratory system is built very differently from our own.

For the sake of reader retention, I’ll oversimplify and say that humans (mammals) have a bidirectional airflow: we inhale oxygen into our lungs, where in smaller air sacs the gas exchange of oxygen and carbon dioxide occurs. It takes our exhale to release that carbon dioxide, making it a moment when we are deprived of oxygen. Birds, on the other hand, need to be more efficient, especially when doing things like flying, and so have evolved non-inflatable lungs with nine air sacs that allow for unidirectional airflow. It requires birds to take two breaths to have a complete respiration cycle, but thanks to this unidirectional flow, birds basically never need to “catch their breath.”

If you want a better explanation, I highly recommend watching a video on YouTube or looking up diagrams, but you can observe this cool avian adaptation just by listening to birds sing. Head out into the woods and listen to the long-winded complex songs of a winter wren – surely not possible to sing in one breath  – or find an open field and listen for brown thrashers mimicking up to 1,000 different songs in a single day.

A browntail moth caterpillar infestation is shown in a chokecherry tree in Brunswick in May. Shawn Patrick Ouellette/Staff Photographer

BIRDS AND BROWNTAILS

The browntail moth is a non-native pest that has become infamous to most Mainers. The adults, with white wings and namesake brown tail, are flying around right now seeking mates and laying eggs, often showing up under porch lights by the dozens if not hundreds. The larvae (caterpillars) are perhaps the better known life stage of browntails, as the hairs over their bodies are known to cause skin irritation and respiratory issues in humans. I’ve written before about the need to properly identify these caterpillars – as their native cousins are often improperly persecuted (read: blowtorched) – but they’ve shown back up in my inbox thanks to a question from Pamela Hargest, from the UMaine Cooperative Extension in Falmouth, asking which animals eat the browntail moth caterpillars. Some may surprise you!

Perhaps the best known hairy-caterpillar eaters are cuckoos. In Maine we have black-billed, and, less commonly, yellow-billed cuckoos that live in deciduous forests, which often contain the same trees that browntail moth caterpillars like eating. Cuckoos will consume large quantities of caterpillars, including our native Eastern tent caterpillars and fall webworms, which, like the browntail, also have spiny hairs over their bodies. The hairs can pierce the stomach lining of the cuckoos and become so dense that once consumed, they can interfere with the bird’s digestion. At this stage the cuckoos have an amazing ability to shed their stomach linings, regurgitating the hairs in a pellet. So instead of blowtorching the next tent caterpillar nest you see, give the cuckoos a chance to clear them out instead.

Black-billed cuckoo. Photo courtesy of Maine Audubon

Blue jays are another oft-reported consumer of browntail moth larvae. A frequently cited source, with a delightful title, is the 1915 U.S. Department of Agriculture Farmers’ Bulletin “Some Common Birds Useful to the Farmer” which covers about 26 bird species and their “usefulness.” In the account on blue jays, it is stated that “Both laboratory investigations and field observations have established the fact that in winter the eggs of the tent caterpillar and the hibernating larvae of the brown-tail moth in New England are eagerly sought.” The bulletin credits blue jays, as “rendering (their) best services to man” in their summer appetite for grasshoppers, along with that late winter taste for browntails.

It seems likely that blue jays are more interested in the early browntail instars, which are smaller, so perhaps the toxic hairs are smaller and easier to digest. The larvae grow through each instar stage, of which there are six to eight, before they pupate. Other birds will reportedly take advantage of these early instars, as Edward Forbush listed in his 1913 publication “Useful Birds and their Protection.”

By watching trees, mostly pears, throughout the spring that had browntail (and other “pests”) on them, Forbush kept note of the different species of birds that visited and if they consumed browntails or not, and if so, how many. In summary, he observed a surprising variety, including: red-winged blackbirds, “Crow Blackbirds,” red-eyed and yellow-throated vireos, black-capped chickadees, Baltimore orioles, American robins, rose-breasted grosbeak, house sparrows, chestnut-sided, yellow and black-and-white warblers. Forbush noted that many of these were able to consume browntail caterpillars even when their detachable hairs were dangerous by shaking or rubbing the hairs off using a branch, as you commonly see vireos do before eating.

It’s nice to know many species are able to eat these non-native pests, but the list of species that benefit from native caterpillars is exponentially longer. For each species Forbush noted eating browntails, there are a handful more that he saw feeding on tent caterpillars. Keep that in mind as you are planting in your backyards: support our birds with native plants, which are feeding native insects, and help the next generation flourish.

Have you got a nature question of your own? Email questions to ask@maineaudubon.org and visit www.maineaudubon.org to learn more about birding, native plants, and programs and events focusing on Maine wildlife and habitat. Doug and other naturalists lead free bird walks on Thursday mornings, 7 to 9 am, at the Gilsland Farm Audubon Sanctuary in Falmouth.