Hummingbirds are promiscuous. Chickadees cheat. Barn swallows sneak their eggs into their neighbor’s nests. As this column disclosed last week, it’s a sexual jungle out there.
Or, in some cases, it’s a hippie commune.
Lately, I’ve been reacquainting myself with the Bicknell’s thrush. This species nests only on mountain slopes in the stunted spruce zone – a zone also called krummholz, which means twisted wood in German. Due to severe weather at higher elevations, growth in spruce trees and balsam firs is stunted. Vegetation is thick and impenetrable. In Maine, that zone is usually at about 3,000 feet. In Cape Breton, Nova Scotia, at the northern end of the thrush’s range, the zone is at a much lower altitude.
I survey these and other alpine birds as a volunteer for the Mountain Birdwatch project, conducted annually by the Vermont Center for Ecostudies in four northeastern states and three Canadian provinces. This study is timely, not only because mountaintops are increasingly eyed for wind power development, but also because deforestation on mountains in the tropics is threatening the existence of some species, including the Bicknell’s thrush. Naturally, because it’s a hard bird to find, I also get called upon to guide folks to see it on occasion.
So I’ve found myself on slopes near Kingfield several times recently, and I’ve seen for myself an unusual mating system. The Bicknell’s thrush uses a breeding strategy called polygynandry. Up to four males hook up with one female, and all share the parenting duties. Each of the eggs in the nest may have a different father. It works both ways: males may have eggs in the nests of more than one female. Males don’t hold strict territories and multiple males may sing from the same area. However, since they are not strongly territorial, they sing little, mostly at dawn and dusk. They utter call notes much more often, letting each other know where they are. However, despite a willingness to share females, I can attest that males are still wary of interlopers who are not part of their cooperative arrangement.
There is a scientific basis for this odd breeding scheme. The krummholz zone may be relatively safe from predators, but it’s a hard place to find food and raise a family. Natural selection favors cooperation over competition. This thrush has many relatives in Maine, including hermit, Swainson’s, and wood thrushes.
None of these other thrushes engage in wife-swapping.
Polygynandry is rare among mammals, but it turns out that it’s a common practice with our closest living relative. Bonobos are great apes in the Congo that are closely related to chimpanzees. Sexual practices and relationships play a big role in bonobo “culture” and any female may be approached and courted by any male, except for her adult sons. Even in bonobo culture, that’s just wrong.
Polygyny occurs when one male forms a mated relationship with multiple females. This practice is documented in about 2% of bird species. Usually it occurs in areas where food and nesting resources are unevenly distributed and a dominant male is able to hold the better territories against other males. In Maine, you can see this behavior in red-winged blackbirds and spruce grouse. Generally, this breeding strategy only works if there is enough food to go around and the fledglings mature fast enough to forage for themselves. Males spend a lot of energy in courtship and defending the territory, but they take little role in raising the family.
One breeding strategy features a complete role reversal. Polyandry occurs when a female establishes a territory and courts multiple males. After she lays the eggs, the fathers are responsible for bringing up the family while she goes off in search of new lovers. Other than laying the eggs, the female has no further responsibility. In Maine, spotted sandpipers engage in this practice. Females of the species are noticeably larger than males. Phalaropes, also polyandric, don’t breed in Maine but they do migrate through our ocean waters in late summer. In the subarctic regions where they breed, the females are brightly colored; the males are drab. Courtship is initiated by the lady, in a love ‘em and leave ‘em sorta way.
I’m guessing by now you have figured out that all of these fancy words are derived from the Greek language in which poly means many. When ascribing names to these sexual couplings, perhaps Latin just didn’t contain words lurid enough to satisfy scientists.
Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at www.mainebirdingtrail.com. Bob can be reached at email@example.com.