TWO IN A BUSH
by Julie Amato
On the first day of spring, I went for a walk at the Stanford Arboretum. Stepping onto the path, I saw a pair of House Finches exploring the nooks and crannies made by strips of bark curling off a eucalyptus. The birds investigated the tree separately, but remained close to each other at all times.
Female House Finch, inspecting a eucalyptus tree at the Stanford Arboretum
As I wandered through the grounds, I saw a pair of American Robins foraging in the deep shadow of trees, and two California Towhees hidden in a tangle of branches, waiting for a dog to pass. A couple of Dark-eyed Juncos shuffled through the dirt under periwinkle vines, side by side, and another two juncos flitted through the trees and gathered dried grasses together. Everywhere I looked, I saw birds in pairs.
Male House Finch in the same eucalyptus where the female was spotted
But what’s in a pair, exactly? And how long do pairs stay together? I came home and cracked open the books.
Let’s start here: what’s a pair bond?
Pairs of birds are usually described as having a “pair bond”.
A pair bond is a social bond between two birds of the same species, usually a male and a female, presumably for the purposes of mating and caring for their young. The duration of the bond varies - in most species, pair bonds only last for one breeding season (or less); in others, they last for more than one season, or even a lifetime.
Bonded pairs of birds often spend a lot of time near each other, especially during the breeding season (roughly late winter to midsummer). As you look out your window or stroll around your neighborhood in the coming month, keep your eye out for birds of the same species in twos, foraging close to each other, perching near each other in a tree or bush, and generally following one another, moving around the landscape together.
American Robins foraging together (look in the blurry distance to see the second bird!)
Sometimes, the pair will exchange contact calls, periodic sounds that tell the listening member of the pair that all is well. Some pairs will also engage in allopreening, the grooming of one bird by another bird (this often appears affectionate to me, and the bird being groomed seems to enjoy it!). Many species also have their own courtship behaviors which are believed to strengthen the pair bond. Pairs often share parental duties, like building and defending a nest, incubating eggs, and feeding and caring for offspring.
While pair bonds are presumed to increase reproductive success, not all birds form pairs. The female Anna’s Hummingbird, for example, builds her own nest, incubates her eggs, and raises her young on her own; the male and female are together only at copulation. Furthermore, not all pair bonds are between individuals of the opposite sex; among Laysan Albatrosses in Hawaii, a third of nests are cared for by two females.
What does the pair bond look like for different species?
Many of our common backyard birds form pairs, running the gamut from seasonal pairings to lifelong partnerships.
The Mourning Dove, the House Finch, and the Spotted Towhee form pairs that last for the breeding season, but not necessarily beyond that. Mourning Dove pairs stay close to each other and are often seen allopreening. The same birds may pair up again in future breeding seasons. House Finches may also stay together for multiple breeding seasons, or even year-round. Spotted Towhees pair up and stay close to each other during the breeding season, but don’t seem to maintain that bond otherwise.
Two American Crows, hanging out together in a street tree
The American Crow, on the other hand, is presumed to form long-term pair bonds, similar to other corvid species. Crow pairs are frequently observed allopreening. The birds live in family groups year-round, often of parents and their offspring, which suggests that the pair bond is ongoing and long lasting. The Western Bluebird also forms enduring pair bonds, and family groups here in California stay together throughout the year.
California Towhee pair, foraging close to one another
Three backyard bird species that are known to form lifelong pair bonds are the Oak Titmouse, the White-breasted Nuthatch, and the California Towhee. In each of these species, the pair bond is maintained year-round, and the members of the pair often stay close to one another. In fact, California Towhee pairs forage so close to each other that one bird often gets debris kicked into its face by the other!
Intriguingly, pair bonds are not well understood in some of our most common backyard species. The Dark-eyed Junco, for example, is observed in monogamous pairs during the breeding season, but whether these bonds last for longer isn’t known.
Dark-eyed Junco pair, foraging together
Katie LaBarbera, Senior Biologist at the San Francisco Bay Bird Observatory, told me that researchers haven’t studied the pair bond in the Oregon Junco, the resident subspecies found here in the Bay Area. She writes: “My gut instinct is that those pairs you see are male-female pairs either building the pair bond that will take them through breeding, looking for nesting sites, or perhaps trying each other out to see if they are a good fit. Most of them look like opposite-sex pairs and they generally act ‘affectionate’ (lots of quiet burbles, paying attention to each other, not flying far from the other when disturbed).”
Similarly, not much is known about pair bonds in the Nuttall’s Woodpecker, a species that is endemic to (only found in) California and Baja California. Nuttall’s Woodpeckers form pairs for at least the duration of the breeding season; pairs are known to spend a great deal of time together in the late winter. Furthermore, each member of the pair stays on the same territory year-round. Does this mean that the birds form long-term pair bonds?
I couldn’t find an answer to this question, but on a recent walk around my neighborhood, I spent some time watching the “woodpecker trees”. Located in a half-block stretch of a nearby street, these old deciduous trees are woodpecker magnets, and our local Nuttall’s are often found here.
Female Nuttall’s Woodpecker, working away at a tree in our local “woodpecker alley”
Standing under one of the trees, waiting quietly, I heard tapping, and looked up to find a female Nuttall’s working her way around the branches. Occasionally, she’d pick up her pace and make a wonderfully resonant drumming sound.
Male Nuttall’s Woodpecker, chipping at another tree close to where the female was
Scanning the area with my binoculars, I discovered a male Nuttall’s, red cap shining in the sun, chipping away at the bark of a nearby tree, his facial feathers disheveled, perhaps from the effort. Spotting him was a surprise - I had never before seen two Nuttall’s Woodpeckers in the same small area at the same time.
Are these birds a pair? I don’t know, but I’ll keep observing, and maybe, just maybe, I’ll figure it out. Welcome, spring!
What birds have you observed in pairs? Write to us at backyardbirds@scvas.org and share your stories!
References:
Birds of the World: S. M. Billerman, B. K. Keeney, P. G. Rodewald, and T. S. Schulenberg, Editors. Cornell Laboratory of Ornithology, Ithaca, NY: https://birdsoftheworld.org/bow/home
The Cornell Lab of Ornithology Handbook of Bird Biology, third edition: Irby J. Lovette and John W. Fitzpatrick, Editors. Published by John Wiley & Sons, 2016.
All photos in the article by Julie Amato
Banner photo: Western Bluebirds by Tom Grey