Cooper Island, Alaska, July 22, 2009 — Black guillemots, like all seabirds, need both parents to incubate eggs and care for young. This mutual participation of males and females in raising young means the breeding success for an individual guillemot depends to a great extent on finding and maintaining a bond with a high quality mate that will share and excel in raising young to fledging.
The cooperation of both parents might be most critical during the current stage of breeding, when chicks are hatching and parents are transitioning from providing nearly continuous warmth (for the egg and young downy nestling) to providing nearly continuous fish (for the rapidly growing chick). During the three-day period the chick breaks out of its shell, parents attend the egg constantly, incubating and turning the egg while also communicating with the emerging chick. After hatching, parents need to both brood and feed the chicks for approximately six days, until they can maintain their own temperature. Parent birds have to do all this while maintaining their own foraging requirements.
Once an individual guillemot has successfully bred with another individual the benefits of retaining a mate are many, but the primary one is likely the demonstrated ability of the mate to share breeding responsibilities and successfully fledge young. That is why on Cooper Island over 95 percent of the breeding pairs remain the same from one year to another, if both members of the pair survived over the winter.
Examining annual survival and mate fidelity is an important part of my initial censoring of the colony in June. Nearly all of the breeding birds are banded with three color bands, allowing individual recognition and rather easy assessment of annual survival and mate fidelity. This year, for instance, orange and yellow-black –black are both back on site. On an adjacent site, yellow-gray-green returned to find that its mate from 2008, gray-yellow-gray did not return so it bred with orange-blue-orange.
Determining who is actually paired with whom can pose some problems, but guillemots frequently reinforce or demonstrate their pair-bond by mutual” head bobbing”. The members of the pair face each other and rapidly move their heads up and down, frequently while circling each other. Guillemot pairs “head bob” when another bird approaches them or when reuniting after a long or short period of separation. Luckily they apparently see me as another bird and when I approach a pair they typically will engage in head bobbing and let me know they are a pair.
When looking at large groups of birds there is another way to determine which are in a pair. While much of the flock might be actively engaged in aggressive displays or pursuing other birds any two birds sitting in close proximity and ignoring each other are almost certainly a stable pair.
After spending the summer at the colony, mainly interacting with guillemots, I have noticed that one could probably use a similar technique to determine pair-bond status in pairs of humans. If you inspect the diners in a restaurant it is not too hard to assess which couples might be having a business dinner (nearly constant but typically unanimated conversation) or a first date (constant, attentive and frequently animated conversation). Couples with long-term pair-bonds certainly will be talking with each other but frequently scan the room or allow periods of silence – hopefully with an expression of contentment.