“Don’t they make a lovely couple!”, is something we might say when two people start a new relationship, but when you see those two carrion crows hopping around your local green, is that a phrase that springs to mind? This week’s blog explores relationships among a few of our well-known species.
A number of UK birds form year round pair bonds and there are several reasons why. Mute swans, house sparrows, jackdaws, marsh tits, tawny owls and others stay together for life for reasons such as the need to share parental care and for sharing the maintenance of a territory. Even among short-lived birds, such as blue tits and blackbirds, it makes sense to stick with a partner with whom you have successfully bred previously. Nuthatch, feral pigeons and carrion crows usually remain together until death also, although separations amongst these species are known, especially with younger pairs. Seabirds such as puffins and fulmars also stay together for many consecutive years, which is remarkable as they only meet up each spring at their breeding site.
The loss of a partner even a long term one tends to produce action, not grief. If a female robin losses its mate it can be re-paired within hours. A territory holding male carrion crow may acquire a new mate within a couple of days, equally, it might also lose its territory before the day is out. Even mute swans are practical, a female may re-pair within a month. Birds bereaved mid-breeding will often attempt to carry on, though are rarely successful.
There are reasons why birds don’t mate for life also. The chosen partner may be of low quality to begin with, or its fitness fades. Birds go to many lengths to select a partner, checking out a potential mate’s physical makeup, its social status song and even its territory quality. These may also go up or down.
While a male and female will combine in a breeding attempt, it is also common for these same individuals to seek copulations outside the pair bond. This is especially frequent among coal tits and reed buntings, but occurs at low percentages within many species. Mallards, while monogamous, are famous for their forced copulations, and rooks do the same. It makes sense for males to increase their paternity, while for females it can be sensible to acquire some top quality DNA from the attractive mate next door.
Various species follow less conventional arrangements than monogamy. Male wrens, cetti’s warblers, pied flycatchers, corn buntings and starlings often pair up with several females in a season, often overlapping. These individuals may help feed the subsequent young. Female dotterels and red-necked phalaropes may mate with several males and then let each one get on with the childcare alone. Ruffs and black grouse have no pair bond at all, other than the physical act of copulation.