This is the season to see chicks. This is also the season to notice a rather odd behavioural difference between birds that show sexual dimorphism and those that don’t.
A species with sexual dimorphism exhibits different characteristics beyond the differences in their sexual organs. Indeed, the male and female often look markedly different. In a species with sexual monomorphism the male and female are difficult to distinguish.
Some birds are monomorphic; some are dimorphic. Among dimorphic species, it is usually the male that has the brighter and fancier plumage, but now and then it is the other way around (kingfisher). Sexual differences in appearance seem to have evolved as a result of the mate selection by females.
Curiously, the visual difference is accompanied by a behaviour difference in parenting:
The lesser the similarity in appearance, the lesser the involvement in breeding duties by the more colourful bird (usually, but not always, the male). The greater the similarity in appearance, the greater the equality in breeding duties.
I was reminded of this yesterday while watching an impressive group of Wood Duck chicks and someone in the group wondered why the chicks weren’t accompanied by both parents. The short answer is the Wood Duck is sexually dimorphic.
The first picture shows the female (left) and male (right) Wood Duck (appropriately) perched in a tree. They are strikingly different in appearance (March 28, 2016).
Consequently, during yesterday’s observation of thirteen Wood Duck chicks, we expect to see only the mother with them. (The most Wood Duck chicks I had previously seen were three.)
Similarly, the (sexually dimorphic) Common Mergansers seen five days ago only shows mommy with the chicks.
Contrastingly, Canada Geese parents look much the same, and both tend chicks (May 31, 2011).
The similarity of Osprey parents implies that both have parental duties (July 19, 2013).