Shorebird Dimorphism

Sexual dimorphism is a phenomenon exhibited by a wide variety of animal taxa, including birds. What this refers to is differences in morphology that divide species roughly along the lines of gender. The aspect of avian sexual dimorphism familiar to many of us is plumage colour dimorphism; in many species, the body feathers of male birds are more brightly coloured than in females. Sometimes the males also show elaborate ornamentation which the females lack. This is epitomized by members of the family Paradisaeidae (birds-of-paradise).

A lesser-known aspect of sexual dimorphism in birds is the difference in average bill length between males and females in many shorebird species. Though we tend to associate greater size with the male of the species, in this instance the female has the longer bill (on average).

Dunlin (♂)

Dunlin (♀)

The photographs above are a nice illustration of the difference in bill length between two Dunlin that Hannah & I observed yesterday at Blenheim Sewage Lagoon. In diagnosing these birds as male and female based on bill length, it is important to add the qualification that these differences are based on statistical averages and there is a degree of overlap. Nevertheless the difference in bill length in the two birds depicted above is acute enough that we can ascribe gender to each bird with a reasonable amount of confidence.