Sexual Dimorphism in nestling woodpeckers

Most woodpecker species are sexually dimorphic, meaning simply that the sexes look different. This dimorphism varies: in some species it is obvious, in other more subtle (and in a few species it hardly exists). In some cases, plumage dimorphism becomes evident as soon as chicks begin to acquire feathers and young woodpeckers can be sexed visually before they leave their cavity. Here is an example, a female Williamson's Sapsucker (Sphyrapicus thyroideus) nestling, just before fledging (Oregon, USA, July 2014). Adults of this species are very different visually, in fact they are one of the most sexually dimorphic woodpeckers (so much so that they were once thought to be different species) and this is the case with nestlings, too.