By Tony Davison
As I’ve said before, it never ceases to amaze me how often I see many comparisons between bird, moth and butterfly. The following analysis is in no way intended to be scientific, it is simply my observations relating to to three examples of colour forms in bird, moth and butterfly, one in particular being dimorphism.
The Skuas are renowned for having dark, light and intermediate colour phases. In these plumages, identification can prove to be difficult. Glyn Sellors© has kindly supplied me with two images of Long-tailed Skua – Stercorarius longicaudus, to show a dark phase (juvenile) and a pale phase (adult).
In moths, one of the best examples of colour form is in the Peppered Moth – Biston betularia. These days the usual variety, especially in rural areas, is white “peppered” with black spots and speckling across the wings and body. The dark melanic colour form – carbonaria, is sooty black, with tiny white spots at the base of the forewing and was once very common being associated with industrial areas where there were high levels of pollution. With cleaner air controls in place and smokeless zones etc, the melanic form is now only present in small numbers throughout populations and is on the decline.
The Map Butterfly – Araschnia levana, is well noted for its seasonal dimorphism. The spring brood is predominantly orange-brown (form levana), whilst the second brood, that emerges in the summer months, is black (form prorsa). To the untrained eye, they can appear to be two completely different species.These colour forms are determined by the length of day in the larva stage and it is not clear what, if any, advantages this holds for the species. My thanks go to Barrie Staley© for providing the photo of the spring colour form taken in Poland during May 2007. The summer colour form is one of my photographs, taken during my recent trip to Bulgaria in July 2014.