A recent discussion on the ID-Frontiers listserver involved an immature gull photographed in Utah (photo by Norman Jenson here). The consensus (and I agree) is that it is a Western Gull based on plumage and shape. But questions arise from the fact that it looks barely larger than the California Gulls next to it – abnormally small for a Western Gull.
Those of us who didn’t see the bird in life might like to ignore the apparent size as an illusion of the photographs, but the observers report that the bird really did look small. Can it still be a Western Gull? Yes. Since size is the only thing suggesting that it’s not a Western Gull, I think we have to go with the identification as a very small Western Gull. But is it a “dwarf”, or just the small extreme of normal variation?
Much of the discussion about this bird and other unusually small individuals has referred to them as “runts”, but technically that is the wrong term. A runt usually means a young animal, still growing (the smallest of a litter of puppies, for example), that is smaller than its siblings. This is common in birds, caused by poor health or poor nutrition, but if runts survive they can grow to full size indistinguishable from their nest-mates. Unusually small adult birds should be called “dwarfs”. ((A runt Common Tern studied by Coulter (1982) was the smallest in the nest, and seemed unlikely to survive, but eventually grew to normal size and fledged, albeit about ten days later than its nest-mates. On the other hand, continued poor nutrition results in birds that never reach full size and remain smaller than normal, as several studies on Snow and Canada Geese have shown.))
Peter Pyle reported on ID-Frontiers that gull expert Larry Spear held the opinion that there is no such thing as a dwarf bird, and that an individual like the Utah gull is just the rarely seen tail end of normal variation in Western Gull. In the same way that full-grown humans under five feet (or over seven feet) tall simply represent the extremes of normal variation.
In humans dwarfism is neither well-defined nor simple. Dwarfism is defined by an arbitrary threshold along the continuum of adult sizes, and over 200 causes of dwarfism have been identified (Wikipedia). It seems likely that birds are similar.
An informative study by Hicks (1934; thanks to Steve Mlodinow for the tip) carefully examined over 10,000 starlings in the hand. Unusually small and large birds that caught the researchers’ attention were measured. In this sample of 10,000 birds there were seven “giant” and six “dwarf” individuals that measured about 10% larger or smaller than the average of “normal” birds measured.
Unfortunately not all 10,000 birds were measured, only about 500 randomly selected “normal” birds were carefully measured, along with the 13 individuals that were strikingly large or small. Only total length measurements are given, but the largest dwarf measured only 9mm (about 5%) smaller than the smallest “normal” female. Furthermore, all of the giant birds were males (the larger sex), and five of the six dwarfs were females. It seems likely that, if all 10,000 individuals had been carefully measured, the data would fill in the relatively small gaps between the normal birds and the dwarf and giant birds.
Another documented case, with direct application to identification, is that of an unusually small Great Crested Flycatcher trapped and collected in New Jersey (Murray, 1971). This individual was immediately suspected of being an Ash-throated Flycatcher based on size, but careful study confirmed it to be a very small Great Crested. Its measurements are over 10% smaller than the average for Great Crested, and smaller than the minimum given by Pyle (1997). It is even a little too small for Ash-throated Flycatcher! In addition, it has a disproportionately short tail, while its wing measurement is just 1mm below the minimum for Ash-throated, its tail measures 9.5mm shorter than the smallest Ash-throated measured by Pyle.
It’s really a semantic question. Documented cases of unusually small birds exist and must be considered when identifying rare species. We could set an arbitrary threshold that categorizes them as “dwarfs” or just consider them the extremes of normal variation. The impact on bird identification is the same either way.
Coulter, M.C., 1982. Development of a Runt Common Tern Chick. Journal of Field Ornithology, 53(3), pp.276–279.
Hicks, L.E., 1934. Individual and sexual variations in the European Starling. Bird-Banding, 5(3), pp.103–118.
Murray, B.G., 1971. A Small Great Crested Flycatcher: A Problem in Identification. Bird-Banding, 42(2), pp.119–119.
Pyle, P. et al., 1997. Identification Guide to North American Birds: Columbidae to Ploceidae, Slate Creek Press.