Partial albinism and natural selection in the hooded crow Corvus corone cornix

Abstract

We studied the frequency of partial albinism amongst hooded crows in Norway, mostly in a population at Trondheim. A total of 3461 birds were collected from the field. About 5% of fledged young showed albinoid markings on their third right‐hand primary; the frequency decreased to 2% among yearling birds and to only 1% among birds that had acquired their adult plumage. No significant difference was found in relation to sex. The partially albinoid birds were typically small‐sized, both as regards their bone structure (ulna, tibia, tarsus) and feathers (wing, tail). Their feathers bore more defective markings than those of birds of normal plumage coloration. A study of the plumage of preserved specimens of hooded crows in the collections of Natural History museums in Norway also indicated that a differential mortality takes place in the wild, such albinoid birds disappearing rapidly from the population. Experimental interchanges of eggs and hatchlings between nests indicated that the occurrence of partial albinism may be related to the feeding conditions during the nestling stage, rather than to any genetical differences. It is not known, however, whether the former was due to starvation, to an unbalanced diet, or to eating poisonous food.

Partial Albinism in a Semi-Isolated Population of Great Reed Warblers

“…In a study of hooded crows Corvus corone cornix, Slagsvold et al (SLAGSVOLD et al 1988) found that albinistic individuals were on average smaller than individuals of normal plumage coloration. Partially albinistic barn swallows Hirundo rustica have been found to exhibit lower between year survival than normally coloured individuals (ELLEGREN et al 1997).…”

“…Hence, by monitoring partial albinism in long term or geographically large scale studies, we might be able to identify populations exposed either to inbreeding or environmental stress. An advantage with the use of partial albinism as a stress indicator is that museum collections can be used as reference populations (SLAGSVOLD et al 1988;ELLEGREN et al 1997) to establish whether the observed level of albinism is atypical relative older samples or samples from other geographical regions.…”

“…It is believed to result from the expression of recessive alleles that offset the normal melanin pigmentation at feather development (SAGE 1962). Plumage albinism occurs infrequently but regularly also in natural populations (SAGE 1963;SLAGSVOLD et al 1988;ELLE-GREN et al 1997), often affecting just some feather tracts, a phenomenon called partial albinism. In natural populations, a few studies have reported that partial albinistic individuals are particularly frequent in small and isolated populations as compared to large continuous populations, both in birds (HOLYOAK 1978) and in mammals (PARSONS and BONDRUP-NTELSEN 1995).…”

Leucistic plumage as a result of progressive greying in a cryptic nocturnal bird

“…Morphological abnormalities are therefore infrequent in natural populations. Current evidence for the action of natural selection on morphological abnormalities is often indirect 5 , 6 in part because of the difficulty in obtaining large samples of rare phenotypic traits. Consequently, empirical data to test theoretical predictions and measure the magnitude and direction of predicted effects are virtually non-existent.…”

“…For simplicity, in this article, the term ‘leucism’ (from the Greek leukos , ‘white’) should be interpreted according to its original and all-embracing meaning, as denoting the absence of melanin in the integument, regardless of the underlying process 7 . Lastly, non-genetic (environmental) factors, such as injury 17 and nutritional constraints on melanization 5 may also cause loss of pigmentation. Many questions remain about the causes and biological significance of leucism, yet the relative ease of detection of this trait can facilitate data collection and provide useful information to understand the emergence and maintenance of colour aberrations in natural populations 18 .…”