Prealternate molt in nuthatches

Nuthatches of the genus Sitta constitute one of the passerine groups in which prealternate (prenuptial) molt is generally believed not to occur. Dwight (1900: 298) discussed this genus with the family Paridae and stated: "All the members of this family have only the annual moult. They are peculiar in assuming a juvenal plumage closely resembling the pale nuptial plumage which results from extreme fading of the winter dress. " Witherby (in Witherby et al. 1938: 244) remarked that in S. europaea the summer plumage is attained by abrasion rather than by molt. Following Dwight's analysis, Bent (1948) recorded that the prenuptial molt did not occur in S. carolinensis or S. canadensis. Although Norris (1958: 246) found several females of S. pygmaea that were replacing feathers in the nuchal region in March and April, he regarded this as a regrowth of feathers lost in social interactions rather than as prealternate molt; the replacement of contour feathers "over nearly every part of the body" in early April by one female of that species was considered to be "an aberrant instance of prenuptial molt." Norris (op. cit.) stated categorically that "There is no prenuptial molt in Sitta pusilla." For most species in the genus, however, I have been unable to find any statement, even a negative one, concerning the occurrence of this molt. On the other hand, there is a partial prealternate molt in the seasonally dimorphic Tichodroma muraria, usually considered a member of the family Sittidae but in a different subfamily. While studying Red-breasted Nuthatches, S. canadensis, for other reasons (Banks 1970), I noticed a spring-taken female that was replacing several ventral feathers. Examination of additional specimens revealed that a partial prealternate molt is of common occurrence in that species. To determine the occurrence, extent, and timing of the molt in the genus Sitta, I subsequently examined most of the seasonally appropriate specimens of nuthatches in the U.S. National Museum of Natural History and the American Museum of Natural History. Through the courtesy of George Watson, I was able to examine specimens from the Yale Peabody Museum that he had collected in the Mediterranean region. Additional specimens were studied at the Musee Royal d'Historie Naturelle in Brussels. A combination of several problems necessarily makes a survey such as this less complete than is desirable (Lanyon 1975). Relatively few persons collect extensively in the months just prior to the onset of breeding, so that population samples are small in the months when prealternate molt might be expected. For some species the material available is too scanty to permit any conclusions to be drawn. Pin feathers are often lost as a specimen is prepared, and if only a few feathers are involved in a partial molt, as seems often to be true in nuthatches, this loss may obscure the occurrence of the molt. Lack of precise information on the time of breeding and of the annual prebasic molt in a species renders it impossible to look specifically for prealternate molt; this situation occurs particularly where there is reason to believe that the onset of breeding is determined by factors other than those that operate in the temperate Palearctic regions. In general, I examined birds taken in the months of February through May for evidence of prealternate molt. This selection of dates proved inadequate for some nuthatch populations of southern Asia and the Philippine Islands, where the nesting season differs from that of nuthatches in temperate areas. Where prealternate molt occurs in a very small proportion of a population, and where it may involve only a few feathers in the molting individuals, it is difficult to distinguish between a true molt and the replacement of feathers fortuitously lost. Some errors of judgement may have resulted from this difficulty, particularly in species represented by small samples. For the reasons mentioned, and various combinations thereof, the fact that I did not find evidence of prealternate molt in certain species or populations of nuthatches should not be taken as a definitive statement that no such molt occurs. It is hoped that the preliminary results reported here will stimulate study by those who have access to appropriate material. The arrangement of the species in the accounts that follow is not meant to be phylogenetic or to imply any concept of relationship. For convenience and because the information is more complete, North American species are listed first. Not all forms in the genus were examined. Species listed by Greenway (1967) but not in this paper are considered to be races of species discussed. Sitta canadensis. -Prealternate molt in the Red-breasted Nuthatch varies from slight (perhaps none in some individuals) to extensive. Most feather replacement occurs on the ventral surface, with the malar