Ptilochronology: Feather Growth Bars as Indicators of Nutritional Status

-Ptilochronology is the study of the growth rates of feathers by the measuring of growth bars. Growth bars are cross-bands on feathers that denote 24-h periods of growth. If a rectrix is plucked from a bird that is released, and then recaptured more than a month later, the width of the growth bars on the replacement (or induced) rectrix can provide a day-by-day record of the nutritional regime under which the bird had lived. Growth rate of induced rectrices varies among ages, sexes, and species of birds. I measured growth bars on feathers induced in Downy Woodpeckers (Picoides pubescens) that wintered in Ohio woodlots without supplemental food. On a daily basis, females grew feathers significantly more slowly than males. In other woodlots, where the woodpeckers were given supplementary food, daily feather growth did not differ between sexes. I concluded that male Downy Woodpeckers normally have a better nutritional status than females during the winter. A difference in nutritional status may explain why male Downy Woodpeckers use their social dominance to exclude females from parts of the species niche during winter. Ptilochronology could permit new insights into the nutritional ecology of free-ranging birds. Hypotheses that predict even minor variation in the nutritional status of birds should become accessible to testing. Received 5 April 1988, accepted 7 October 1988. BIRDS molt their plumage at yearly or more frequent intervals (Mathew and Naik 1986, Rohwer 1986). When birds lose feathers, however, replacements are promptly grown (HallCraggs and Evans 1985, Payne 1972). Selection apparently favors the expenditure of energy and specific nutrients (sensu King and Murphy 1985) to maintain a complete complement of body and flight feathers. However, feather growth can be reduced to compensate (sensu King and Murphy 1985) for the nutritional demands of other activities. Molting periods tend not to overlap with either breeding or migration. In some species, the molting sequence brackets reproductive episodes (Mathew and Naik 1986) or migratory periods (Stresemann 1967). Replacement of feathers is slowed or deferred during times when feather production would compete for limited resources. For several weeks after a rectrix is lost (either during molt or mechanically) or is plucked by an experimenter, a new rectrix grows from that follicle if the intake of energy and specific nutrients is adequate for feather formation. A feather grows from the collar zone of the papilla (Watson 1963, Lucas and Stettenheim 1978) to produce "growth bars" on the feather oriented roughly perpendicular to the rachis. Growth bars are alternating darkerand lighter-appearing bands (Riddle 1908). Darker bands are apparently derived from material laid down during the day and lighter bands consist of material laid down during the night (Wood 1950). Wood (1950) suggested that the appearance of these bands is due to differences in optical properties brought about by differential pigment incorporation into the keratin matrix of the feather during the day and night. However, darkerand lighter-appearing bands are found on white feathers (pers. obs.) and white feathers apparently do not contain pigments of any kind. Whatever their cause, each pair of bands constitutes a 24-h period of feather growth (Michener and Michener 1938). The breadth of a growth bar along the axis of the feather seems to be related to the bird's nutritional status. During a 24-h period if a bird is stressed nutritionally, it produces a growth bar deficient in barbules. This growth stage has a "fault bar" noticeable to the unaided eye (Newton 1968). Fault bars are observed commonly in the rectrices of young raptorial birds with sporadically insufficient food supplies during feather growth (J. A. Smallwood pers. comm.). Under more severe nutritional conditions, there may be insufficient energy or specific nutrients available even for adequate deposition of keratin in the central rachis and the entire feather may break off where the rachis is "pinched" (Riddle 1908). 314 The Auk 106: 314-320. April 1989 This content downloaded from 157.55.39.78 on Sat, 25 Mar 2017 22:29:23 UTC All use subject to http://about.jstor.org/terms April 1989] Ptilochronology 315 If one plucks a rectrix and releases the bird, then recatches the same individual more than a month later and plucks the newly grown replacement feather, that replacement (or induced) feather can provide a day-by-day record of the nutritional regime of the bird during the previous weeks. Individual growth bars (one dark band plus one light band) can be counted and measured to establish the number of days taken to grow the feather and which days were more nutritionally constrained than others. Correlations can be sought between daily increments of feather growth and mean daily temperature, wind velocity, day length, and other factors. Matching induced rectrices from two birds that had feathers plucked at the same time and were then released in the same habitat could be used to determine the relationship of species, sex, age, dominance status, kinship, and other factors to nutritional status. The induced feather is retained until the next molt. In the case of most temperateand borealzone birds, rectrices are replaced annually during the pre-basic molt of late summer and autumn (Svensson 1975, Pettingill 1985). A bird plucked in late autumn will grow the induced feather and will carry this record of nutritional status to be collected any time during the next 6-8 months. I use Ptilochronology (literally, the study of feather time) to define the study of the growth rates of feathers. Ptilochronology focuses on the daily growth bars of feathers from much the same perspective that dendrochronology focuses on the annual growth rings of trees (Fritts 1976). The potential of ptilochronology as an experimental method stems from its ability to detect variation in the rate of experimentally induced feather growth. Artificial diets below the maintenance requirements of Whitecrowned Sparrows (Zonotrichia leucophrys) molting in captivity reduced the rate of feather growth (Murphy and King 1987). Because each growth bar delimits one day's feather growth (Michener and Michener 1938), it seems logical to conclude that the captive sparrows on an inadequate diet must have grown feathers with narrower than normal growth bars. Nevertheless, controlled experiments are needed to verify the quantitative relationship between inadequate diets and the width of growth bars on induced feathers. Ptilochronology uses the growth rate of feathers to assess the nutritional status of birds. The extent to which feather growth in nature falls below that of birds living in optimal, artificial conditions could be used as an index of nutritional status. Furthermore, comparisons of feather growth could be used to indicate the probability of starvation. King and Murphy (1985) lump energy (in the form of stored lipids), protein, minerals, and vitamins under the term, nutrient. It seems probable, however, that feather growth is constrained primarily, if not entirely, by the amount of available energy. During the winter of 1987-1988, I tested the hypothesis that the ultimate cause of sex-specific foraging in Downy Woodpeckers (Picoides pubescens) is the increase in nutritional status to be gained by foraging in the males' microhabitat. Sex-specific foraging niches have been found in reptiles (Schoener 1974) and mammals (Bartholomew 1970, Krebs and Davies 1981), and are particularly well-known in woodpeckers (Selander 1966). In Downy Woodpeckers, both and males and females wintering in deciduous woodland changed their foraging behavior in response to ambient weather conditions (Grubb 1975, 1977, 1978), but males generally foraged higher in the forest and on smaller diameter branches than did females. When tested individually in a laboratory aviary, however, females preferred smaller diameter branches than did males (Pierce and Grubb 1981). Finally, when all males were removed from a woodlot, the females foraged on branches of smaller diameter and greater height above the ground than did females in a control woodlot. Reciprocally, males in the absence of females foraged like control males (Peters and Grubb 1983). These studies support the hypothesis that sexspecific foraging niches in Downy Woodpeckers are caused by female avoidance of the foraging microhabitat of socially dominant males. I predicted that under normal circumstances the growth bars on induced feathers would be wider in males than in females, but that the difference should lessen or disappear if woodpeckers were provided with supplemental food.