A Quantitative Study of the Foraging Ecology of Downy Woodpeckers
TL;DR: Both male and female Downy Woodpeckers use sub—surface foraging techniques to a greater extent during the winter and superficial techniques during the warmer months, and greater use of dead trees during winter is also indicated.
Abstract: Studies of the foraging ecology of Downy Woodpeckers (Dendrocopos pubescens) at the University of Kansas Natural History Reservation indicate that there is intersexual partitioning of the foraging niche, seasonal variation in the relative frequency of the modes of foraging, variation in the mode of foraging on live versus dead trees, and seasonal variation in the use of live and dead trees. Partitioning of the foraging niches by the sexes is accomplished behaviorally by a differential use of the available substratum according to limb height and diameter. The degree to which these unisexual subniches are expressed varies on live versus dead trees. Males tend to forage on small branches, generally 5 cm in diameter or less; females tend to forage on the trunk and larger limbs. The mean foraging height of males in live trees (6.0 m) is significantly different from that of males in dead trees (8.9 m); the mean foraging height of females in dead trees (8.4 m) is not significantly different from that of females in live trees (8.1 m) or of males in dead trees. The mean foraging height of males in live trees is significantly different from that of females in live trees. A similar relationship exists among the heights of the trees in which Downy Woodpeckers forage. Both male and female Downy Woodpeckers use sub—surface foraging techniques to a greater extent during the winter and superficial techniques during the warmer months. Greater use of dead trees during winter is also indicated. Dead American elms (Ulmus americana) are used as foraging sites by both sexes to a greater extent than expected by chance. Some other tree species seem to be favored and some avoided by one or both sexes.
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TL;DR: If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual sizeDimorphism in many other animal species may well also have been influenced by ecological factors, and it may be premature of dismiss this hypothesis.
Abstract: Can sexual dimorphism evolve because of ecological differences between the sexes? Although several examples of this phenomenon are well known from studies on birds, the idea has often been dismissed as lacking general applicability. This dismissal does not stem from contradictory data so much as from the difficulties inherent in testing the hypothesis, and its apparent lack of parsimony, in comparison to the alternative explanation of sexual selection. The only unequivocal evidence for the evolution of sexual dimorphism through intersexual niche partitioning would be disproportionate dimorphism in trophic structures (e.g., mouthparts). This criterion offers a minimum estimate of the importance of ecological causes for dimorphism, because it may fail to identify most cases. A review of published literature reveals examples of sexually dimorphic trophic structures in most animal phyla. Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it.
1,312 citations
Cites background from "A Quantitative Study of the Foragin..."
...…bill and tongue are excavates 1976, 1977a, 1978 longer, tail shorter nest cavity Kilham, 1970, 1978 Ligon, 1968 Short, 1970 Peters & Grubb, 1983 Jackson, 1970, 1971 Williams, 1980 Askins, 1983 Order PASSERIFORMES Male dioch Male eats Ward, 1965 (Quelea) has large seeds larger bill Male monarch…...
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TL;DR: A long-term approach to optimal foraging strategy in response to a regularly varying environment is presented, with parameters and assumptions chosen to match qualitatively the food availability and time-energy demands of actual animals.
Abstract: A long-term approach to optimal foraging strategy in response to a regularly varying environment is presented. A dynamically optimal animal uses this regularity to produce the best strategy vis-a-vis a single long-range objective. (No long-range planning ability need be postulated, however. There need only be selection for behaving via environmental cues as if planning occurred.) This dynamic optimization approach is applied to a model of the African weaver bird Quelea quelea (documented by Ward 1965a, 1965b, 1971). This model includes the effect upon the animal's condition of past and present behavior and variations (with time) in environmental and foraging characteristics, but contains a number of simplifications: (1) considering food availability only in terms of calories, ignoring other differences between foods; (2) ignoring the distinction between components (e.g., skeleton, fat) of body weight; (3) assuming that Quelea's sole control over its food energy input is its daily feeding time, which impli...
181 citations
Cites background from "A Quantitative Study of the Foragin..."
...are formulated mathematically by Katz (1974). The following exemplify effects that can be included in a dynamic optimization model, as constraints and/or goals to optimize....
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TL;DR: As downy woodpeckers spend less time on vigilance, they devote more time to foraging, thereby increasing their foraging efficiency.
172 citations
TL;DR: A simple model based on certain bioenergetic phenomena accounts for seasonal patterns of sexual dimorphism, size related changes in the timing of reproduction and geographic variation in body size of Darwin's finches.
Abstract: A simple model based on certain bioenergetic phenomena accounts for seasonal patterns of sexual dimorphism, size related changes in the timing of reproduction and geographic variation in body size of Darwin's finches.
155 citations
TL;DR: The metabolic rates of birds are inversely correlated with ambient temperature, but the metabolic cost of low temperature is moderated by artificial insolation, and air movement is added and derived theoretical "climate".
Abstract: Terrestrial organisms are surrounded by a microclimate composed of four independent variables: humidity, radiation, wind velocity, and air temperature (Porter and Gates 1969). The metabolic responses of animals to some of these climatic variables have been studied in the laboratory. For instance, below the lower critical temperature (about 250C; Helms 1968) metabolic rates of birds are inversely correlated with ambient temperature (Kendeigh 1949, 1969, Steen 1958, Brooks 1968, Helms 1968, Kontogiannis 1968), but the metabolic cost of low temperature is moderated by artificial insolation (Lustick 1969, Lustick et al. 1970). Porter and Gates (1969) added air movement and derived theoretical "climate
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TL;DR: Evidence of an adaptive function of sexual dimorphism in size in woodpeckers is presented by relating degrees of morphologicalDimorphism and sexual divergence in foraging behavior in two melanerpine species, the stronglyDimorphic Hispaniolan Woodpecker of Haiti and the Dominican Republic and the moderately dimorphic Golden-fronted Woodpeker of continental North and Central America.
Abstract: Adaptive radiation has been defined as the evolutionary divergence of members of a phyletic line into different niches or adaptive zones (Mayr, 1963:633). Although it has been customary to think of adaptive radiation solely in terms of species or races, a growing body of evidence indicates that some degree of radiation occurs also within populations, as individuals come to occupy different subniches or adaptive subzones, subdividing and, perhaps, expanding the total niche or zone utilized by the population. Probably all species show some degree of ecological variation, either polymorphic or continuous. But this phenomenon is being studied in only a few groups of organisms, notably in Drosophila, in which chromosomal polymorphism has been interpreted as a. means of adaptation of populations to heterogeneous environments (Dobzhansky, * 1961, 1963, 1965). Theoretical bases for research on ecological variation in animal populations have been provided by Ludwig (1950), Levene (1953)) da Cunha and Dobzhansky (1954), Dempster (1955), Li (1955), Carson (1959), and Levins (1962, 1963). In birds, as in other vertebrates, the sexes usually differ in size if not also in proportions of body parts, including those used in feeding (Amadon, 1959) ; and, especially where the degree of sexual dimorphism, which is a form of polymorphism (Ford, 1961: 12), is marked, it seems probable that the morphological divergence has ecological significance in adapting the sexes to different subniches. However, there is only an occasional reference in the literature to sexual dimorphism in relation to niche utilization (e.g., Pitelka, 1950; Rand, 19.52), and, in general, the whole problem of ecological variation in populations has been neglected by vertebrate ecologists. The primary purpose of this report is to present evidence of an adaptive function of sexual dimorphism in size in woodpeckers by relating degrees of morphological dimorphism and sexual divergence in foraging behavior in two melanerpine species, the strongly dimorphic Hispaniolan Woodpecker (Centurus striatus) of Haiti and the Dominican Republic and the moderately dimorphic Golden-fronted Woodpecker (Ce&zmus awifrons) of continental North and Central America. In addition, the paper surveys other evidence that sexual dimorphism in birds is related to differential niche utilization. Finally, some evolutionary aspects of sexual dimorphism and ecological variation are considered.
789 citations
TL;DR: Although the females of all three species average larger than the males, the sexual difference in size was greatest in the smallest species, the Sharp-shinned Hawk, and least in the largestspecies, the Goshawk.
Abstract: THE three widespread North American bird hawks, the Sharp-shinned Hawk (Accipiter striatus velox), Cooper's Hawk (A. cooperii), and the Goshawk (A. gentilis atricapillus) differ greatly in size. In the course of studies on weight, wing area, and skeletal proportions of these three species (Storer, 1955), it became apparent that although the females of all three species average larger than the males, the sexual difference in size was greatest in the smallest species, the Sharp-shinned Hawk, and least in the largest species, the Goshawk. To determine more precisely the degree of sexual dimorphism in these three species, I measured the wing length (arc) of study skins in the collection of The University of Michigan Museum of Zoology. The series used were large enough to provide both a good estimate of variation within the species and an accurate mean. All birds measured were in adult plumage and were collected in the region between Grafton, in extreme eastern North Dakota, and Point Pelee, Ontario, Canada. The sample of the Sharp-shinned Hawk consisted entirely of birds taken in Michigan; those of the other species included birds from most of the broader area. Mis-sexed accipiters, especially Goshawks, appear to be not infrequent in collections, and I strongly suspect that a few mis-sexed specimens have led Friedmann (1950: 150-152) and possibly others to describe the variation both in measurements and plumage as overlapping more than actually will be found to be the case. Much of the mis-sexing probably results from the collectors' mistaking the paired ovaries (usual in birds of this genus) for testes. In our collections, four Goshawks sexed as males by the collectors measure 348, 357, 360, and 361 mm in wing length. All were taken in the winter months (December to February) when the gonads are small, and all have the heavy black streaking on the under parts, which is usually found in adult females. A fifth Goshawk, sexed as a female, measures 324 mm in wing length, has the fine barring characteristic of adult males, and lacks heavy black streaking. These five birds were omitted from the samples, and even so the coefficient of variation is over 10 per cent larger for both sex groups of the Goshawk thlan for those of the other species (Table 1). A Cooper's Hawk sexed as a male but measuring 261 mm in wing length and having the brownish plumage of the upper parts characteristic of adult females was also omitted from the data. The measurement data are summarized in Tables 1 and 2. Judging froim the difference between the means for males and for females and from
295 citations
TL;DR: The Red-cockaded Woodpecker (D. borealis) and the Arizona woodpecker(D. arizonae) differ greatly in the degree of sexual dimorphism of body size and bill length.
Abstract: Two recent studies on woodpeckers (Kilham, 1965; Selander, 1966) support the hypothesis of Rand (1952), which states that within a species sexual dimorphism may aid in reduction of competition for food. In an extensive discussion of intraspecific differences in foraging, Selander (1966: 143-145) diagrams the solutions open to birds faced with decreased availability of food, and Kilham (1965) discusses possible causes, other than those directly related to food supply, for the evolution of traits that reduce intersexual competition for foraging sites. Neither author discusses in detail the problem of origin of stereotyped behavioral differences between the sexes. Sexual differences in foraging behavior of two species of Dendrocopos, the Red-cockaded Woodpecker (D. borealis) and the Arizona woodpecker (D. arizonae), are described in this report, and a discussion of the possible origin of this phenomenon in woodpeckers is presented. These two species differ greatly in the degree of sexual dimorphism of body size and bill length, to which much importance has been attached (Selander and Giller, 1963; Selander, 1965, 1966). The Red-cockaded Woodpecker inhabits open pine forests of the southeastern United States. It is found only in areas where pines predominate and is probably most common on the Atlantic coastal plain. The range of this species extends northward to Virginia and Kentucky, and westward to Oklahoma and Texas (A.O.U., 1957). The range of the Arizona Woodpecker extends from extreme southern Arizona and New Mexico, where it is associated with oak woodland, south into Michoacan (Davis, 1965: 537).
113 citations