BREEDING-SITE SELECTION AND COLONY FORMATION IN DOUBLE-CRESTED AND PELAGIC CORMORANTS
1986; Oxford University Press; Volume: 103; Issue: 1 Linguagem: Inglês
ISSN
1938-4254
AutoresDouglas Siegel-Causey, George L. Hunt,
Tópico(s)Animal Ecology and Behavior Studies
Resumo230 dean Venezuela, and specimens from the western te- puis of Territorio Amazonas are somewhat grayer than are specimens from the eastern tepuis of the Gran Sabana. The definitive female plumage in the two subspe- cies is similar to that of the second basic male plum- age of the same subspecies. Between the subspecies, the definitive male plumage of duncani is similar to, but still somewhat browner than, the definitive fe- male or second basic male plumage of homochroa. Similarly, the second basic plumage of duncani is more similar to the first basic plumage of homochroa. Thus, reduced stripedness and gray coloration probably represent the derived condition, and heavily striped brown plumage is probably the primitive condition. If this is true, then I would suggest that duncani, which inhabits the tops of the far older land mass of the tepuis, would have been the parental stock that “hopped” to and colonized the Andes after they were formed. A paler juvenile plumage and grayer defini- tive plumage was part of their adaptation to the drier paramo conditions. Males of C. h. areaphila, the isolated Santa Marta population, in juvenile and first basic (in second pre- basic molt) and three females in second basic plum- age are separable from the corresponding plumages of nominate homochroa in being somewhat paler brown. The type of oreophila was compared with the above three females by K. P. Parkes. It is distinctly grayer and probably represents the definitive basic plumage. In contrast to our knowledge of plumage se- quences for North American birds, one can gener- alize that such sequences are poorly known for neo- tropical species. The recognition of four sequential Short Communications [Auk, Vol. 103 plumages in male Hemithraupis flavicollis was so star- tling at the time that reviewers of Parkes and Hum- phrey's (1963) manuscript questioned the phenome- non (Parkes pers. com.). The recognition of four sequential plumages in male Icterus gularis and now in both sexes of Catamenia homochroa perhaps indicate that a higher frequency of complex plumages is to be found among neotropical species. I thank the curators of the following collections for the loan of material in their care: Carnegie Mu- seum of Natural History, Field Museum of Natural History, Louisiana State University Museum of lo- ology, Museum of Comparative Zoology, and the U.S. National Museum of Natural History. Mr. William H. Phelps, Ir., and Ramon Aveledo kindly hand car- ried the entire series of duncani from the Coleccion Phelps, Caracas to New York for this study. Kenneth C. Parkes reviewed the age plumage classes with me and made useful suggestions. The Photography De- partment at the American Museum of Natural His- tory prepared the illustration. LITERATURE CITED PARKE. K. C., & P. S. HUMPHREY. 1963. Geograph- ical variation and plumage sequence of the tan- ager Hemithraupis flavicollis in the Guianas and adjacent Brazil. Proc. Biol. Soc. Washington 76: 81-84. SMITH!-I, F. B. 1975. Naturalist’s color guide, part I. New York, Amer. Mus. Nat. Hist. . 1981. Naturalist’s color guide, part III. New York, Amer. Mus. Nat. Hist. Received 12 March 1985, accepted 26 August 1985. Breeding-site Selection and Colony Formation in Double-crested and Pelagic Connorants DOUGLAS SIEGEL-CAUSEY’ AND Gsoncs L. HUNT, JR.‘ ‘Museum of Natural History and Department of Systematics and Ecology, University of Kansas, Lawrence, Kansas 66045 USA, and ‘Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA The success of a breeding pair to raise eggs to fledged birds does not appear to be uniform over an entire colony; instead, there are certain regions (cen- tral area, steepest slopes, etc.) where the occupants have enhanced reproductive success (Coulson 1968, Nettleship 1972, Ryder 1980, Panov and Zykova 1982, Kharitonov 1983b). Fretwell and Lucas (1970) pro- posed a mechanism by which colonial birds could assess and choose optimal breeding sites from among a range of choices. A series of habitats are identified, each ranked by a unique suitability function that is a density-dependent measure of average reproduc- tive success. That is, the average success of breeding birds decreases through the effect of density-depen- dent factors such as disease, neighbor conflict, pred- ator awareness, etc. as the density of the habitat in- creases. At some density, the expected reproductive success in the optimal habitat will equal that expe- rienced in some lesser, unoccupied habitat. Incoming adults intent on breeding should then colonize both habitats at an equivalent rate, rather than saturate the optimal habitat. Where breeding must be initiated within a narrow time frame, or nest sites are changed at a loss of the previously invested time and effort,
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