Philip S. Humphrey

Can Natural History Museums Capture the Future

This is the publishers version, also available electronically from http://bioscience.oxfordjournals.org/content/50/7/611.

FLIGHTLESSNESS IN STEAMER-DUCKS (ANATIDAE: TACHYERES): ITS MORPHOLOGICAL BASES AND PROBABLE EVOLUTION

Flightlessness in Tachyeres is caused by wing‐loadings in excess of 2.5 g·cm–2, which result from the large body size and small wing areas of the flightless species. Reduced wing areas of flightless species are related to absolutely shorter remiges, and to relatively or absolutely shortened wing bones, although these reductions differ among species. Reduced lengths of the ulna, radius, and carpometacarpus are associated most strongly with flightlessness. Pectoral muscles and the associated sternal keel are well developed in all species of Tachyeres, largely because of the use of wings in “steaming,” an important locomotor behavior. Relative size of these muscles was greatest in largely flighted T. patachonicus; however, sexual dimorphism in wing‐loadings results in flightlessness in some males of this species. Proportions in the wing skeleton, intraspecific allometry, and limited data on growth indicate that the relatively short wing bones and remiges of flightless Tachyeres are produced developmentally by a delay in the growth of wing components, and that this heterochrony may underlie, in part, skeletal sexual dimorphism. Increased body size in flightless steamer‐ducks is advantageous in territorial defense of food resources and young, and perhaps diving in cold, turbulent water; reductions in wing area probably reflect refinements for wing‐assisted locomotion and combat. Flightlessness in steamer‐ducks is not related to relaxed predation pressure, but instead was permitted selectively by the year‐round habitability of the southern South American coasts. These conditions not only permitted the success of the three flightless species of Tachyeres, but at present may be moving marine populations of T. patachonicus toward flightlessness.

Sexual Dimorphism in Continental Steamer-Ducks

Analyse du dimorphisme sexuel chez les trois especes continentales (133 specimens, biometrie du squelette et des muscles, mensurations externes): Tachyeres pteneres, T. leucocephalus, T. patachonicus et recherche des causes de ce dimorphisme

Territoriality and interspecific aggression in steamer-ducks

indicates that Burrowing Owls modify prairie dog burrows used as nest sites. Presumably, sandy soil would facilitate enlarging burrow passageways. Coulombe (1971) stated that in California burrow diameters averaged 20 cm, and suggested that owls may modify burrows that have been abandoned by rodents. In addition, sandy soils drain rapidly, which would reduce nest flooding during frequent spring and summer rainstorms.

Genetic differentiation among steamer-ducks (Anatidae: Tachyeres): an electrophoretic analysis

Electrophoretic and isoelectric focusing analyses of liver proteins of the steamer-ducks, Tachyeres patachonicus, T. pteneres, T. brachypterus, and T. leucocephalus, how these species to be distinct genetically, with the latter three species being more closely related to one another than any one of them is to T. patachonicus. There is also significant differentiation among populations of T. patachonicus. Estimates of the average calculated heterozygosity per species are high: 0.185, 0.160,O. 165, and 0.084, respectively, and observed heterozygosities are 0.115 I 0.090, 0.202 f 0.118, 0.201 i0.085, and 0.080 f 0.069, respectively. The genetic distances of Cavalli-Sforza and Edwards, Nei, and Rogers were estimated and used with a Wagner tree algorithm to prepare a consensus tree based on 1,000 subsets of allelic frequency data prepared by bootstrapping over loci. Estimates of F,, for comparisons among populations of T. patachonicus and estimates of Nm based on the distribution of private alleles indicate that gene flow between some pairs of populations is significantly reduced. Values of F,, are much higher than those prev-iously found for other pairs of avian populations. Genotypic distributions that deviate significantly from equilibrium expectations are found for several of the polymorphic loci of one or more species.

Nest, Eggs, and Downy Young of the White-Headed Flightless Steamer-Duck

Nest, eggs, and downy young of the recently described White-headed Flightless Steamer-Duck (Tachyeres leucocephalus) are described and compared with those of other species of steamer-duck. Nests of T. leucocephalus, like those of other steamer-ducks, are shallow bowls lined heavily with down, and typically located in dense woody shrubs on peninsulas and off-shore islands. Egg size of T. leucocephalus is most similar to that of T. brachypterus, but eggs of all species of Tachyeres overlap substantially in their dimensions. Downy young of T. leucocephalus differ from those of congeners in eyelid and back color, and facial pattern. These characters are used to construct a key to the downy young of the four species of Tachyeres. RESUMEN. Los nidos, huevos, y pichones de Tachyeres leucocephalus estan descritos y comparados con los de las otras especies del genero. Los nidos de T. leucocephalus semejan cuencos pocos profundos forrados con plum6n y estan situados tipicamente abajo de arbustos ubicados en islotes y peninsulas. Los huevos de T. leucocephalus tienen un tamano mas semejante a los de T. brachypterus pero las dimensiones de los huevos de todas las especies de Tachyeres se superponen. Los pichones de T. leucocephalus se distinguen de otros del mismo gi nest attendance; deposition of down; diameter and depth of nest bowl; height and composition of cover; distances from nest to shore and to another nest; habitat type; clutch size; stage of incubation (Weller 1956); length and width of eggs. We also recorded the size and approximate ages of broods of T. leucocephalus encountered during field work. Age classes of broods and skin specimens were defined as follows: class Idowny young with no ju venal feathers visible; class IIyoung covered by a mixture of down and juvenal feathers; and class IIIyoung birds completely covered with feathers, no down visible. Our data for T. leucocephalus were supplemented by those collected by Boswall and Prytherch (1972), Boswall (1973), Boswall and Maclver (1979), and Daciuk (1976) at other localities in coastal Chubut. Eggs described in these papers were identified as T. patachonicus by their authors, but clearly pertain to T. leucocephalus based on a photograph of an incubating female in Daciuk (1976), distributional information, locomotor behavior, and, to a lesser extent, dimensions of the eggs. The data for T. leucocephalus were compared to published descriptions and our own information on nests and eggs of the Flying Steamer-Duck (T. patachonicus) and Magellanic Flightless Steamer-Duck (T. pteneres) and published descriptions of the Falkland Flightless Steamer-Duck (T. brachypterus).