Factors Affecting Incubation Rhythms of Northern Shovelers

Abstract

-Nesting behavior of wild Northern Shovelers (Anas clypeata) was studied in 1974 and 1975 near Delta, Manitoba, Canada. Laying and rhythms are described and adaptive aspects of the rhythm discussed. Factors affecting total time spent off the nest per day, recess frequency, and recess duration are investigated with simple correlation and multiple regression analysis. The data support my prediction that in smaller individuals, environmental factors increasingly affect anatid rhythms. Small size and concomitant critical dependence on food resources during have apparently been important in the evolution of the Northern Shoveler's behavior. The relationship of fasting endurance to body size has probably been of fundamental importance in the evolution of avian behavior. Avian behavior has probably evolved in relation to at least three major factors: (1) physical requirements of the embryos for development; (2) metabolic requirements of the parent(s); and (3) predation on eggs and the parent(s). The requirements of the embryos must be met through behavior that compensates for fluctuating environmental conditions, allow the parent(s) to maintain and/or acquire sufficient energy to support body metabolism, and reduce the probability of predation. Environmental variables have little or no effect on rhythms of Canada Geese (Branta canadensis) or Trumpeter Swans (Cygnus buccinator) (Cooper 1978, 1979). These large waterfowl fast during (MacInnes et al. 1974, Cooper 1978, 1979, Raveling 1979) and their large eggs cool relatively slowly; hence, these birds are able to incubate unaffected by brief environmental fluctuations. Fasting endurance, or the period of time an individual can survive on stored energy, decreases with body size (Calder 1974). Small species are probably unable to subsist exclusively on body reserves during and therefore, must rely more on environmental food resources (Skutch 1962, White and Kinney 1974, Afton 1978, 1979a). Thus, I predict that in smaller anatids as compared with larger, environmental factors should have more effect on rhythms. This paper describes laying and rhythms of the Northern Shoveler (Anas clypeata). Factors affecting the rhythm are examined in order to evaluate my prediction. METHODS The 777-ha study area was located 12 km east of Delta, Manitoba, Canada and has been described in detail by Caldwell (1976). Nesting behavior of wild Northern Shovelers was studied in 1974 and 1975 by direct observation and with strip chart, thermistor-event recorders (Rustrak 2133) that synchronously recorded hen attentiveness and temperatures. The h 's presence at the nest was detected with an infrared photoelectric relay (Microswitch MLS-3). Detailed descriptions of the nest monitoring system and its operation are given in Cooper and Afton (in press). Nests were found by watching females fly to their nests or by flushing hens from cover with a dog or a chain drag device (Higgins et al. 1969). Clutch size and relative amounts of nest down were recorded upon discovery and subsequent visits. In nests found with complete clutches, stage was estimated by opening one egg to determine the age of the embryo. Embryos were aged by comparison with known-age photos as in Caldwell and Snart (1974). Air temperature was continuously recorded on the area with a thermograph (Marshalltown 1000A)+housed 1.2 m above ground in a standard meteorological shelter. Records of daily sunshine duration were obtained from the University of Manitoba Delta Marsh Field Station, 18.5 km west of the study area. Additional weather data were provided by the Canadian Forces Base at Portage la Prairie, 21 km to the south. Although actually begins before the clutch is completed (Afton 1979b), 1 of incubation is defined as the day of clutch completion in order to allow comparisons with previously published information. Periods spent on the nest by the hen are termed sessions, while periods off the nest are recesses. Incubation constancy is the average percent of time spent on the nest per day (Skutch 1962). Multiple regression analysis was used to investigate factors affecting the rhythm. Data were analyzed using the backward elimination procedure (Draper and Smith 1966:167) with the MULTREG statistical program (Weisberg 1977). A regression model which included all independent variables was calculated as the first step of this procedure. Then, the least important variables, as judged by the reduction of sum of squares unexplained by regression, were individually excluded in subsequent models until all remain-