Sexually Distinct Daily Activity Patterns of Blue-Eyed Shags in Antarctica

Abstract

Sexually distinct rhythms of nest attendance were documented for Blue-eyed Shags (Phalacrocorux atriceps) in Antarctica. Males attended the nest from approximately 00:00-l 2:00 and foraged approximately from 12:00-24:O0. Female activity patterns were the opposite. Timing of male courtship occurred around 12:00 during the prelaying and laying-incubation periods, and timing of chick feeding was also sexually distinct. Males fed chicks in the early morning and late evening while females fed chicks in the afternoon. The rhythms were colony-wide and, at least, partially influenced by times of sunrise and sunset. Although specific adaptive pressures influencing these patterns are unknown, intersexual foraging competition is presented as a possible explanation relative to other theories. Field studies have documented diurnal activity rhythms in several species of colonial seabirds by measuring the timing and periodicity of certain behaviors (e.g., Palmgren 1949, Cullen 1954, Muller-Schwarze 1968, Delius 1970, Drent 1970, Burger 1976, Galusha and Amlaner 1978, and Conover and Miller 1980). Few studies, however, have found rhythms within breeding pairs. Additionally, synchronized, colony-wide, daily rhythms for each sex have seldom been reported (Snow 1963; Vestjens 1977; Gaston and Nettleship 1981; van Tets, pers. comm.; T. R. Birkhead, pers. comm.). During our studies at a colony of Blueeyed Shags (Phalacrocorax atriceps), however, the existence of such behavioral rhythms became apparent. Although Murphy (1936) had noted “highly interesting” rhythms in this species, no further research had been conducted. Such rigid activity patterns would be of interest in measuring energy allocation between sexes (Bernstein and Maxson, unpubl. data) and also in answering questions concerning selection pressures on the Blue-eyed Shags and other colonial birds. Accordingly, we quantified timing of basic behavioral rhythms within breeding pairs to determine if the timing differed between sexes, and to determine how widespread individual rhythms were within the colony. STUDY AREA AND METHODS We collected over 3,000 bird-hours of time budget data from 15 January 1979 to 1 April 1979 and from 23 September 1979 to 15 March 1980 at a colony of Blue-eyed Shags on Cormorant Island, 5 km southwest of Palmer Station, Anvers Island, Antarctica (64”46’S, 64”03’W). There were 485 nests on 3 1 December 1978 and 326 on 19 December 1979. Nests (2-10) were observed simultaneously every 30 s, at the tone of a metronome (see Wiens et al. 1970) either from dawn to dusk or for 24 h during periods of continuous daylight. Behaviors were categorized as: present at the nest (preening, resting, guarding chicks, etc.), incubating, brooding chicks, off the nest but gathering nest material (algae) nearby (Bernstein and Maxson 1982), and feeding chicks. Other behaviors that consumed little time, such as courtship or pair-bond maintenance, were considered as present at the nest if the shag was not simultaneously incubating or brooding. Shags were assumed to be foraging when absent from the nest or from the vicinity of the colony. Data were grouped by sex for each of the following stages of the breeding cycle: prelaying, laying-incubation, brooding, and fledging periods. Laying and incubation were combined because incubation began during the interval between the laying of the first and second eggs (Williams and Burger 1979, Shaw 198 1). The brooding period was further divided into three stages determined by size of the largest chick in a nest. Ulna length was the primary criterion (Dunn 1975b), and divisions were based on separations within the typical sigmoid growth curve. The early chick rearing stage was defined by chicks weighing less than 500 g and with ulnas less than 55 mm, the middle chick rearing stage by chicks of 5001,000 g and ulnas of 55-l 20 mm, and the late rearing stage by larger chicks until they fledged. Early chick rearing ended approximately on day 12, the time also given by Dunn (1976) SEXUALLY DISTINCT ACTIVITY PATTERNS IN SHAGS 153