The Behavioral Ecology of Breeding American Coots in Relation to Age

Abstract

-Time-activity budgets were determined for breeding male and female American Coots (Fulica americana) of three age-classes. Time apportioned to foraging and courtship decreased with age, while time spent in defending territory and feeding young increased with age. We hypothesize that differences in time spent foraging, defending the territory and feeding young are the result of older coots occupying better-quality territories. Such a hypothesis is consistent with the generally assumed wide variability in spatial distribution of energy resources in marsh ecosystems. Such variability seems to be a primary proximate and ultimate factor affecting the reproductive success of American Coots. Several recent theoretical papers predict that reproductive effort should increase with age or decrease with residual reproductive value (Williams 1966, Gadgil and Bossert 1970, Charlesworth and Leon 1976, Pianka 1976). Residual reproductive value is an animal's age-specific expectation of all present and future offspring, being discounted by the probability of adult mortality preceding the production of offspring (Fisher 1958). Younger birds have greater average residual reproductive value than do older birds. The theory predicts that younger birds should expend less effort than older ones on reproduction, thereby reducing risk and conserving energy, which will enhance survival and successful future reproduction. Empirical studies have shown that clutch size, size of eggs, chronology of laying, and success in fledging young are related to age; older birds lay more and larger eggs earlier and are more successful in producing young (Coulson and White 1958, Brakhage 1965, Coulson et al. 1969, Mills 1973, Davis 1975, Ryder 1975, Coulson and Horobin 1976, Crawford 1977, Blus and Keahey 1978). Crawford (1975) found similar age-related phenomena in American Coots (Fulica americana). An animal's apportionment of time, energy, and matter determines how it adjusts to its environment (Pianka 1974:201). Studies of age-related biology lead to the prediction of differences in the apportionment of time for various activities during the reproductive period. We wished to test this prediction by comparing quantitatively the behavior of three age-classes of breeding American Coots. We further attempted to relate environmental conditions to observed age-specific behavior, and to assess possible factors affecting reproductive success and a aptive behavior of coots. STUDY AREA AND METHODS Fieldwork was conducted in 1976 and 1977 at Smith's Slough, Dewey's Pasture, and Hottes Lake, wetlands in northwestern Iowa. Smith's Slough and Dewey's Pasture were described by Bennett (1938), Hayden (1943), and Low (1945). Conditions at the time of this study, and a description of Hottes Lake, are given by Ryan and Dinsmore (1979). Time-activity budgets were determined for both sexes within three age-classes (one, two, and three years) during prelaying, laying-incubation, and broodrearing periods. We made spot-observations of behaviors of known individuals every 30 s in one-hour time blocks distributed throughout the day (06:00-20:00). We used a metronome to time sampling intervals (Wiens et al. 1970). Usually both members of a pair of coots were watched simultaneously. At Smith's 'Slough and Dewey's Pasture in 1976, and at Hottes Lake in 1977, coots were watched from an automobile or from locations far outside the marshes. In 1977, we used four four-m tower blinds in Smith's Slough and two water-level blinds at Dewey's Pasture. Coots were watched through a spotting scope or binoculars. Thirty males and 25 females were visible 77.3% (46.4 min/h) of 467 bird-hours of observation. Coots were sexed in the field by voice (Gullion 1950) and assigned to age-classes by tarsal color (Crawford 1978). Some individuals were identified by coded nasal discs; others were identified by morphological characteristics, such as distinctive white feathering at various places on the body or the unique shape of the callus (Gullion 1951). The prelaying period commenced with the initiation of territorial aggression and ended with deposition of the first egg . The laying-incubation period ended with hatching of the first egg. Observations during the brood-rearing period continued until we were no longer able to find the brood, or until feeding of the young by the parents was markedly reduced (approximately 20 days after the first egg hatched; Gullion 1954, Ryan and Dinsmore 1979).