Parental behaviour and role differentiation in the black oystercatcher, Haematopus bachmani
Date
1985
Authors
Purdy, Margaret Anita
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Abstract
An understanding of patterns of evolution and adaptive radiation of parental behaviour within a group such as shorebirds (sandpipers, plovers and their kin) requires adequate documentation of the characteristics and existing patterns of variation in parental behaviour, both within and among species. To this end, monogamously breeding Black Oystercatchers Haematopus bachmani were studied on Cleland Island, British Columbia from April-September, 1982 and 1983. The emphasis of the study was a comparison of the behaviour of males and females during six stages of the breeding season. Data were collected during day-long (16-h) sampling periods on thirteen pairs, simultaneously recording the behaviour of both pair members.
Prior to egg-laying, females spent approximately 6 per cent more time foraging than did males. During this period, males were more aggressive, spending almost twice as much time in piping behaviours with conspecifics. Males were also more alert than females, as indicated by comparison of the amount of time they spent in alert standing behaviours ( females 23%, males 30 %), and by the shorter bouts of foraging, preening, and sitting, which were considered to be non-vigilant behaviours. Both sexes were more aggressive toward conspecifics during this period.
Males did almost all the egg-covering during egg-laying. During the first half of incubation, females incubated twice as much as males did, but the sexes shared incubation duties about equally in the latter half. There was no diel sharing of incubation between the sexes. Females incubated in longer bouts (average 58 min) than did males (39 min). During incubation, oystercatchers rarely sat when off the nest and most other behaviours showed a corresponding decrease in the percentage they comprised of the total time-budget during these stages. Males continued to respond to intruding oystercatchers even if it meant leaving the nest. This was in contrast to females, which responded to fewer of these intruders and usually did so only after their mates had responded.
Chicks were brooded almost continuously for about the first three days after hatching and intermittently until they were 15 days old. Females did almost 70 per cent of the brooding. During the brooding period, males took a dominant role in feeding and foraging for the chick, but the sexes participated equally in this after the chicks ceased to be brooded. After hatching, both sexes showed a sharp increase in interspecific aggression, particularly toward gulls. Both sexes were more alert during chick rearing stages than during previous ones, as indicated by their shorter foraging, preening and sitting bouts and the increase in the amount of time spent in alert standing behaviours. Chicks continued to be fed even after they fledged, at approximately 40 days of age.
Behind the general trends in role differentiation there was considerable inter-pair variability. This was exemplified by the arrangement of individuals in two-dimensional space by Multidimensional Scaling. These analyses showed that, prior to egg-laying and during chick-rearing stages when chicks were brooded, pair members were more similar to each other than to birds of the same sex in other pairs. In contrast, during incubation and early chick-rearing stages the sexes tended to be grouped together.
It appears that, even though males and females adopt different parental roles, parental investment is equalized between sexes over a given breeding season. When compared with other shorebirds, the pattern of the parental behaviour exhibited by oystercatchers appears to be related to their relatively large size, long-term mate fidelity, longevity, and sedentary life style.