| Popis: |
I timed the activities of individually colour-banded South Island Robins Petroica australis australis of known age at Kowhai Bush, Kaikoura (August 1976 to December 1978) and on Outer Chetwode Island, Pelorus Sound (April to June 1978) to establish and investigate their time-budgets and diurnal patterns of behaviours. The number of Robins present in the Kowhai Bush study area gradually declined from a peak of about 100 in January 1977 to 37 in July 1978. Probably, about 120 Robins were present on Outer Chetwode Island during my visits there (Flack and Lloyd 1978). The birds seemed to be active only during the day, and no data on sleeping and roosting were collected. Generally, each behaviour included several activities: for example, body-maintenance behaviour was made up of stretching, beak-wiping, body-shaking, scratching, preening, bathing, anting and sunning. Many activities were timed separately and descriptions of them are given. To determine the proportion of time Robins devoted to a behaviour, the time they spent in the various activities making up the behaviour were combined. Throughout the year Robins foraged for more than half the daylight hours. It was evident that foraging to find enough food to meet their maintenance requirements was their first priority. Once this need had been met, extra time was devoted to body-maintenance and resting. The behaviours of lowest priority were those associated with reproduction. In the non-breeding season (January to July inclusive), adult males and adult females foraged least of all just before starting to moult (December and January), spending about 55% of observation time on feeding. Over the year they foraged most in July, 90-94% of their time. The shortest day and lowest monthly mean temperature occurred in June, but both factors increased little to July. Since the Robins devoted more time to foraging during the course of autumn (April and May) and winter (June and July), they spent gradually less time in resting and body-maintenance. Adults spent the least time in partner-interactions, territorial defence and vocalizing when moulting (January to March). These behaviours increased as a proportion of their time-budgets thereafter. From January to June, both immature males and immature females foraged more and spent less time on body-maintenance and resting than did adult males and adult females respectively. Furthermore, from January to April, immature males sang less than did adult males, but in June and July the reverse was true. In the breeding season (August to December inclusive), the time-budgets of males were determined to a large extent by the number of trips with food per hour they made to their mates and/or progeny; the higher the rate of food-trips the more time males foraged. When rearing nestlings, males made an average of five food-trips per hour to their young and foraged for 83.3% of observation time, significantly greater for both factors than at any other stage of the breeding cycle. Paired males foraged least of all during the nest-site selection stage (60.8%), but this value was significantly higher than that for bachelors (57.4%). As only females built nests, incubated and brooded, their time-budgets were greatly influenced by the stage of the breeding cycle they were engaged in. They foraged most when tending juveniles (82.0%). In contrast to their partners, females raising nestlings foraged for only 42.6% of time because they spent 46.7% of their time brooding. Females foraged least of all in the incubation stage (12.9%) when they incubated for about 80% of their time. In general, the results of this time-budget study support the findings of investigators into the time- and energy-budgets for other species. Without a change in food availability or energy content per food item, when birds require more energy they spend more time foraging. Winter was the season when Robins spent almost all their time in behaviours associated with self-maintenance, particularly foraging. At that time of year they devoted less time to behaviours associated with reproduction than they did during summer and autumn. It seems that Robins began breeding as soon as the environmental conditions enabled them, in the daylight hours available, to meet the energy costs of self-maintenance and breeding at the same time. Other investigators have shown that flights and yolk formation require large energy inputs to sustain. In support of this finding, female Robins that build nests and form yolks simultaneously, foraged for as much of their time as possible and had their food intake supplemented by that provided by their partners. Similarly, parent birds feeding nestlings or juveniles devoted most of their time to foraging. The moult, a process known to be of considerable energetic cost to birds, was undertaken by Robins at the most favourable time of year for self-maintenance and they stopped breeding in order to do so. Thus, when moulting, they spent a minimum of time meeting their self-maintenance requirements and so were able to devote a large proportion of their time to foraging in order to find enough food to meet the energy costs of feather replacement. |
