Aspects of the breeding biology of Black Shags ( Phalacrocorax carbo ) near Lake Kohangatera, Wellington
|Title||Aspects of the breeding biology of Black Shags ( Phalacrocorax carbo ) near Lake Kohangatera, Wellington|
|Publication Type||Journal Article|
|Year of Publication||1999|
|Authors||Powlesland, RG, Reese, PJ|
|Type of Article||paper|
|Keywords||Black Shag, breeding success, colony history, fledgling dependence, nesting activities, Phalacrocorax carbo, Wellington|
The nesting activities and breeding success of Black Shags (Phalacrocorax carbo) near Lake Kohangatera, Wellington, were studied from 1993 to 1998. The colony was used during November-July by a mean of 67 birds per night, but in August-October numbers increased to a mean of 98 birds when fledglings were present. Courtship and nest- building began in March, and nesting continued until October-November when the last chicks fledged. Most clutches (85% of 185) were laid in April-May (early nests), the remainder being laid in June-September (late nests). The mean estimated laying date of early nests varied from 14 April in 1998 to 3 May in 1995, the overall mean (1993-98) being 24 April. During the day typically the male took two incubation stints, including the first, and the female one or two. The mean length of incubation stints by females was 3 h 46 min, over an hour longer than that of males. However, the mean time females and males were absent from the colony to forage, 2 h 39 mins and 2 h 21 mins respectively, did not differ significantly. Three types of changeovers seen during incubation are described, as are the activities of adults and chicks during nestling rearing. Fledglings took their first flights when 49-60 days old, but continued to be fed by their parents for 40 to 80 days afterwards, the oldest fledged young seen fed being about 140 days old. Of 185 breeding attempts during 1993-98, 83% were successful, the majority resulting in one or two fledglings per nest. Mean brood size at fledging varied with year, from 1.1 in 1997 to 1.7 in 1998. Overall, the mean brood size was 1.4 fledged young per nest, and 1.7 for successful attempts. Early clutches were more productive than late ones. We conclude that a pair of Black Shags would be unable to successfully rear two broods and complete their moult within a year, and that late nestings were replacement clutches.