BEHAVIOR AND SEX ROLES OF NESTING ANHINGAS AT SAN BLAS, MEXICO

Breeding behavior and sex roles of nesting Anhingas were studied in San Blas, Nayarit, Mexico during July and August 1975. The Anhingas nested in a mixed species colony of egrets, herons, and cormorants. The egg-laying period for Anhingas was 21 days. Anhingas built nests in the open areas of trees near exposed perch sites and nested closer to conspecifics than to other species. We observed 14 nests for 483 h during the incubation period. When data were summedfor all nests, males incubated for 55% of the time, and females incubated for 45% of the time, although these differences were not significant. However, males did incubate for significantly more time during days 1-5 and 26-30. From 06:00 until 15:00 there was an equal probability of finding a female or male incubating, but significantly more males incubated at sunrise and sunset. Males incubated 65% of the nighttime. At a given nest, the same sex did not always incubate on successive nights. Mates often remained near the nest when not incubating, and there were no sexual differences in time spent nearby. The amount of time a mate was present when non-incubating decreased seasonally. During days 1-5 post egg-laying, mates were present 56% of the time, by days 11-15 they were present 14% of tile time, and by day 25 they returned only to exchange. Nest relief, which usually occurred from 06:00 to 08:00 and from 14:00 to 15:00, always involved vocalizations. Males made significantly more trips for nest material than did females. Males brought nest material with leaves 75% of the time, whereas females brought material with leaves only 20% of tile time; 85% of the nest material trips occurred after nest relief. Most (95%) of the nest material trips occurred during the incubation period, although some occurred while chicks were in the nest.Aggressive encounters were brief and usually involved displacing an intruder. Males performed significantly more aggression (86%) than did females. The non-incubating mate performed 50% of the aggression from days 1-10 and 100% thereafter. Conspecific aggression decreased seasonally. Heterospecific aggression accounted for 25% of the aggressive encounters. Eighty-one % of the eggs laid in 21 nests hatched. Egg loss occurred through dismantling of the nest and by eggs being knocked in the water from active nests. Of the chicks that hatched, 95% survived until at least 16 days of age. Males and females spent equal amounts of time brooding chicks. One adult brooded the chicks at all times until they were 12 days of age, when the parents began to leave the chicks alone. Both sexes fed the young. Breeding chronology, success, and sex roles are discussed.

JOANNA BURGER, LYNNE M. MILLER, D. CALDWELL HAHN, Wilson Bulletin: Vol. 90, No. 3, July-September, 1978