Factors Influencing Egg Laying and Feeding in Black-naped Terns Sterna sumatrana

Abstract

The Black-naped Tern breeds on cays and continental islands along the Great Bamer Reef, Australia. Studies of breeding and feeding were conducted at Eagle Island in the tropical north. Nesting was recorded from September to January. Egg laying was protracted over considerable periods in two of the three seasons of study. In all three seasons the first clutches for the breeding season, and for each subsequent wave of laying, were laid within a four-day period of the new moon and the number of eggs laid per day generally peaked within a few days of the full moon. Adults fed their chicks principally on the silver schooling fish belonging to the families Engraulidae, Atherinidae and Hemiramphidae. Feeding by adults occurred mostly in association with the reef surrounding Eagle Island. Adult terns hunted most actively during the early morning and late afternoon. Chick feeding rates were affected by (1) time: rates were highest in the late afternoon and lowest in the early afternoon; and (2) tides: rates were highest on flooding tides and lowest on ebbing tides. During the summer at Eagle Island, flooding tides occurred in the early morning and late afternoon at the time of new and full moons. Food seemed particularly abundant for chicks at this time. This may also explain why egg laying was linked to moon phases, because the male must courtship-feed the female all of her fish requirements just before egg laying.