only 20 adult Common Terns. The terns stayed on that pier until July 7th and attempted to rear young as did the gulls. The sightings on July 14th showed only the presence of the Gulls and it remained that way for the rest of the study. The Common Tern is very protective of its young and will display aggressive behavior toward any object it considers a predator. The gulls which did occupy the same nesting site as the terns were observed being harassed by the terns quite often. This type of behavior takes the tern away from its nesting and feeding activities which could cause a reduced fledging success rate. The number of chicks noted on pier #7 appeared to drop to 50% over this time period. Actual predation by the gulls was not seen by the author but was reported by others. The number of the terns using the colony showed a steady decrease from 228 adult terns on June 17th to 96 adult terns on July 28th.

Dan McAskill (pers. comm.) reported the presence of at least 6 pairs of Common Terns exhibiting nesting behavior on Pier # 10 in late May. By the time observations were started in this study those terns had abandoned any attempt to nest on this pier although terns were often observed harassing the adult Great Black-backed Gulls and young on this pier during the period of the study. 0n the remaining piers, the terns and gulls did not occupy the same pier and, while the adult terns did spend time harassing the Great Black-backed Gulls on the piers on which the gulls were nesting, on the light standards, and on the service road, the gulls were not seen landing on these other piers.

While the tern and gull chicks were difficult to see as they tended to settle in the lower or grassy areas of the piers a comparison between the tern piers with and without gulls present suggests greater nesting and hatching activity on piers without Great Black-backed Gulls.

Double—crested Cormorants were located on pier #3 throughout the study. 1988 was the first year that cormorants remained on the piers throughout the tern breeding season. A pair of adult Great Black-backed Gulls was observed rearing young on that same pier. The Double-crested Cormorant's interaction with the Common Terns was restricted to competition for space on the piers and, possibly, competition for food.

Based on the observations made during this study there appears to be a link between the tern and gull populations in this colony. Not only do they compete for food, they also compete for nesting space and the terns must prevent the gull from predating its nests and young which, if they remain in the same breeding area, will continue to create a problem for the tern population.

(Editor's Note:

1. This article was excerpted from the Natural History Society of Prince Edward Island's 1988 SEEDS Project Report. This project was implemented by Pamela MacEachern and Shelley Barrett who were employed by the Society with funds provided by the Canada Employment and Immigration Commission and the Prince Edward Island Conservation Strategy.

2. Wildlife managers must base decisions to intervene in species management on p0pulation data and a judgement of need. In the case of endangered species such as the Whooping Crane, there has been strong support for this intervention. A recent decision to intervene on behalf of the terns on Sable Island was withdrawn due to public opposition over the proposed gull kill. The situation in the Hillsborough Tern Colony raises a similar controversy. Should the Great Black-backed Gulls on these piers be removed or prevented from nesting on the piers so that the Common Terns will face less predation and so they can reoccupy their former nesting territory? What do you think? I will explore this matter further in a future issue. J.D.M.)

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