General Biology

The Common Tern feeds on small fish by diving from low altitudes. It is usually a colonial nester utilizing offshore islands or undisturbed beaches. The nest is a small, shallow depression sometimes lined with grass, twigs, and broken shells. The tern usually lays three eggs which are incubated for approximately 26 days by both parents. The chicks are a light brown in colour but they soon change to the same colouring as winter plumage adults.

The Great Black-backed Gull is a scavenger and an opportunistic predator. This gull is distinguished by its black mantle and wings, white under belly, yellow beak with red tip, and flesh coloured legs. Great Black-backed Gulls usually nest in areas close to the water such as coastal islands or cliffs. They nest either “ singly or in colonies" (Godfrey, 1986), laying 2-3 eggs which are incubated by both parents for a 26 - 30 day period.

Population Variation

The known census data was compiled using information from the Northcott - Creamer study of Colonial Nesters on P.E.I in 1987, personal communications with Bruce Johnson (CNS), Margaret Mallett (former Secretary/Treasurer of the Natural History Society), and Bruce Pigot (a birder and NHS member).

Table 1. Census data for Common Terns on the Hillsborough Bridge Piers Tern Colony from 1948 to 1988.

Date Observer Number of Adults Number of Young Number of Nests

July 3/48 J.E.V. Goodwill 50 8

July 4/65 Bruce Pigot 30 V

June 12/66 Bruce Pigot 110 80 1972 Margaret Mallett 246 61

1973 Margaret Mallett 341 NR

July 8/74 Margaret Mallett 400 106

May 10/75 Margaret Mallett 402 NR

June 24/75 Margaret Mallett 402 107

June 23/76 Margaret Mallett 310 20

July 20/87 Northcott/Creamer 231 NR

June 13/88 MacEachern/Barrett 228 NR

NR = not recorded

This data indicates substantial variations in the population between 1948 and 1988. The rapid growth between 1965 and 1972 is possibly attributable to the displacement of terns from highly disturbed colonies particularly the Pownal Islands' colony (Pigot, pers. comm.).

Gulls verses Terns

A pair of Great Black—backed Gulls was first noted nesting on pier #10 in 1984 (Dan McAskill, pers. comm.). This gull is a natural predator of the Common Tern and its presence during a period when the tern population is thought to be declining caused concern among some Society members.

During the initial period of this study piers #1, #2, #4, #5, and #6 were occupied by the Common Tern, Piers #3 was occupied by Double-crested Cormorant, and Piers # 3, # 6, #7, and #10 by the Great Black-backed Gull. On June 17th an adult Great Black-backed Gull was noted on pier #6 where 30 adult Common Terns were exhibiting nesting behavior. During the next observation period (June 24th) a pair of adult Great Black-backed Gulls with 1 young were seen and

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