HILLSBOROUGH BRIDGE PIER TERN COLONY by Pam MacEachern

The Hillsborough Bridge was constructed between 1901 - 1903 primarily for rail transport. The bridge extended 4,496 ft. in length from "The Old Shipyards Point" on the Charlottetown side to "Mutches Point" on the Southport side. The piers were numbered 0 to 11 from Charlottetown to Southport (Anonymous, 1900) and that numbering system is still used. The steel section of the bridge was removed after the new bridge was opened in 1963. The steel span was successfully removed but efforts to move one of the stone piers resulted in its collapse. The Natural History Society of Prince Edward Island lobbied for the retention of the remaining piers as a nesting site for the terns which were already using the north and south ends on the bridge (Bruce Pigot, Pers. Comm.). A letter to Mr. Bruce Pigot from J.E.V. Goodwill dated October 6th, 1965 indicated that terns were nesting on the rock filled crib to the north-east side of the swing section of the Hillsborough Bridge on July 3rd, 1948. He counted 50 adults and 8 young on that date. In 1965, Bruce Pigot noted that "In addition to the north and south colonies, terns are now nesting on the piers of the old Hillsborough Bridge. Twenty-four adults were flushed off these. They very likely are nesting" (The North and South colonies refer to the pentagon shaped rock piers adjacent to the swinging portion of the bridge).

The discarded piers had become home to a colony of Common Terns. Margaret Mallett kept records of the number of birds in the colony from 1972 to 1976. In 1984 a pair of Great Black-backed Gulls established a nest on pier # 10 and some members of the Natural History Society of Prince Edward Island became concerned for the tern colony at this site.' That pair was successful in rearing young and a pair was successful in rearing young on that pier each year from 1984 to 1988. In 1987 Great Black- backed Gulls also established a nest on pier # 7 and a pair occupied pier # 6.

Bruce Pigot recorded the early dates of occurrence for Common Terns as May 22 to June 3 over an eleven year period ending in 1967. E. Holdway recorded the early dates of arrival of the terns at Wood Islands between May 16th and May 22nd during the 10 year period 1956 to 1965 (Pers. Comm. to B. Pigot from E. Holdway). Dan McAskill (pers. comm.) recorded the dates that he first observed terns on the Hillsborough Bridge Piers in four of the last five years. These were: May lst in 1984; May 3rd in 1985; May 5th in 1987; and May 6th in 1988. The date for 1986 was not recorded. He noted that the Great Black-backed Gulls occupied their nests several weeks before the terns returned. Bruce Pigot estimated the total population of Common Tern in Prince Edward Island in 1966 to be 2,874 based on the number of occupied nests in 32 colonies. He noted the probable displacement of Common Tern colonies from the Pownal Islands by the Great Black-backed Gulls.

The objective of this study was to ascertain whether or not the presence of the Great Black—backed Gulls on the Hillsborough Bridge Piers was having a negative impact on the number of Common Tern utilizing the piers or on the number of young being produced from this colony. This concern was raised by two Natural History Society members concerned over reported declines in the tern population in the region (Dan McAskill, pers. comm.).

Common Tern