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 
     
    
   
   
    
     _ 4 _