Eight comparisons of adult/nest ratios were made at Massachusetts 

 double-crested cormorant colonies over a 2-year period. Adults were often 

 underestimated (Table 26). This finding is surprising considering that cor- 

 morants are one of the most conspicuous species included in the survey. 



Accurate nest counts were difficult to obtain at wading bird colonies. 

 Great egrets and great blue herons required no correction factor since their 

 tree- (or shrub-) top nesting habit enabled direct nest counts to be obtained 

 either from aerial photographs or visual counts. At 10 colonies in Massachu- 

 setts, Maryland, and Virginia, the black-crowned night heron adult-to-nest 

 ratio was only 0.24 (i.e., For every adult seen from the helicopter, four 

 active nests were actually found in the colony.). A combination of dense 

 vegetation, cryptic nests and plumage, and the tendency to "sit tight" when 

 disturbed all resulted in gross underestimation of numbers (Portnoy 1977, 

 Buckley pers. comm.). A conservative correction factor of 2.5, rather than 

 4.0, was applied since in a number of colonies multiple adult counts were 

 made. These counts showed that differences between maximum and minimum 

 estimates (both air and ground) were not great enough to warrant applying a 4x 

 correction factor to the aerial estimates. When censuses were made by flush- 

 ing birds on the ground, a factor of 1.0 was used. 



A factor of 1.0 was applied to all other herons, egrets, and ibises. 

 This factor has been applied elsewhere (Custer and Osborn 1976, P. A. and F. 

 G. Buckley pers. comm.) even though its validity has not been established. 

 Limited data at snowy egret and glossy ibis colonies in Virginia and Maryland 

 indicate that adult estimates underestimate nest counts. Glossy ibises often 

 nest near or on the ground in dense Phragmites in the mid-Atlantic region. 

 The 1:1 adult-to-nest factor is probably 'very conservative. All the dark- 

 bodied herons (Louisiana, little blue, and green) are probably underestimated 

 because of their inconspicuousness. 



Aerial Photography 



In 1976 and 1977, aerial photographs were taken with hand-held 35 mm (55 

 mm lens) and 70 mm (105 mm lens) format single lens reflex cameras from both 

 fixed-and rotary-wing aircraft. Fine-grained (VPS) color and Ektachrome film 

 were used. Most photos were taken from altitudes of 100-200 m. In 1977, a 

 Cessna 172 fixed-wing aircraft was used with bottom-mounted K17 (9' format) 

 and Hasselblad (70 mm) cameras. Kodak Plus X aerographic film was used. 

 Colonies of all species were photographed to assess the feasibility of obtain- 

 ing accurate census data from prints. 



Photography proved to be of limited value in censusing waterbird colo- 

 nies. Among the 28 species included in the survey, only double-crested 

 cormorants, great blue herons, great egrets, and royal and sandwich terns 

 could be counted from either contact or enlarged prints (Figures 13 & 14). In 

 some beach colonies, black skimmers and herring and great black-backed gulls 

 can also be counted. For other species, either the small size (most terns), 

 concealed nests (eiders, petrels, guillemots), or dense vegetation (herons, 

 egrets, ibises, gulls) limited the usefulness of photography. Photographs can 

 be valuable, however, in documenting the nesting habitat and the colony area 

 used by the birds which could provide a baseline for comparison with photos 

 taken during subsequent years. 



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