DISCUSSION 



The thoroughness of observations and accuracy of counts on any given day 

 were influenced by an assortment of variables. Overall viewing conditions 

 were influenced by sea and sky conditions (Table 6). Ideal viewing conditions 

 occurred on calm days when the sky was thinly overcast with no reflective 

 solar glare. These conditions allowed 360° of visibility and provided a high 

 contrast of birds against the water. Bright clear days with slightly choppy 

 seas often rendered distant small, dark colored birds such as storm petrels 

 and alcids nearly invisible, but spouts of large whales and pale colored 

 seabirds such as gannets and gulls became more conspicuous. Viewing condi- 

 tions were poorest on cloudy, rough days when viewing was restricted to the 

 stern of the smaller boats, which were only a few feet above the water line. 

 This resulted in many birds and cetaceans being overlooked. 



Trips when several experienced observers were on board yielded better 

 coverage, although the thoroughness was proportional to sea and sky condi- 

 tions. Generally, the larger the vessel the more complete the coverage due to 

 the advantage of being able to view from a higher level. However, cetaceans 

 were more easily observed and identified from the more maneuverable small 

 charter boats. 



The accuracy of marine bird census ing is somewhat questionable because 

 "chum" were dispersed on most trips. When this procedure succeeded in attract- 

 ing birds to the boat, it thereby created a bias in the sampling. Generally, 

 chumming was most successful during cold weather months (October to May) when 

 herring gulls were the predominant seabird. Continuous chumming while moving 

 during the winter months proved highly successful in obtaining a relatively 

 large sample of birds along seaward transects. Gulls tended to follow longer 

 as the distance from shore increased. In turn, gulls often attracted pelagic 

 species to the boat, including fulmars, shearwaters, gannets, jaegers, and 

 skuas. 



The distribution of seabirds was apparently affected by commercial fish- 

 ing vessels. These included small American fishing trawlers, and clam and 

 scallop draggers, most often encountered along the 30 fathom contour; and the 

 fleet of large foreign trawlers most often observed along the edge of the 

 Continental Shelf at Baltimore Canyon. Foreign trawlers had the greatest 

 effect during the winter months. 



Seabird distribution may also be influenced by winds (Tuck 1961, Rowlett 

 1973b). Strong winds which accompany hurricanes and intense extra-tropical 

 storms temporarily carry some birds well beyond their normal range or usual 

 feeding areas. Southern hemisphere shearwaters migrate northward in spring 

 and pass through the northern Chesapeake Bight rather quickly to reach nutri- 

 tionally productive waters over the Georges and Grand Banks. Most of these 

 birds probably pass through unnoticed and are usually detected only when an 

 observer along the shore sights a flight which has been blown inshore 

 (Teulings 1972, Buckley 1973). 



63 



