Abstract
GULLS (Family Laridae) are generalized foragers that utilize many kinds of food items ranging in size from insects to the carcasses of whales. They forage in the air, on foot in fields and the intertidal zone, and by swimming and diving for underwater prey. Although species and individual gulls may show considerable specialization in foraging techniques and food preferences, most species of gulls forage at least occasionally over a large range of available habitats and use a wide variety of feeding techniques.
Highlights
Field work during three breeding seasons in Maine and 5 weeks during July and August in northwestern Europe yielded a total of 45,604 gulls counted from 24 and 26 days of surveying respectively (Table 1)
Comparisons were made of the partitioning of foraging habitats by three species of gulls in Maine (Larus marinus, L. argentatus, and L atricilla) and six species in Scotland and Norway (L. marinus, L. fuscust L. argentatus, L. ridibundus, L. canus, and Rissa tridactyla)
Feeding habitat diversity was found to be generally similar in Maine and Europe, species diversity within each foraging habitat was greater in two areas studied in Europe
Summary
We studied foraging gulls by censusing those visible from coastal roads. In Maine (see Figure 1) we surveyed the shoreline between Belfast, Waldo County, 827 The Auk 90: 827-839. Jn Norway we visited most areas only once Except for those studies designed to examine the effect of tide level on foraging activity, all surveys in Maine were conducted within 2 hours of low water. In Scotland most observations were made within 2 hours of low water except those made while driving between study areas. Data for the analysis of partitioning of the intertidal substrate by changes in the tide level were gathered by making repeated observations of foraging gulls at a given location from high until low tide. These were areas where at least one-half the surface was covered with clumps of mussels 45,604 substrate diversity of each gull species, and the average foraging habitat overlap between gull species in each area. Substrate overlap between species was measured by the formula where Px, , and P,., 1 are the frequencies for species x and y respectively, for the jth category (Schoener 1968)
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