Colony Site Dynamics and Habitat Use in Atlantic Coast Seabirds

Abstract

-Seabird colony sizes and movements were documented in the DelMarVa coastal region in 1976-1977 and in New Jersey in 1978-1979. Most colonies were found on marsh and dredge deposition islands and on barrier island beaches. For the traditionally beach-nesting Herring Gull, Common Tern, and Black Skimmer, larger, more stable colonies were found on barrier beaches than on marsh islands. In marsh habitats, rates of colony-site change of marshnesting Forster's Tern and Laughing Gulls were similar to those of the former nesters. Several adaptations have evolved in marsh specialists to cope with a high risk of reproductive failure due to flooding, but both Herring Gulls and Common Terns also appear to be very adaptable in nesting under various habitat conditions. New colonies and those abandoned between years may be pioneering attempts by younger or inexperienced birds, because they are often smaller than persistent colonies, although patterns differ among areas and habitats. Colony-site dynamics are complex and result from many selective factors including competition, predation, physical changes in site structure, and flooding. The invasion of Herring Gulls into marshes along the mid-Atlantic coast has had an impact on new colony-site choice by associated seabirds. Calculating colony-site turnover rates allows for comparisons among species, habitats, and regions and may give useful insights into habitat quality and change and alternative nesting strategies. Received 10 November 1980, accepted 20 February 1981. COLLECTIVELY, colonially nesting seabirds (here referring to gulls, terns, and Black Skimmers, Rynchops niger) nest in a variety of habitats along the North and mid-Atlantic coast, including beaches and dunes, salt marshes, dredge deposition islands, rocks, etc. (Bent 1921, Stone 1937, Erwin 1979a). Some species are habitat specialists. Forster's Terns (Sterna forsteri) and Laughing Gulls (Larus atricilla), for instance, nest nearly exclusively in salt marshes in the mid-Atlantic coastal region (Montevecchi 1978, Storey 1978, Erwin and Korschgen 1979). Others such as Common Terns (Sterna hirundo) and Herring Gulls (Larus argentatus) nest in several different habitat types ranging from marshes to rocky islands (Burger 1977, Burger and Lesser 1978, Erwin 1980). Recently, many North and mid-Atlantic coast colonies of Herring Gulls, Common Terns, and Black Skimmers have shifted from their (Bent 1921, Stone 1937) outer beach-dune habitats to salt marshes and bay islands, due largely to human disturbances along beaches (Buckley and Buckley 1977, in press; Burger 1977; Burger and Lesser 1978; Erwin 1980). Recent coastal surveys (Erwin 1979a, Erwin and Korschgen 1979, Parnell and Soots 1980, Buckley and Buckley in press) showed that in the states with the greatest coastal zone development, the use of marsh and dredge deposition islands by is highest. Major changes in breeding habitat use patterns pose a number of ecological and evolutionary challenges to the individual organism. More specifically for nesting seabirds, the major factors affecting the relative fitness of the individual in different 550 The Auk 98: 550-561. July 1981 This content downloaded from 157.55.39.180 on Tue, 27 Sep 2016 05:45:12 UTC All use subject to http://about.jstor.org/terms July 1981] Seabird Colony-site Dynamics 55 1 habitats include the risk of flooding of eggs or young (Greenhalgh 1974, Montevecchi 1978, Burger 1979a, Burger and Lesser 1979), the physical stability of sand, marsh, and rocky substrates (McNicholl 1975, Southern 1977), predation (Buckley and Buckley 1972, Burger 1979b, Burger and Lesser 1978), and competition for nest sites (Crowell and Crowell 1946; Nisbet 1971, 1973; Burger and Shisler 1978; Erwin 1980). For ground-nesting seabirds in disturbed areas, the shift from use of open, dry duneand beach-sand substrates to the more vegetated wet marshes of lower elevation represents an abrupt change; both Common Terns and Herring Gulls, however, have demonstrated sufficient plasticity to make at least one important adjustment-nest building behavior (Burger 1979a). Other species, such as Black Skimmers, may not be quite as plastic, as they are nearly exclusively sand nesters. In addition to the change in the physical regime of the nesting site, the biotic environment might also change with more intense competitive and predatory interactions occurring as the amount of occupiable habitat becomes limiting. Large, aggressive Herring Gulls usurp nest sites by breeding earlier than most other species and also prey on the young of other species (Hatch 1970, Burger 1977, Burger and Lesser 1978). Both Common Terns (Burger and Lesser 1978, 1979; Erwin 1980; see Nisbet 1973 and Drury 1973, 1974 for historical review) and Laughing Gulls (Nisbet 1971; Burger 1977, 197 9a; Burger and Shisler 1978) have been shown to be adversely affected by the presence of nesting Herring Gulls. Ultimately, the ability of each species to cope with these changes should be reflected in the reproductive output of the individuals affected. Low reproductive success in seabirds is often associated with declining colony size or abandonment of colony sites (Crowell and Crowell 1946, Morris and Hunter 1976, Burger unpubl. data). Because these habitat shifts have occurred mostly in the past 30-50 yr (Stone 1937, Buckley and Buckley 1977, Burger 1977, Erwin 1980), we would expect selection still to be acting rather strongly (a nonequilibrium condition). We propose here to compare the dynamics of colony-site use and colony sizes among five seabird species in New Jersey, a region of intense coastal development, with that on the Delaware-Maryland-Virginia (hereafter DelMarVa) coast, an area comprised largely of undisturbed barrier islands (Erwin 1980). In New Jersey, virtually all nesting of the five species is on salt marsh islands or dredge deposition islands, while in Virginia Herring Gulls, Common Terns, and Black Skimmers still nest primarily on barrier island beaches and dunes under relatively pristine conditions (Erwin 1980). Given the above scenario, we pose the following questions: (1) Are beach-dune colonies of the traditional beach nesters any different in size or stability from year to year than those colonies in marshes or on dredge islands? (2) Are colonies of the marsh-adapted species, i.e. Forster's Tern and Laughing Gull, more consistent in site use from year to year than the marsh invaders? Presumably, the latter species have not had sufficient time to acquire fine-tuned adaptations and, hence, are possibly inferior in judging high quality sites in marshes. (3) Do new colonies or colonies abandoned between years differ in any way from those that persist? (4) How does the presence of the competitively dominant and predatory Herring Gull influence the colony-site choice of other species? We respond to these questions by examining comprehensive census data collected since 1976. This content downloaded from 157.55.39.180 on Tue, 27 Sep 2016 05:45:12 UTC All use subject to http://about.jstor.org/terms 552 ERWIN, GALLI, AND BURGER [Auk, Vol. 98