Acadia National Park Colonial Waterbird Inventory
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ACADIA NATIONAL PARK COLONIAL WATERBIRD INVENTORY 1985 \ 1, i I gO~~Pt'~t>"'t i.Yt::: ~~emt~_,:!!;:_ No n Principal Investigator: ex 1600-4-0003 DF' t Wi 1 1 i am H. Dt"ur"Y Effective Date: April 16, 1985 College of The Atlantic Bar Harbor~ Maine Expiration Date: Dec. 31, 1985 Submi t.teci: David C. Folger" Research Associate College of The Atlantic Bar Harbor~ Maine / INTRODUCTION This report has been prepared for Acadia National Park through a cooperative research agreement between Acadia NPS and the College of The Atlantic, Bar Harbor, Maine. It sumarizes our observations on the present status of colonial waterbirds nesting on islands which Acadia NPS owns, holds easements on, or proposes to acquire. Species surveyed in this report. include: Black Guillemot CE'pph~:~~_ 9.CyJJ..e, Doubl e-crested Cormorant F·ha~crocora:·~ ._aur~~~L~_, Great Cormorant ~ar~~, Common Eider So~~teri~_ moll i ss2.~~, Leach's Storm Petrel Q.ceanodrama l~2:~orh~.~, Herri ng Gull Lc~!::y.s ar-qer:!. .:L~!:_~:...~' Greater Bl ack-backed Gull ~~... ~€:l.rin~~, Laughing Gull L atricilla, Arctic Tern §!:....er!..:-!~~_.E~!..::~di sae~, Common Tern S. hi runcl.~., Roseate Tern S •. _~<:!LI.(1.al....:~.2.2.., and Great Blue Heron f.:irdea herodias. Additionally nesting Osprey Pandion haliaetu~ and Bald Eagl e H~l i aeetus 1 eucocepllal us WE?re noted when obser-v·ed cm islands visit.ed in the survey. Objectives and tasks of this project involved the following: 1. a review of available historical information on nesting colonial waterbirds for each of the islands surveyed. 2. an field inspection of park island holdings for all colonial waterbird species mentioned above. 3. a description of vegetation and physical features associated with each nesting colony. 4. an identification of threats and potential threats to waterbirds and their nesting habitat. 5. a review of, and recommendation for the mitigation of, identified threats. I This report summarizes information collected during the c. 1985 nesting season. It represents the first survey to be carried out by Acadia NPS and College of The Atlantic researchers to monitor seabird colonies within the park's jurisdiction. A total of thirteen days were spent in the field, using on average three people to assist in censusing. Further analysis, synthesis and writing up of the information required approximately six weeks for one person. For this project we have used survey methods which can be easily replicated. Additional surveys will further our understanding of population changes on park holdings and decrease the error inherent from single estimates. ·····1··_· To assist in this process the following format has been used: / -Part 1. a discussion of methods--survey techniques employed in the field, subjective techniques used in evaluating potential threats, and format used in presenting the data collected for each island. -Part 2. Results-- island-by-island descriptions, historical accounts, identified threats, and a subjective assesment of perceived value for each island in reference to regional and local significance. -Part 3. Discussion-- potential threats identified in the survey, areas needing further attention, and recommendations for mitigation of potential threats to colonial waterbird colonies. Part 1. METHODS: ,.. Techniques for censusing seabirds have been a subject of much interest among ornithologists (see Nettleship, 1976; Drury, 1973; Nisbet, 1972; Wilbur, 1969; Buckley & Buckley, 1980). An on-going discussion involving bird surveys revolves around questions of rigor, repeatability and the suitability of results for interpretation. To go into the specifics of each of these questions is unnecessary. However, it should be recognized that conducting surveys of nesting colonies is complicated by a variety of factors. some specific to a colony, others specific to the biology of the species being surveyed. To achieve a "good" count sE"!vet-C'd considerations need to bE~ made. A primary element is the timing of a census. Counts of breeding pairs are strongly influenced by when they are made in the breeding phenology of the species. This often varies from island to island as well as from season to season, and greatly influences comparisons of counts between years. It is generally held that for most colonial nesting species, surveys made just prior to when eggs are hatching include the greatest number of actual breeding pairs (Nisbet & Drury, 1972; Korshgen, 1979). This rule works well, however it neglects late arrivals, pairs that have lost their clutch early in the season, and sometimes first time breeding pairs. In addition to seasonal variation, the time of day a census is conducted will also greatly influence the number of individuals seen around a colony. Generally, surveys made in the early morning or at evening include the highest number of birds around an island. In contrast, counts made around mid-day and early afternoon will include incubating adults and fewer non-breeders. Presumably those birds that do not have a nest are likely away feeding. Similarly, the state of the tide will effect the number of birds; gulls will oft~n leave and feed at low tide away from a colony, while at high tide they will gradually appear and roost. A second major consideration includes the size of a colony, as well as its access. These features often dictate the type of survey method which is appropriate. For example, usually small colonies located on small offshore islands I J with low vegetation are better estimated from a boat than by landing and disturbing nesting birds. This is in contrast to large colonies that are typically spread out through a variety of habitats and better sampled by landing on the island. Estimates may be made in a variety of ways for either counts made from the deck of a boat or from the nesting island. Generally they can include total coverage involving direct nest and individual counts of the birds on an island; or they can involve estimates made by extrapolating from counts made within sub-samples of a colony. Additionally, counts may include the number of nests, the number of incubating birds, or the number of individuals. Despite efforts to systematize survey procedures, techniques need to be responsive to the physical featUres surrounding a colony and the ecology of the species being counted. Standardized sampling applied uniformily to all nesting species and all colonies is an ideal which is difficult to obtain and is often in conflict with field logistics and what can actually be counted without causing undue stress to the nesting species. Because nesting populations change over the breeding season, and because typically in both nest counts and counts of individuals human error involves 10-15 percent, interpretation of single estimates needs to accept 20-25 percent sampling error. Further counts made in a season will reduce this error, while counts made over several years will reveal trends which override annual variations. Ultimately, seabird surveys involve maintaining a data base that acts as an index from which general trends can be deciphered. The techniques we employed in the field varied from species to species and from island to island. We aimed to get the most accurate estimate possible with the least amount of disruption to the nesting birds. Certain species lend themselves to counting nests, others require counting individuals around the island. For each species in this survey the following methods were employed: Black Guillemot-- we counted the number of individuals around an island, preferably in early mornings in May when the greatest number of individuals associated to an island are concentrated within 200 meters offshore. l Double-crested Cormorant and Great Cormorant-- we counted the number of nests seen from the best vantage point around e~ch colony. This often meant counts from the deck of our boat or a promentory on the island. Common Eider-~ we counted the number of male and females seen in early May in the water and on land at each nesting island. Additionally, we made a subjective estimate of the number of nests. (Because eider nest in dense vegetation and they flush easily from their nests, leaving eggs unprotected and vulnerable to gull predation, we avoided entering eider nesting areas.) -4- Laughing, Herring, and Greater Black-backed Gull-- In small colonies ({25 pairs) direct nest counts were made. / In larger, yet consolidated, colonies counts of individuals appearing to defend nesting territory or incubating eggs were made from outside the colony, either from the deck of our boat or from a promontory on the nesting island. In large, dispersed, colonies adults in smaller sub-sample areas were counted either by flushing adults or, counting individuals on their nests while standing at the periphery of the nesting area, and then further extrapolations were made to match the whole colony. Arctic, Common and Roseate Tern-- Tern nests and eggs are well camouflaged and difficult to count. Experience has shown us that in colonies of less than 50 pairs good estimates can be made by counting the total number of individuals flushed from the nesting area. We have found our estimates approximate the total number of nests usually within 10%. In larger colonies we use direct nest counts and employ a capture/recapture technique to assess sampling i. error (see Folger & Drennan, 1984). To separate species, we estimate the proportions of adult Arctic and Common Terns flying over the colony and infer this ratio in the final estimate. For Roseate Terns, their few numbers allow direct counts. Great Blue Heron-- Herons typically nest in the tops of trees. Direct counts from below can be employed as well as counts from aerial photos. Tyler (1977), however, reports greater accuracy from ground counts. In this survey we employed direct ground counts.