Julia Ambigis '01, Master's Thesis

The purpose of my study was to learn more about Leach's Storm Petrels. We need more information to be able to census and monitor petrel population fluctuations, which will allow us to better assess the overall health of these populations. To this end, my study had three main objectives; to estimate the population size of petrels on Great Duck Island, Maine, to compare different ways to census petrels, and to gain a further understanding of petrel habitat preferences.

Great Duck Island off the coast of Maine is a 109-hectare island of mixed open and forested areas. Censuses on this island have reported a range of 5,000 to 16,000 breeding pairs (Table 1). No two surveys have been done the same way and none of the surveys are comparable (Cowger 1976, Folger and Wayne 1986, Huntington 1963).

I sampled 13 open and nine forested plots. This was a total of 55,000m² or 7.5% of suitable petrel habitat on the island. I used a Shreco Flexible Camera system to determine burrow occupancy. This camera is designed to examine the inside of sewage pipes, in this application the camera was inserted into the burrows. The camera, attached to the end of a flexible cable, was maneuvered with the cable and a thin cord attached to the head of the camera. This presented a picture of the burrow. Roots, cones, nesting material, bird, eggs, and even insects were discernable. A burrow was determined to be active if it contained an adult, an egg, a chick, or an eggshell. I counted each active burrow as one breeding pair.

I found that Great Duck Island, Maine, has 9297.3 + 6500 (90% CI) breeding pairs of Leach's Storm-Petrels.

I compared three techniques for censusing Leach's Storm Petrels. Estimates of Leach's Storm Petrel population densities range widely even from the same island. Difficulties in studying this species are that they nest in burrows, are nocturnal, and spend most of their life out at sea, only coming in to land to breed. I compared traditional "grubbing" (reaching in the burrow with ones arm), to use of a camera on a cable to visually inspect the burrows, and playback recordings to elicit a response from occupied burrows. Traditional grubbing proved to be inefficient and detected the fewest active burrows. Playbacks detected as many burrows as the camera method, took less time and required less equipment. The playback method is less intrusive than the other two methods because it allows detection of a bird with no need to get in the burrow itself.

I found that on Great Duck Island, Leach's Storm-Petrels nest more in the forest than in the open. They group their nest sites around tree root systems, man-made structures, or fallen trees. Active burrows often appear unoccupied due to spruce cones, leaves and other debris clogging the entrance. Leach's Storm Petrels are typically reported at occupancy rates ranging from 55-65% and occupancy of Great Duck averaged 30% in 2000. This lower occupancy rate raises the potential of a declining population on Great Duck Island, although overall census estimates suggest a stable or even slightly growing population.

On Great Duck and possibly other islands, petrel densities may be decreasing. Concern has been expressed about whether small peripheral breeding colonies have been decreasing (Huntington et al. 1996, Drury 1973). While my results show an increase in numbers of petrels, the low occupancy rate may be cause for concern. The rise in numbers of breeding pairs cannot be said necessarily to reflect a population increase because of the different methods used in previous studies and because of high variance within studies. This means that the Great Duck colony and others like it should continue to be studied in a regulated way to be able to compare changes from year to year and between colonies. Improvements in monitoring Leach's petrels will help seabird management groups in Maine and elsewhere assess the hundreds of islands for which we do not know the status of Leach's Storm-Petrels.

Literature Cited:
COWGER, J. 1976. The nesting habitat of Leachís Storm-Petrel in Maine & its relevance to the Critical Areas Program. Maine State Planning Office.

DRURY, W.H. 1973. Population changes in New England Seabirds. Bird Banding 44: 267-276.

FOLGER, D.C., P.M.WAYNE. 1986. A biological inventory of Great Duck Island, Hancock County, Maine. Report to Nature Conservancy.

HUNTINGTON, C.E. 1963. Population dynamics of Leach's Petrel, Oceanodroma leucorhoa. Proceedings XIII International Ornithological Congress: 701-705.

HUNTINGTON, C.E., R.G.BUTLER, R.A.MAUCK. 1996. Oceanodroma leucorhoa Leach's Storm Petrel. The Birds of North America, No. 233.