Ecology of Coastal Raccoons (Procyon lotor) on the Queen Charlotte Islands, British Columbia, and evaluation of their potential impact on native burrow-nesting seabirds
Date
1993
Authors
Hartman, Lisa Helen
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Abstract
This three-year study investigated the potential impact of introduced raccoons (Procyon lotor) on native, burrow-nesting seabirds of the Queen Charlotte Islands (QCI). Raccoon diet, home range size, and patterns of movement and activity were documented on Vertical Point (VP), Louise Island, where no seabirds breed, and on adjacent East Limestone Island (EL), site of an Ancient Murrelet colony containing roughly 1,215 breeding pairs. Spring-summer diet was compared between these locations based on fragment counts of scats collected biweekly along 3 00 m shoreline latrine circuits, from VP (n=55) and EL (n= l2). Intertidal invertebrates, particularly shorecrab (Hemigrapsus nudus) and amphipods, occurred in all VP scats, and comprised 92% of the total fragment count, whereas they occurred in only 57% of EL scats, and comprised 20% of the total fragment count. Seabird remains were absent from VP scats, but occurred in all EL scats, comprising 43% of the fragment count.
Home range, movement and activity patterns were determined using telemetry and direct observations. Five males (M) and 4 females (F) were located a total of 614 times, divided between day and night. Four animals (2M, 2F) were intensively monitored for two summer field seasons, the remaining five for one. Data obtained for 2 raccoons from EL (lM, lF) were compared with data for animals on VP . Single season, 100% minimum convex polygon home ranges on both VP and EL ranged from 32.0 - 96.6 ha for males (X +/- S.D . = 60.2 +/- 23 .0 ha), and from 17.1 - 39.5 ha for females (X = 28 .3 +/- 8.3 ha). Home ranges included 1,825 to 6,245 m of shoreline. Home ranges overlapped both between and within sexes. The ability of raccoons to survive within small home ranges, and their non-territorial behaviour, contribute to their ability to persist on small islands.
Raccoons were primarily nocturnal, inactive for 75% of day locations, and active for 93% of night locations. Daily denning usually occurred within 150 m of shore, and up to a maximum of 750 m inland. Active locations were concentrated on the shoreline at VP, but occurred both on the shoreline and within the boundaries of the seabird colony on EL. The nocturnal behaviour of raccoons and their association with the shoreline bring them in contact with breeding seabirds, and their activity within the colony indicates a response to seabirds. Raccoon predation on seabirds was characterized and measured using site investigations, predation transects and carcass monitoring. Eleven of 15 site investigations tied active raccoon locations to recent evidence of predation on seabirds, which included dug burrows, broken eggs and single or multiple headless carcasses. Predation levels were estimated by walking 7, 20-m-wide belt transects (17% of colony area) every 3 days, from April 9 to June 8 1991 . Six headless carcasses, 77 feather piles, 30 broken eggs, 2 chicks and 12 dug burrows were found on the transects, representing the loss of approximately 488 adults and 188 juveniles. Extrapolation of carcass numbers, corrected for losses due to scavenging, yielded an estimated 252 breeding and non-breeding adults killed by raccoons during the period of study (approx. 1 seabird/raccoon/night). Losses due to raccoon predation likely exceed recruitment, and could cause substantial declines in colony size, or extinction of the colony.
I surveyed 61 islands in 1990 and 1991 for evidence of raccoons, using latrine and spotlight circuits. Raccoons have reached at least 35 of these islands, requiring overwater crossings of up to 950 m +/- 100 m. Thus, approximately 80% of all colonies and 75% of the burrow-nesting seabird population of the Queen Charlotte Islands are potentially at risk. The accessibility of many colony islands to raccoons, and the severity of raccoon predation on burrow-nesting seabirds, indicate a pressing need for monitoring and control efforts aimed at preventing the raccoon's colonization of seabird islands.