Disturbance and roosting ecology of California brown pelicans (Pelecanus occidentalis californicus) on East Sand Island in the Columbia River Estuary Public Deposited

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  • The California brown pelican (Pelecanus occidentalis californicus) is listed as an endangered species under the U.S. Endangered Species Act. The Recovery Plan for this subspecies outlines the need to "Assure long-term protection of adequate food supplies and essential. . .roosting. . . habitat throughout the range." I investigated the effects of disturbance to California brown pelicans roosting on East Sand Island in the Columbia River estuary to determine if disturbances were degrading the quality of the roost site. During the summers of 2001 and 2002, I examined potential differences in pelican numbers, distribution, and behavior on East Sand Island before and after disturbances caused by research activity, non-research anthropogenic factors, and natural factors. East Sand Island was the site of the largest known post-breeding roost for California brown pelicans, with peak numbers of 10,852 counted on the island in September 2002. Year, date, and tide height accounted for over 90% of the variation in pelican numbers on East Sand Island. Research activity on the island was the only category of disturbance that had a detectable effect on the total number of pelicans roosting on East Sand Island. In 2001 the magnitude of research disturbance had a marginally significant negative association with total pelican numbers (P = 0.09), but in 2002 there was no significant association (P = 0.31). Research activity and non-research anthropogenic disturbance were negatively associated with pelican numbers in localized parts of the roost near the west end of East Sand Island, where these disturbance factors were most prevalent. I measured the effects of disturbance on the behavior of roosting California brown pelicans by collecting time-activity budget data for pelicans roosting in a study plot that encompassed 136 m of shoreline. All three categories of disturbance affected the time-activity budgets of roosting pelicans, as indicated by changes during the half hour following a disturbance: the proportion of resting pelicans decreased and the proportion of attentive pelicans increased. Disturbances caused by research activity on the island appeared to have a greater and longer lasting impact on time-activity budgets of pelicans roosting in the study plot, compared to non-research anthropogenic disturbances (i.e., watercraft) or natural disturbances (mostly by bald eagle fly-bys). Although natural disturbances did not appear to have as pronounced an effect as did anthropogenic disturbances on the numbers, distribution, and behavior of pelicans roosting on East Sand Island, natural disturbances may gradually deter pelicans from using the island as a roost. Bald eagles caused more pelicans to flush from East Sand Island than all other types of disturbance combined. Bald eagle disturbance rates (pelicans flushed/hour) steadily increased from 2001 to 2003. It would be useful to monitor use of East Sand Island by nomadic sub-adult bald eagles, which use the island more than breeding adults, to determine trends. In addition, the island should be carefully monitored to detect any large mammalian predators that access the island and all large predators should be removed immediately to prevent abandonment of the island by roosting and nesting colonial waterbirds. East Sand Island is closed to the public to protect breeding and roosting waterbirds from unrestricted human disturbance. Recreational boaters should be educated that the island is closed during the seasons when waterbirds are aggregated on the island so they understand and are more likely to comply with the restrictions. Although researchers took several precautions to minimize disturbance to brown pelicans, as stipulated by a protocol approved by the U.S. Fish and Wildlife Service (USFWS), there was still an apparent negative effect of this source of human activity on pelican numbers and behavior. Unrestricted access by researchers would certainly have a larger impact than observed during this study, so restrictions should continue. Researchers should only access beaches with low pelican densities (Camp to Dike, North and East Beaches; unless otherwise specifically permitted by USFWS), access the west end bird blinds only within 2 hours of low tide, and postpone or cancel any research activities that could potentially flush greater than 15% of the pelicans roosting on East Sand Island.
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