Abstract
Western and Clark’s Grebes are iconic colonial nesting waterbirds of North America most famous for their complex courtship rituals on water. Western and Clark’s Grebes breed on inland lakes from the west coast to the Great Plains of the United States including 18 U.S. States and the 4 western Canadian provinces. Western and Clark’s Grebe populations are thought to be declining across their range but rigorous monitoring data to estimate population trends are lacking. Numerous threats likely impact breeding productivity at grebe breeding colonies. We currently lack sufficient data on Western and Clark’s Grebe abundance, distribution, and population trends due to the challenges of surveying and monitoring these species. For Chapter 1, we conducted an exhaustive review and compiled published and unpublished data summarizing the status of current, recent, and historical grebe populations on breeding lakes across North America. We summarized count numbers at 770 breeding colonies for adult grebes, nests, and chicks during the breeding season including all known nesting colonies in North America over the past century to better document population status and document regional variation in abundance. We reviewed over 290 studies and summarized specific threats to breeding productivity and persistence of grebes that were identified by the authors. The largest concentrations of adult grebes (maximum count of >1000 adults) during the breeding season in any survey year were concentrated in only 5% (36) of the 770 breeding lakes identified rangewide. Most (73%) of these breeding lakes supported only small numbers of breeding adults (maximum count of <100). We hope this information will inform management of these two species of conservation concern at state, provincial, and regional scales across their range. Idaho is one of many U.S. states where Western and Clark’s Grebes are considered Species of Greatest Conservation Need. Periodic surveys of Western and Clark’s Grebes have been conducted at 3 lakes in Idaho where grebes have large nesting colonies. Cascade Reservoir supports the largest Western Grebe breeding colony in Idaho and past surveys suggest a high rate of early nest abandonment in most years but the cause of these widespread nest failures is not known. In 2020 and 2021, these nesting colonies were surveyed by a drone affixed with a high-resolution camera at an altitude that did not disturb nesting adults. Obtaining nest data by drone imagery was a non-invasive way of censusing grebe nests without researchers entering the colony. We reconstructed nest history and estimated nest fate for >3800 nests whose fate we assessed on repeat drone flights and photo imagery throughout each of two nesting seasons. For Chapter 2, we used the data from these drone flights to estimate nest survival and examine the effects of numerous environmental variables on nest fate including daily average temperature (Celsius), daily peak wind speed (km/hour), daily precipitation (cm), and daily water level (m). Our results indicate that daily peak wind direction, daily peak wind speed, daily precipitation, daily water level, and date explain variation in nest fate. Daily nest survival decreased as the nesting season progressed and was also low on days when precipitation combined with winds blew into the colony. A better understanding of the factors that affect grebe nest survival will help managers design and implement tangible actions that can potentially increase recruitment and slow or reverse grebe population declines in Idaho and elsewhere.