Management and Conservation Article Population Growth and Demography of Common Loons in the Northern United States JASON S. GREAR, 1 Atlantic Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, 27 Tarzwell Drive, Narragansett, RI 02882, USA MICHAEL W. MEYER, Science Services, Wisconsin Department of Natural Resources, 107 Sutliff Avenue, Rhinelander, WI 54501, USA JOHN H. COOLEY, JR., Loon Preservation Committee, P.O. Box 604, Moultonboro, NH 03254, USA ANNE KUHN, Atlantic Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, 27 Tarzwell Drive, Narragansett, RI 02882, USA WALTER H. PIPER, Department of Biology, Chapman University, 1 University Drive, Orange, CA 92866, USA MATTHEW G. MITRO, Science Services, Wisconsin Department of Natural Resources, 2801 Progress Road, Madison, WI 53716, USA HARRY S. VOGEL, Loon Preservation Committee, P.O. Box 604, Moultonboro, NH 03254, USA KATE M. TAYLOR, Loon Preservation Committee, P.O. Box 604, Moultonboro, NH 03254, USA KEVIN P. KENOW, Upper Midwest Environmental Sciences Center, United States Geological Survey, 2630 Fanta Reed Road, La Crosse, WI 54603, USA STACY M. CRAIG, LoonWatch Program, Sigurd Olson Environmental Institute, Northland College, 1411 Ellis Avenue, Ashland, WI 54806, USA DIANE E. NACCI, Atlantic Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, United States Environmental Protection Agency, 27 Tarzwell Drive, Narragansett, RI 02882, USA ABSTRACT We used recent developments in theoretical population ecology to construct basic models of common loon (Gavia immer) demography and population dynamics. We parameterized these models using existing survival estimates and data from long-term monitoring of loon productivity and abundance. Our models include deterministic, 2-stage, density-independent matrix models, yielding population growth- rate estimates (l) of 0.99 and 1.01 for intensively studied populations in our Wisconsin, USA, and New Hampshire, USA, study areas, respectively. Perturbation analysis of these models indicated that estimated growth rate is extremely sensitive to adult survival, as expected for this long-lived species. Also, we examined 20 years of count data for the 2 areas and evaluated support for a set of count-based models of population growth. We detected no temporal trend in Wisconsin, which would be consistent with fluctuation around an average equilibrium state but could also result from data limitations. For New Hampshire, the model set included varying formulations of density dependence and partitioning of stochasticity that were enabled by the annual sampling resolution. The best model for New Hampshire included density regulation of population growth and, along with the demographic analyses for both areas, provided insight into the possible importance of breeding habitat availability and the abundance of nonbreeding adults. Based on these results, we recommend that conservation organizations include nonbreeder abundance in common loon monitoring efforts and that additional emphasis be placed on identifying and managing human influences on adult loon survival. (JOURNAL OF WILDLIFE MANAGEMENT 73(7):1108–1115; 2009) DOI: 10.2193/2008-093 KEY WORDS common loon, count-based population model, demography, density dependence, Gavia immer, matrix population model, population growth rate. Common loons (Gavia immer) are among the most conspicuous species on lakes in the northern United States and Canada and are the focus of numerous summer monitoring studies by conservation organizations and government agencies. World population size for the common loon is estimated at 607,000 to 635,000 (Evers 2007), with .50% of breeding activity likely occurring in Canada. Based on Breeding Bird Surveys, population increases were described for several states in the United States during the period of 1969 to 1989 (McIntyre and Barr 1997). These increases, which translate to annual growth rates (l) from 1.03 to 1.04, may represent recovery from declines described for the mid-20th century. However, population growth in some areas of the northern United States appears to have slowed during the past 2 decades. The presence of, and explanations for, such slowing of common loon population growth deserve further attention. Potential explanations for changes in population growth include impacts from summer tourism (Robertson and Flood 1980, Hemberger et al. 1983), land-use change (Lindsay et al. 2002), acid deposition (DesGranges and Darveau 1985, Parker 1988, McNicol et al. 1995), mercury toxicity (Fimreite 1974, Kenow et al. 2007), and lead poisoning from fishing tackle (Pokras and Chafel 1992, Pokras et al. 1998). These and lesser known problems associated with water level management (DeSorbo et al. 2007), changes in availability of forage fish, fishing net entanglement (Smith and Morgan 2005), coastal oil spills (Ken Munney, United States Fish and Wildlife Service, personal communication; also see Environment Canada 2007), and disease (Brand et al. 1983, 1988; Moccia 2005) may affect specific vital rates either directly or through interactions with one another. Alternately, or in addition, changes in population growth may indicate the increasing influence of density-dependent factors as loon populations recovered from lows in the mid 1990s. This explanation would be consistent with the frequent aggressive encounters 1 E-mail: grear.jason@epa.gov 1108 The Journal of Wildlife Management N 73(7)