research paper Wader Study 124(1): 000 –000. doi:10.18194/ws.00053 Recent Snowy Plover population increase arises from high immigration rate in coastal northern California Mark A. Colwell 1*, Elizabeth J. Feucht 1, Matt J. Lau 1, David J. Orluck 1, Sean E. McAllister 2 & Amber N. Transou 3 1Wildlife Department, Humboldt State University, Arcata, CA 95521, USA 2Sean E. McAllister & Associates, Eureka, CA 95503, USA 3California State Parks, North Coast Redwoods District, Eureka, CA 95503, USA *Corresponding author: [email protected] Colwell, M.A., E.J. Feucht, M.J. Lau, D.J. Orluck, S.E. McAllister & A.N. Transou. 2017. Recent Snowy Plover population increase arises from high immigration rate in coastal northern California. Wader Study 124(1): 000 –000. The Pacific coast population of the Snowy Plover Charadrius nivosus nivosus is Keywords listed as threatened under the U.S. Endangered Species Act, which requires Snowy Plover demographic data to inform management directed at increasing the population. Accordingly, we summarized a 16-year dataset on population size and growth, Charadrius nivosus return rates, and productivity of a color-marked breeding population in coastal population growth northern California, one of six recovery units for the listed population. The geo - graphically isolated population varied annually in size (19–74 breeding adults), threatened with an early nine-year decline ( λ = 0.92 ± 0.30 ) followed by a seven-year increase vital rates (λ = 1.22 ± 0.19 ). Overall, productivity averaged 0.85 ± 0.29 chicks fledged per male, which is well below that identified by viability analyses to maintain the population. Adult return rates, an index of survivorship, varied greatly among years (30–95%). Immigrants comprise 63 ± 5% of the population over the past 12 years when we have marked approximately 95% of breeding adults. We conclude that the northern California population is a demographic sink, and that management actions currently are insufficient to increase the population; recent growth stems from immigration. INTRODUCTION of individually-marked birds has provided valuable data to inform conservationists of population genetic structure Conservation of biodiversity, whether at the level of (Koenig 1988), or to evaluate captive-rearing efforts (Neu - ecosystem, community or species, requires detailed knowl - man et al . 2016) or actions to lethally remove predators edge derived from monitoring to inform management to bolster populations (Dinsmore et al . 2014, 2017). decisions. Of the world’s ~10,600 species of bird (Clements 2007), 220 are shorebirds; 59 of these shorebird species In 1993, the United States government listed the Pacific require conservation action based on IUCN criteria (e.g., coast population of the Snowy Plover Charadrius n. Near reatened, Vulnerable, Endangered or Critically nivosus as threatened (United States Fish & Wildlife Endangered; http://www.iucnredlist.org/ ); many (17) are Service 1993; hereaer USFWS) based on numerous plovers (Charadriidae). e causes of this extinction risk factors, including evidence of a small and declining pop - are varied, but most are associated with humans. For ulation (Page & Stenzel 1981, Page et al . 1991). e instance, the Spoon-billed Sandpiper Calidris pygmaea recovery plan (USFWS 2007) specified that delisting for population is estimated to be several hundred; hunting this species hinges on two demographic metrics: (1) pop - in the species’ winter range is a principal cause of its Crit - ulation exceeding 3,000 breeding adults sustained for 10 ically Endangered status (Zöckler et al . 2010). For endemic consecutive years; and (2) per-capita reproductive success shorebirds of New Zealand (e.g., Black Stilt Himantopus exceeding 1.0 fledged young (per male) for five years. novaezelandiae , Wrybill Anarhynchus frontalis ), populations Since the species’ listing almost a quarter century ago, a are imperiled owing to introduced predators that have number of studies have been initiated to provide data increased adult mortality and lowered reproductive success necessary to evaluate these criteria. For example, in 2005 (Dowding & Murphy 2001). In many cases, monitoring USFWS personnel began an annual, coordinated, range- 2 -- Wader Study 124(1) 2017 Fig. 1. Breeding locations of Snowy Plovers along ~80 km of ocean-fronting beaches and ~15 km of gravel bars along the Eel River in coastal northern California. Filled and open circles (500 m radius) represent the presence or absence, respectively, of breeding based on at least one nest over 16 years ( n = 917 nests; 2001–2016 in Humboldt County). Some stretches of rocky intertidal habitat were not surveyed owing to unsuitability of habitat for breeding. Occasional nesting occurred in Mendocino ( n = 9) and Del Norte ( n = 3) counties (not shown). Colwell et al. l Snowy Plover breeding population demography -- 3 wide effort to estimate population size, which has provided permit TE-73361A-1; USFWS banding permits #23844 insight into population growth in relation to management and #10457, HSU IACUC 14/15.W.07-A), and state (CA actions (Eberhart-Phillips et al . 2015). At a finer spatial Dept. Fish & Wildlife Scientific Collecting permit #SC0496; scale, research across the Pacific coast range of the plover CA Dept. Parks & Recreation research permit #16-635- is required to quantify vital rates (e.g., apparent survivorship, 017) permits. annual and lifetime reproductive success) necessary to In the field, observers surveyed habitat for breeding predict population growth (Stenzel et al . 2007, 2011, plovers from March into September. We surveyed most Mullin et al . 2010, Herman & Colwell 2015), as well as to sites once per week, but visits increased (oen 2–3 a evaluate effectiveness of management actions aimed at week) when we detected breeding adults. e last date ameliorating factors that limit population recovery (e.g., on which chicks fledged varied annually (mid-Aug to Dinsmore et al . 2014, 2017, Eberhart-Phillips et al . 2015). mid-Sep), which determined our last survey, and, hence, Here, we provide details on the vital rates of a small, geo - duration of breeding season. Observers worked mostly graphically-isolated subpopulation of Snowy Plovers that alone to survey suitable habitat (e.g., sandy ocean-fronting we have monitored for 16 years (2001–2016). beaches or gravel bars; not rocky intertidal habitats) by walking slowly, stopping occasionally, and using binoculars METHODS and a spotting-scope to scan for plovers. At a few sites occupied consistently by more plovers, observers oen Study area surveyed in pairs. Occasionally observers used all-terrain We studied Snowy Plovers in Del Norte, Humboldt and vehicles to survey long stretches of beach. When observers Mendocino counties, California (Fig. 1), which is Recovery detected plovers, they observed them from a distance to Unit 2 of the species’ recovery plan (USFWS 2007). e evaluate their breeding status (e.g., courting, copulating, delisting criteria for the northern California subpopulation scraping, incubating, brooding). Also, we routinely are 150 breeding adults maintained for 10 years and 1.0 scanned sandy substrates for plover tracks and courtship fledgling per male for 5 years (USFWS 2007). is is the scrapes in dry, sand substrates to indicate the presence smallest subpopulation within the listed population of breeding activity. is was not possible on riverine segment (USFWS 2007, Eberhart-Phillips et al . 2015). gravel bars. Plovers bred in two types of habitat. roughout the 16 In 2005, we began using a personal data assistant (PDA) years, plovers have bred on ocean-fronting beaches char - equipped with a global positioning system (GPS) to acterized by fine, homogeneous (i.e., sandy) substrates record locations of adults, nests, and broods. We sum - sparsely vegetated with native (e.g., Leymus mollis , Abronia marized the presence of plover breeding activity (Fig. 1) spp., Umbellata breviflora ) and invasive ( Ammophila are - based on the occurrence of at least one nest within an naria , Carpobrotus spp.) plants. Early in the study, plovers array of 500 m radius circles established in a systematic- also bred on coarse, heterogeneous (i.e., gravel) substrates random fashion using a geographic information system of the lower Eel River, amidst sparse vegetation dominated (GIS). Observers routinely determined the reproductive by willow ( Salix spp.) and white sweet clover Melilotus status by observing adults courting or tending eggs or albus . Detailed descriptions of habitats are provided else - chicks. When we did not detect an adult incubating a where (Colwell et al . 2010, Brindock & Colwell 2011, clutch, we approached to a distance where we could Herman & Colwell 2015). ese two habitats differ determine with binoculars whether eggs were present. markedly in habitat quality as gauged by per-capita repro - We categorized a nest as successful if at least one egg ductive success (Colwell et al . 2010, Herman & Colwell hatched. We determined that a clutch failed if eggs dis - 2015), with riverine substrates affording greater crypsis, appeared, were buried or abandoned prior to the predicted and hence survival, of eggs and chicks (Colwell et al . hatch date based on the sequence of egg laying or egg 2007a, 2011). flotation (Westerkov 1950). Observations of adults re- nesting oen confirmed our determination that a repro - Field methods ductive attempt had failed. For failed nests, we categorized Intensive monitoring began in 2001. Each subsequent the