The distribution of,and relation among,mercury and methylmercury,organic carbon,carbonate, nitrogen and phosphorus,in periphyton of the South Florida ecosystem

Periphyton samples from Water Conservation Areas, Big Cypress National Preserve, and Everglades National Park in south Florida were analyzed for concentrations of total mercury, methylmercury, nitrogen, phosphorus, organic carbon, and inorganic carbon. Concentrations of total mercury in periphyton decrease slightly along a gradient from north‐to‐south. Both total mercury and methylmercury are positively correlated with organic carbon, nitrogen and phosphorus in periphyton. In horizontal sections of periphyton mats, total mercury concentrations tend to be largest at the tops and bottoms of the mats. Methylmercury concentrations tend to be the largest near the bottom of mats. These localized elevated concentrations of methylmercury suggest that there are “hot spots”; of methylmercury in periphyton.     

Genetic structure of the southeastern United States loggerhead turtle nesting aggregation: evidence of additional structure within the peninsular Florida recovery unit

The southeastern United States supports one of two large loggerhead turtle (Caretta caretta) nesting aggregations worldwide and is therefore critical to global conservation and recovery efforts for the species. Previous studies have established the presence of four demographically distinct nesting populations (management units) corresponding to beaches from (1) North Carolina through northeastern Florida, (2) peninsular Florida, (3) the Dry Tortugas, and (4) northwest Florida. Temporal and geographic genetic structure of the nesting aggregation was examined utilizing partial mitochondrial control region haplotype frequencies from 834 samples collected over the 2002 through 2008 nesting seasons from 19 beaches as well as previously published haplotype data. Most rookeries did not exhibit interannual genetic variation. However, the interannual variation detected did significantly impact the interpretation of spatial genetic structure in northeastern Florida. Based on pairwise F _ST comparisons, exact tests of population differentiation, and analysis of molecular variance, the present study upholds the distinctiveness of the four currently recognized management units and further supports recognition of discrete central eastern, southern (southeastern and southwestern), and central western Florida management units. Further subdivision may be warranted, but more intensive genetic sampling is required. In addition, tools such as telemetry and mark-recapture are needed to complement genetic data and overcome limitations of genetic markers in resolving loggerhead turtle rookery connectivity in the southeastern USA.     

Wood Thrush Populations Are Not All Sinks in the Agricultural Midwestern United States

Abstract: Conservation strategies for forest-breeding, Neotropical migratory birds focus on the identification and preservation of source habitats. In sources, populations maintain relatively high levels of reproductive success, whereas in sinks, reproductive success is consistently below net replacement rate. In agricultural regions of the midwestern United States, forest patches of various sizes appear to act as sinks for Neotropical migrants. In 1994–1996 I monitored Wood Thrush (   Hylocichla mustelina ) nests in four forest patches in an agricultural landscape in northern Indiana. Given the prevailing estimates of adult and juvenile survival, I report productivity data that show substantial annual variation and that are sufficient to suggest the maintenance of temporal sources. Indeed, some Wood Thrush populations may act as source habitats in regions with relatively high levels of Brown-headed Cowbird (   Molothrys ater ) parasitism and nest predation because pairs produce multiple clutches and have the ability to fledge cowbirds and their own young from parasitized nests. My results and those from other studies of Wood Thrushes in landscapes with fragmented forests illustrate the variable ways in which source-sink dynamics are affected by habitat fragmentation. In some agricultural regions, populations in small forest patches may act as sources for Wood Thrushes and may be a legitimate focus of conservation efforts aimed at some migratory songbirds.     

Estimation of age at maturation and growth of Atlantic green turtles (Chelonia mydas) using skeletochronology

Despite the vast amount of research on threatened and endangered green turtle populations, some uncertainty regarding stage durations, growth rates, and age at maturation remains. We used skeletochronology to address this gap in knowledge for green turtle populations in the North Atlantic Ocean that use coastal waters along the southeastern U.S. as developmental habitat. Oceanic stage duration was estimated at 1–7 years ( [`(\textX)] \overline{\text{X}}  = 3 years). Several growth models, including von Bertalanffy, logistic, Gompertz, and power functions were evaluated for describing sex-specific length-at-age data. Ages at maturation estimated using mean size at nesting for females from each genetic sub-population contributing juveniles to this neritic foraging area were 44 years (Florida), 42.5 years (Costa Rica), and 42 years (Mexico), which were higher than previously reported ages. This implies that nesting populations comprising primarily individuals utilizing foraging grounds in the southeastern U.S. may take longer to recover than previously estimated.     

Fine-scale distribution of juvenile Atlantic and Mediterranean loggerhead turtles (Caretta caretta) in the Mediterranean Sea

Loggerhead turtles nesting in the Mediterranean Sea exhibit remarkable genetic structuring. This paper tests the hypothesis that young loggerhead turtles from different rookeries do not distribute homogeneously among the major Mediterranean foraging grounds, due to a complex pattern of surface currents. We extracted long fragments of mitochondrial DNA from 275 stranded or bycaught juvenile turtles from six foraging grounds (Catalano-Balearic Sea, Algerian basin, Tyrrhenian Sea, Adriatic Sea, northern Ionian Sea and southern Levantine Sea). We used a Bayesian mixed-stock analysis to estimate the contributions from rookeries in the Mediterranean, the North-west Atlantic and Cape Verde to the studied foraging grounds. Differences were found in the relative contribution of juvenile turtles of Atlantic and Mediterranean origin to each foraging ground. A decreasing proportion of Atlantic juveniles was detected along the main surface current entering the Mediterranean, with a high prevalence of turtles from eastern Florida in the Algerian basin and lower numbers elsewhere. In regard to the turtles of Mediterranean origin, juveniles from Libya prevailed in central and western Mediterranean foraging grounds other than the Algerian basin. Conversely, the Adriatic Sea was characterised by a large presence of individuals from western Greece, while the southern Levantine Sea was inhabited by a heterogeneous mix of turtles from the eastern Mediterranean rookeries (Turkey, Lebanon and Israel). Overall, the distribution of juveniles may be related to surface circulation patterns in the Mediterranean and suggests that fisheries might have differential effects on each population depending on the overlap degree between foraging and fishing grounds.     

Seasonal patterns in size and abundance of <Emphasis Type="Italic">Phyllorhiza punctata</Emphasis>: an invasive scyphomedusa in coastal Georgia (USA)

Phyllorhiza punctata, commonly called the Australian white spotted jellyfish, invaded the Caribbean in the 1960s, becoming established there and subsequently in the United States in the northern Gulf of Mexico (by 2000) and eastern Florida (2001). With the prevailing Loop Current flowing clockwise around the Gulf of Mexico and joining the Gulf Stream along eastern Florida, potential transport of P. punctata along the eastern seaboard of the USA could be facilitated. P. punctata medusae were collected in small numbers along the entire Georgia coast during May–November in 2007 and 2008. Medusa bell diameters increased both years from ca. 10 cm in May to ca. 33 cm in autumn. Specimens lacked zooxanthellae, as reported for medusae in the northern Gulf of Mexico and Florida. It is possible that the P. punctata medusae observed were transported from established populations to the south; however, whether or not this species is established along the Georgia coast has yet to be determined.     

Foraging habitats and migration corridors utilized by a recovering subpopulation of adult female loggerhead sea turtles: implications for conservation

From 1998 to 2008, 68 adult female loggerhead sea turtles (Caretta caretta) were instrumented with platform transmitter terminals at nesting beaches in Georgia, North Carolina (NC) and South Carolina (SC) on the East Coast of the United States of America (30°48′N, 81°28′W to 33°51′N, 77°59′W). The majority of post-nesting loggerheads (N = 42, 62 %) migrated to foraging habitats in the Mid-Atlantic Bight during May–October, with a subsequent migration occurring during November–March to foraging habitats south of Cape Hatteras, NC. Nine (13 %) loggerheads initially foraged in the near-shore, coastal areas of the South Atlantic Bight, but moved to offshore habitats—closer to the Gulf Stream—during November–March, while fourteen (21 %) loggerheads remained in foraging areas along the mid-continental shelf off of the eastern coast of Florida and/or continued southward to Florida Bay and the Bahamas. The present study delineates important, post-nesting foraging habitats and migration corridors where loggerheads may interact with commercial fisheries—providing managers opportunities to develop and implement optimally effective conservation actions for the recovery of this threatened species.     

Effects of parasites on foraging and defense behavior of a termitophagous ant,Pheidole titanis Wheeler (Hymenoptera: Formicidae)

Summary Pheidole titanis Wheeler, an ant that occurs in desert and deciduous thorn forest in the southwestern United States and western Mexico, is a predator on termites. In the dry season well-coordinated raids against termite foraging parties occur early in the morning or late in the afternoon, whereas in the wet season most raids occur at night. This seasonal shift in the timing of raids is due to the increased activity of a fly (Diptera: Phoridae) that is a specialist parasitoid on P. titanis workers and soldiers. When parasitic flies discover P. titanis nest entrances or raiding columns, workers stop foraging and defend themselves against oviposition attacks. Flies are only active during the day and never interfere with foraging at night. However, P. titanis does not increase the frequency of raids at night and, as a result, colonies collect less food in the wet season compared to the dry season. Presence of parasitic flies also interferes with normal defense behavior of P. titanis against conspecific and heterospecific enemy ants. Dissections of P. titanis workers and soldiers suggest that the parasitism rate by flies is less than 2% and observations indicate that parasitic flies are much rarer than their host workers and soldiers. Nonetheless, these parasites exert a strong ecological impact on their host.     

Feeding habits and phenotypic changes in proboscis length in the southern oyster drill, Stramonita haemastoma (Gastropoda: Muricidae), on Florida sabellariid worm reefs

The southern oyster drill, Stramonita (= Thais, Kool 1987) haemastoma, is a common intertidal and subtidal predator in the southeastern United States. It uses specialized feeding structures and foraging strategies to bore holes through the shell of its bivalve prey. However, on the east coast of Florida, S. haemastoma, is common on sabellariid worm reefs constructed by the polychaete Phragmatopoma lapidosa (Walton Rocks Beach, Florida, 27°17′N, 80°12′W), a habitat where the snail’s typical prey are scarce. From 1999 to 2001, we examined the feeding habits of S. haemastoma on sabellariid reefs and the behavioral and morphological responses of S. haemastoma that accompanied switching from a diet of bivalves to sabellariids. On worm reefs S. haemastoma feeds on P. lapidosa by inserting the proboscis deep into a worm’s tube. Worm-feeding snails had longer proboscises (~3.7 times shell height) than bivalve-feeding conspecifics (~2.0 times shell height). Snails raised on different diets showed significant differences in proboscis length suggesting that the proboscis length is phenotypically plastic. Whereas typical oyster drills must bore holes for days before ingesting prey, S. haemastoma on worm reefs avoids boring and attacks and consumes worms in 15–50 min. In the laboratory, oyster drills consumed 1.7 worms per day, spending <1 h each day feeding. On sabellariid reefs, differences in feeding, handling costs, and prey value, are likely to have a significant effect on the ecology and life history of S. haemastoma in this habitat.     

Climate Change,Multiple Stressors,and the Decline of Ectotherms

Climate change is believed to be causing declines of ectothermic vertebrates, but there is little evidence that climatic conditions associated with declines have exceeded critical (i.e., acutely lethal) maxima or minima, and most relevant studies are correlative, anecdotal, or short‐term (hours). We conducted an 11‐week factorial experiment to examine the effects of temperature (22 °C or 27 °C), moisture (wet or dry), and atrazine (an herbicide; 0, 4, 40, 400 μg/L exposure as embryos and larvae) on the survival, growth, behavior, and foraging rates of postmetamorphic streamside salamanders (Ambystoma barbouri), a species of conservation concern. The tested climatic conditions were between the critical maxima and minima of streamside salamanders; thus, this experiment quantified the long‐term effects of climate change within the noncritical range of this species. Despite a suite of behavioral adaptations to warm and dry conditions (e.g., burrowing, refuge use, huddling with conspecifics, and a reduction in activity), streamside salamanders exhibited significant loss of mass and significant mortality in all but the cool and moist conditions, which were closest to the climatic conditions in which they are most active in nature. A temperature of 27 °C represented a greater mortality risk than dry conditions; death occurred rapidly at this temperature and more gradually under cool and dry conditions. Foraging decreased under dry conditions, which suggests there were opportunity costs to water conservation. Exposure to the herbicide atrazine additively decreased water‐conserving behaviors, foraging efficiency, mass, and time to death. Hence, the hypothesis that moderate climate change can cause population declines is even more plausible under scenarios with multiple stressors. These results suggest that climate change within the noncritical range of species and pollution may reduce individual performance by altering metabolic demands, hydration, and foraging effort and may facilitate population declines of amphibians and perhaps other ectothermic vertebrates. Cambio Climático, Estresantes Múltiples y la Declinación de Ectotermos     

Impact of minimum winter temperatures on the population dynamics of Dendroctonus frontalis.

Predicting population dynamics is a fundamental problem in applied ecology. Temperature is a potential driver of short-term population dynamics, and temperature data are widely available, but we generally lack validated models to predict dynamics based upon temperatures. A generalized approach involves estimating the temperatures experienced by a population, characterizing the demographic consequences of physiological responses to temperature, and testing for predicted effects on abundance. We employed this approach to test whether minimum winter temperatures are a meaningful driver of pestilence from Dendroctonus frontalis (the southern pine beetle) across the southeastern United States. A distance-weighted interpolation model provided good, spatially explicit, predictions of minimum winter air temperatures (a putative driver of beetle survival). A Newtonian heat transfer model with empirical cooling constants indicated that beetles within host trees are buffered from the lowest air temperatures by approximately 1-4 degrees C (depending on tree diameter and duration of cold bout). The life stage structure of beetles in the most northerly outbreak in recent times (New Jersey) were dominated by prepupae, which were more cold tolerant (by >3 degrees C) than other life stages. Analyses of beetle abundance data from 1987 to 2005 showed that minimum winter air temperature only explained 1.5% of the variance in interannual growth rates of beetle populations, indicating that it is but a weak driver of population dynamics in the southeastern United States as a whole. However, average population growth rate matched theoretical predictions of a process-based model of winter mortality from low temperatures; apparently our knowledge of population effects from winter temperatures is satisfactory, and may help to predict dynamics of northern populations, even while adding little to population predictions in southern forests. Recent episodes of D. frontalis outbreaks in northern forests may have been allowed by a warming trend from 1960 to 2004 of 3.3 degrees C in minimum winter air temperatures in the southeastern United States. Studies that combine climatic analyses, physiological experiments, and spatially replicated time series of population abundance can improve population predictions, contribute to a synthesis of population and physiological ecology, and aid in assessing the ecological consequences of climatic trends.     

Simulating a natural fire regime on an Atlantic coast barrier island complex in Florida, USA

The HFire fire regime model was used to simulate the natural fire regime (prior to European settlement) on Kennedy Space Center, Merritt Island National Wildlife Refuge, Canaveral National Seashore, and Cape Canaveral Air Force Station, Florida. Model simulations were run for 500 years and the model was parameterized using information generated from previously published empirical studies on these properties (e.g., lightning fire ignition frequencies and ignition seasonality). A mosaic pattern of frequent small fires dominated this fire regime with rare but extremely large fires occurring during dry La Niña periods. This simulated fire size distribution very closely matched the previously published fire size distribution for lightning ignitions on these properties. A sensitivity analysis was performed to establish which parameters were most influential and the range of variation surrounding empirically parameterized model output. Dead fuel moisture and wind speed had the largest influence on model outcome. A wide range of variance was observed surrounding the composite simulation with the least being 6% in total burn frequency and the greatest being 49% in total area burned. Because simulation modeling is the best option for fire regime reconstruction in many rapidly growing shrub dominated systems, these results will be of interest to scientists and fire managers for delineating the natural fire regime on these properties, the southeastern United States and other fire adapted shrub systems worldwide.     

Can forest management be used to sustain water-based ecosystem services in the face of climate change?

Forested watersheds, an important provider of ecosystems services related to water supply, can have their structure, function, and resulting streamflow substantially altered by land use and land cover. Using a retrospective analysis and synthesis of long-term climate and streamfiow data (75 years) from six watersheds differing in management histories we explored whether streamflow responded differently to variation in annual temperature and extreme precipitation than unmanaged watersheds. We show significant increases in temperature and the frequency of extreme wet and dry years since the 1980s. Response models explained almost all streamflow variability (adjusted R2 > 0.99). In all cases, changing land use altered streamflow. Observed watershed responses differed significantly in wet and dry extreme years in all but a stand managed as a coppice forest. Converting deciduous stands to pine altered the streamflow response to extreme annual precipitation the most; the apparent frequency of observed extreme wet years decreased on average by sevenfold. This increased soil water storage may reduce flood risk in wet years, but create conditions that could exacerbate drought. Forest management can potentially mitigate extreme annual precipitation associated with climate change; however, offsetting effects suggest the need for spatially explicit analyses of risk and vulnerability.     

Obligate Cave Fauna of the 48 Contiguous United States

Abstract: We assembled a list of obligate cave-dwelling species and subspecies, their county distribution, and their provisional global conservation rank. A total of 927 species and 46 additional subspecies in 96 families exclusively from cave and associated subterranean habitats have been described in the 48 contiguous states of the United States. The terrestrial (troglobitic) species are concentrated in northeast Alabama (especially Jackson County), with other concentrations in Kentucky, Texas, Virginia, and West Virginia. Only 23 counties, comprising less than 1% of the land area of the 48 contiguous states, account for over 50% of the terrestrial species and subspecies. The aquatic (stygobitic) species are concentrated in Hays County, Texas, with other concentrations in Florida, Oklahoma, Texas, Virginia, and West Virginia. Only 18 counties, comprising less than 1% of the land area, account for over 50% of the aquatic species and subspecies. Endemism is high, with 54% of the species known from a single county. Approximately 95% of the species are listed by The Nature Conservancy as vulnerable or imperiled in the United States. These cave species comprise 50% of all vulnerable or imperiled species listed in databases of the Natural Heritage Program. Less than 4% of these subterranean species have federal status. Conservation can best be accomplished through habitat protection, which must include protection of the associated surface habitat.     

Conservation Science and Forest Service Policy for Roadless Areas

Abstract:  Questions persist regarding whether the science of conservation biology can successfully affect environmental decision making. One of the most prominent fields of intersection between conservation science and environmental policy is public-lands debates in the United States. I reviewed the role of conservation science in the roadless-area policies of the U.S. Forest Service. Since 1971, the Forest Service has systematically evaluated roadless areas on national forests three times, most recently during the Clinton administration's Roadless Area Conservation Review (1998–2000) ( U.S. Department of Agriculture Forest Service 2000b ). Drawing on the agency's environmental impact statements and supporting documents and the internal records of conservation organizations, I examined the changing goals, methodology, and outcome of roadless-area advocacy and policy. Since the 1970s, conservation science has successfully informed public and administrative concern for roadless-area protection. Conservation science has transformed public discourse regarding roadless areas and has changed the scope and rationale of national conservation organizations' goals for roadless-area policy from protecting some to protecting all remaining national forest roadless areas. The Forest Service has increasingly drawn on the lessons of conservation biology to justify its methodology and its administrative recommendations to protect roadless areas. The 2000 Roadless Area Conservation Review resulted in a recommendation to protect all remaining national forest roadless areas, up from 22% of roadless areas in the first roadless review. Despite the scientific merits of recent roadless-area advocacy and policy, however, such initiatives have faced political difficulties. The emphasis on large-scale, top-down, national approaches to conservation policy has rendered such policies politically problematic.     

Specimen‐Based Modeling,Stopping Rules,and the Extinction of the Ivory‐Billed Woodpecker

Abstract: Assessing species survival status is an essential component of conservation programs. We devised a new statistical method for estimating the probability of species persistence from the temporal sequence of collection dates of museum specimens. To complement this approach, we developed quantitative stopping rules for terminating the search for missing or allegedly extinct species. These stopping rules are based on survey data for counts of co‐occurring species that are encountered in the search for a target species. We illustrate both these methods with a case study of the Ivory‐billed Woodpecker (Campephilus principalis), long assumed to have become extinct in the United States in the 1950s, but reportedly rediscovered in 2004. We analyzed the temporal pattern of the collection dates of 239 geo‐referenced museum specimens collected throughout the southeastern United States from 1853 to 1932 and estimated the probability of persistence in 2011 as <6.4 × 10^?5, with a probable extinction date no later than 1980. From an analysis of avian census data (counts of individuals) at 4 sites where searches for the woodpecker were conducted since 2004, we estimated that at most 1–3 undetected species may remain in 3 sites (one each in Louisiana, Mississippi, Florida). At a fourth site on the Congaree River (South Carolina), no singletons (species represented by one observation) remained after 15,500 counts of individual birds, indicating that the number of species already recorded (56) is unlikely to increase with additional survey effort. Collectively, these results suggest there is virtually no chance the Ivory‐billed Woodpecker is currently extant within its historical range in the southeastern United States. The results also suggest conservation resources devoted to its rediscovery and recovery could be better allocated to other species. The methods we describe for estimating species extinction dates and the probability of persistence are generally applicable to other species for which sufficient museum collections and field census results are available.     

Conservation of Stream Fishes: Patterns of Diversity, Rarity, and Risk

Abstract: North America has a rich fauna of freshwater fishes which attains greatest diversity in the central and southeastern United States. Many stream fishes have limited ranges and are locally rare and patchily distributed. Local diversity increases downstream and total diversity follows typical species-area relationships. Drainages linked with the large Mississippi system support more species than those of comparable area flowing directly to the sea and rivers isolated by falls have notably few species. The between-drainage component of diversity is large. Threats to this fauna, which are not addressed by management focused on threatened species, include fragmentation of drainage networks by impoundments and homogenization of faunas by interbasin connections and introductions. Conservation efforts require a biogeographic perspective; they should focus on streams of intermediate size (orders 4–6) plus the upstream portion of each drainage and should attempt to maintain total diversity and, inclusively, populations of rare or threatened species.     

Cover Caption

Cover: Tree pincushion (Leucospermum conocarpodendron viridum), a member of the family Proteaceae and the largest member of its genus, Cape of Good Hope National Park, South Africa. On pages 593‐601, Tucker et al. explore differences in the species richness, phylogenetic diversity, and evolutionary distinctiveness of Proteaceae species in the Cape Floristic Region of South Africa. They found that species richness and phylogenetic diversity typically were highly correlated, but sites with high phylogenetic diversity were more spatially extensive than sites with high species richness. Rare species generally had high levels of evolutionary distinctiveness. Sites near the southern edge of the Cape Floristic Region were occupied by species with high levels of evolutionary distinctiveness and limited geographic ranges, but those sites were not assigned high conservation‐priority rankings on the basis of species richness or phylogenetic distinctiveness. Photographer: Photographer and conservationist Jaime Rojo ( http://www.jaime‐rojo.com ) believes visual storytelling is a powerful tool for conservation. Since moving to Mexico from his native Spain 8 years ago, Jaime has worked with diverse environmental nongovernmental organizations while developing his career as a photographer. He has participated in numerous conservation initiatives including the promotion of El Carmen Big Bend Conservation Corridor, a transboundary protected area along the border between Mexico and the United States; a campaign to protect the San Pedro Mezquital, the last free‐flowing river in the western Sierra Madre; and the celebration of WILD9, the ninth World Wilderness Congress, for which he was executive director in Mexico. Rojo is a trustee of Th e WILD Foundation and an associate of the International League of Conservation Photographers.     

Mitochondrial control region sequence analyses indicate dispersal from the US East Coast as the source of the invasive Indo-Pacific lionfish Pterois volitans in the Bahamas

Lionfish are popular aquarium fish from the Indo-Pacific that have invaded the western Atlantic. Two species, Pterois volitans and P. miles, were well established along the United States east coast before the first lionfish were reported from the Bahamas in 2004, where they quickly dispersed throughout the archipelago by 2007. The source of the Bahamian lionfish invasion has been in question because of the hypothesized low connectivity between Florida and Bahamas reef species as well as the temporal lag in their arrival in the Bahamas. Mitochondrial control region haplotypes (680 bp) were determined and analyzed for lionfish specimens from the Bahamas, North Carolina, and two sites within their native range (Indonesia and the Philippines). Exact tests, pairwise F _st and AMOVA analyses all showed no significant differentiation between the Bahamas and North Carolina specimens. The similarity between the Bahamas and North Carolina lionfish was also reflected in a minimum spanning network and neighbor-joining distance tree generated from the data. Sequence analyses also revealed the presence of only Pterois volitans, as no P. miles were detected in the Bahamian sample. These results indicate that the source of the Bahamian lionfish is egg and larval dispersal from the United States east coast population, and support previous models of reef fish dispersal that suggest a low level of connectivity between the Bahamas and east coast of Florida.     

The Impact of Brown-Headed Cowbird Parasitism on Populations of the White-Crowned Sparrow

Brown-headed Cowbirds, Molothrus ater , are brood parasites on many species of North American Passerines. Cowbird parasitism is frequently cited as a contributing factor in the decline of United States passerine populations, based on three lines of evidence. First, range expansion and population increases by Cowbirds in this century are dramatic and well documented. Second, increasing rates of parasitism have been discovered in a variety of host species populations. Finally, direct negative effects of parasitism on certain vulnerable species notably Kirtland's Warbler, Dendroica kirtlandii , have been demonstrated. It remains unproven, however, that Cowbirds have significant negative effects on more abundant and widely-distributed hosts.
We report here on the impact of Cowbird parasitism on populations of Nuttall's White-crowned Sparrow, Zonotrichia leucophrys nuttalli , in the San Francisco Bay Area. This widespread songbird is now subject to a 40–50% rate of parasitism in San Francisco, an increase from 5% only 15 years ago. Using known mortality and fecundity values, we calculate that this population of White-crowns cannot maintain its numbers when the parasitism rate exceeds approximately 20%. Present levels of parasitism thus appear to threaten the longevity survival of this adaptable Songbird in the San Francisco Bay Area.