C. John Ralph

Density estimation in wildlife surveys

Abstract Several authors have recently discussed the problems with using index methods to estimate trends in population size. Some have expressed the view that index methods should virtually never be used. Others have responded by defending index methods and questioning whether better alternatives exist. We suggest that index methods are often a cost-effective component of valid wildlife monitoring but that double-sampling or another procedure that corrects for bias or establishes bounds on bias is essential. The common assertion that index methods require constant detection rates for trend estimation is mathematically incorrect; the requirement is no long-term trend in detection “ratios“ (index result/parameter of interest), a requirement that is probably approximately met by many well-designed index surveys. We urge that more attention be given to defining bird density rigorously and in ways useful to managers. Once this is done, 4 sources of bias in density estimates may be distinguished: coverage, closure, surplus birds, and detection rates. Distance, double-observer, and removal methods do not reduce bias due to coverage, closure, or surplus birds. These methods may yield unbiased estimates of the number of birds present at the time of the survey, but only if their required assumptions are met, which we doubt occurs very often in practice. Double-sampling, in contrast, produces unbiased density estimates if the plots are randomly selected and estimates on the intensive surveys are unbiased. More work is needed, however, to determine the feasibility of double-sampling in different populations and habitats. We believe the tension that has developed over appropriate survey methods can best be resolved through increased appreciation of the mathematical aspects of indices, especially the effects of bias, and through studies in which candidate methods are evaluated against known numbers determined through intensive surveys.

Multi-scale landscape and seascape patterns associated with marbled murrelet nesting areas on the U.S. west coast

Habitat for wide-ranging species should be addressed at multiple scales to fully understand factors that limit populations. The marbled murrelet (Brachyramphus marmoratus), a threatened seabird, forages on the ocean and nests inland in large trees. We developed statistical relationships between murrelet use (occupancy and abundance) and habitat variables quantified across many spatial scales (statewide to local) and two time periods in California and southern Oregon, USA. We also addressed (1) if old-growth forest fragmentation was negatively associated with murrelet use, and (2) if some nesting areas are more important than others due to their proximity to high quality marine habitat. Most landscapes used for nesting were restricted to low elevation areas with frequent fog. Birds were most abundant in unfragmented old-growth forests located within a matrix of mature second-growth forest. Murrelets were less likely to occupy old-growth habitat if it was isolated (> 5 km) from other nesting murrelets. We found a time lag in response to fragmentation, where at least a few years were required before birds abandoned fragmented forests. Compared to landscapes with little tono murrelet use, landscapes with many murrelets were closer to the oceans bays, river mouths, sandy shores, submarine canyons, and marine waters with consistently high primary productivity. Within local landscapes (≤ 800ha), inland factors limited bird abundance, but at the broadest landscape scale studied (3200 ha), proximity to marine habitat was most limiting. Management should focus on protecting or creating large, contiguous old-growth forest stands, especially in low-elevation areas near productive marine habitat.

DYNAMICS OF HABITAT USE BY SHOREBIRDS IN ESTUARINE AND AGRICULTURAL HABITATS IN NORTHWESTERN CALIFORNIA

Abstract We examined shorebird use of mudflats, marsh islands, and nearby agricultural fields near Humboldt Bay in northwestern California between September 1988 and April 1989. Most species used fields for both foraging and roosting, including some species usually considered to be mudflat specialists. After seasonal rains began in late fall, Dunlins (Calidris alpina), Least Sandpipers (Calidris minutilla), Long-billed Curlews (Numenius americanus), and Marbled Godwits (Limosa fedoa) became opportunists and used fields at intermediate and high tides when mudflats were inundated. Black-bellied Plovers (Pluvialis squatarola) and Greater Yellowlegs (Tringa melanoleuca) were seasonal generalists during the two wettest seasons, using fields at all tides and mudflats at low and intermediate tides. Western Sandpipers (Calidris mauri) were mudflat specialists, and Willets (Catoptrophorus semipalmatus) were salt marsh opportunists that mainly used mudflats, but shifted to salt marsh at high tide. Killdeer (Charadrius vociferus) and Common Snipe (Gallinago gallinago) were field specialists and did not use the other two habitats in significant densities during any season. The presence of short vegetation and the presence or absence of standing water were the two most important characteristics influencing increased use of fields by all species.

BREEDING SEASONS, MOLT PATTERNS, AND GENDER AND AGE CRITERIA FOR SELECTED NORTHEASTERN COSTA RICAN RESIDENT LANDBIRDS

Abstract Detailed accounts of molt and breeding cycles remain elusive for the majority of resident tropical bird species. We used data derived from a museum review and 12 years of banding data to infer breeding seasonality, molt patterns, and age and gender criteria for 27 common landbird species in northeastern Costa Rica. Prealternate molts appear to be rare, only occurring in one species (Sporophila corvina), while presupplemental molts were not detected. Most of our study species (70%) symmetrically replace flight feathers during the absence of migrant birds; molting during this period may limit resource competition during an energetically taxing phase of the avian life-cycle.

RANKING HABITAT FOR MARBLED MURRELETS: NEW CONSERVATION APPROACH FOR SPECIES WITH UNCERTAIN DETECTION

An essential element in the conservation of rare species is the ranking of some aspects of habitat quality. We developed a method to rank the importance of individual habitat patches to Marbled Murrelets (Brachyramphus marmoratus) in 26 old-growth forest stands in northern California, using estimates of stand occupancy as an index of nesting activity. We used survey data collected in the stands from 1992 to 1997. The analysis was based on an adjustment that incorporates uncertainty of detection into a binomial model. Maximum likelihood estimators were used for the proportion P of the stands occupied by murrelets and the conditional probability p of detection with each visit to an occupied survey station, and bootstrapping methods were used for error estimates. We were able to rank a single stand most important, three other stands second in importance, and eight additional stands third in importance to murrelet nesting activity. For the murrelets in our study area, these results provided information useful in negotiations between government agencies and a private company in efforts to preserve some of the stands. Our methodology also has potential application for other flora and fauna of management concern, when sampling for presence or absence with uncertain detection. This technique can be applied at a variety of scales depending upon the species and habitat. Although conservation issues require consideration of many factors, including political, social, economic, and biological, our methods are helpful in providing science-based information from sample data to assist in the decision-making process.

Land and Seascape Patterns Associated with Marbled Murrelet Abundance Offshore

Abstract We measured offshore Marbled Murrelet (Brachyramphus marmoratus) abundance from April through October between 1989 and 1998, in northern California and southern Oregon and investigated its relationships with marine and terrestrial habitats. We found that higher murrelet abundance offshore was strongly related to the presence of large, clustered and unfragmented old-growth forests on nearby inland areas. Murrelets were most abundant offshore of contiguous old-growth forest adjacent to relatively abundant medium-sized, second-growth coniferous forests. Compared to the forest habitat, marine habitat was relatively unimportant in determining murrelet abundance offshore; high marine primary productivity and nutrients were not associated with high murrelet numbers. Tidal flat shorelines were weakly associated with more murrelets, independent of inland habitat. Our findings suggest management efforts to conserve the Marbled Murrelet should focus on protecting or creating large, contiguous blocks of old-growth habitat, features which currently are rare in the study area.

STAND-SCALE HABITAT ASSOCIATIONS ACROSS A LARGE GEOGRAPHIC REGION OF AN OLD-GROWTH SPECIALIST, THE MARBLED MURRELET

Abstract We used two metrics, occupancy and relative abundance, to study forest stand characteristics believed to be important to a threatened seabird that nests in old-growth forests, the Marbled Murrelet (Brachyramphus marmoratus). Occupancy refers to murrelet presence or absence based on observed bird behaviors, while relative abundance refers to categories of low, medium, and high numbers of bird observations per survey in a forest stand. Within the murrelets nesting range in California and southern Oregon, we measured habitat and climatic variables in all old-growth stands surveyed for murrelets between 1991 and 1997. The two bird metrics produced similar results. In California, murrelets most often occupied, or were abundant in, redwood (Sequoia sempervirens) stands with large trees (>100 cm diameter at breast height) located on gentle, low-elevation slopes or on alluvial flats close to streams. In stands of the less flood-tolerant Douglas-fir (Pseudotsuga menziesii) in southern Oregon, murrelets most often occupied, or were abundant on, gentle, low-elevation, west-facing slopes that were not close to streams. Murrelets tended to use areas farther from roads. The important climatic requirements for murrelet stands in both states were cool temperatures and high amounts of rainfall.

Enhancing the value of the breeding bird survey : Reply to sauer et al. (2005)

Abstract Bart et al (2004a) proposed several approaches for enhancing the considerable value of the Breeding Bird Survey (BBS). Sauer et al. (2005) critiqued some of these approaches, and emphasized alternative goals for the survey. We agree with many of the suggestions of Sauer et al. (2005); notably that multispecies, large-scale surveys such as the BBS are most valuable for bird conservation if they achieve multiple objectives. Nevertheless, we strongly assert that estimation of long-term trends is of fundamental importance for identifying important conservation issues and determining which species represent priorities for conservation efforts, as has been repeatedly demonstrated in the past. We are confident that our recommendations for enhancing the ability of the BBS to detect trends—reducing bias, explicitly recognizing that all bias cannot be eliminated, and increasing sample size in poorly covered areas—can only enhance, and not detract from, the value of the BBS for other purposes.

Postbreeding elevational movements of western songbirds in Northern California and Southern Oregon

Abstract Migratory species employ a variety of strategies to meet energetic demands of postbreeding molt. As such, at least a few species of western Neotropical migrants are known to undergo short‐distance upslope movements to locations where adults molt body and flight feathers (altitudinal molt migration). Given inherent difficulties in measuring subtle movements of birds occurring in western mountains, we believe that altitudinal molt migration may be a common yet poorly documented phenomenon. To examine prevalence of altitudinal molt migration, we used 29 years of bird capture data in a series of linear mixed‐effect models for nine commonly captured species that breed in northern California and southern Oregon. Candidate models were formulated a priori to examine whether elevation and distance from the coast can be used to predict abundance of breeding and molting birds. Our results suggest that long‐distance migrants such as Orange‐crowned Warbler (Oreothlypis celata) moved higher in elevation and Audubons Warbler (Setophaga coronata) moved farther inland to molt after breeding. Conversely, for resident and short‐distance migrants, we found evidence that birds either remained on the breeding grounds until they finished molting, such as Song Sparrow (Melospiza melodia) or made small downslope movements, such as American Robin (Turdus migratorius). We conclude that altitudinal molt migration may be a common, variable, and complex behavior among western songbird communities and is related to other aspects of a species’ natural history, such as migratory strategy.

Decline of the Black Tern (Chlidonias niger) Population in the Klamath Basin, Oregon, 2001–2010

Abstract We monitored the Black Tern (Chlidonias niger) population at Agency and Upper Klamath Lakes, in the Klamath Basin, Oregon, from 2001–2010. We estimated that the population of adult Black Terns declined at these 2 joined waterbodies by 8.4% annually. In contrast, our analysis of Breeding Bird Survey data for the Bird Conservation Region 9/Great Basin during the same period did not detect a trend. Knowledge of local short-term trends in the Klamath Basin can contribute to understanding regional and continental population declines of this species. Water levels, which may affect both habitat availability and suitability, were relatively stable from 2001–2009, but dropped in 2010 and remained depressed from 2010–2014. The direct effect of water-level fluctuations on habitat availability (hectares of habitat loss) is not well understood. Water allocation is a management challenge in this region and has been exacerbated by recent drought conditions. In combination, understanding trends and causal factors can inform water management, restoration, and habitat protection, all of which may contribute to reversing the decline of breeding terns.