J. Field Ornithol., 72(4):556±572 CORVID SURVEY TECHNIQUES AND THE RELATIONSHIP BETWEEN CORVID RELATIVE ABUNDANCE AND NEST PREDATION JOHN M. LUGINBUHL,JOHN M. MARZLUFF AND JEFFREY E. BRADLEY College of Forest Resources, University of Washington, Seattle, Washington 98195 USA MARTIN G. RAPHAEL U.S. Forest Service, Paci®c Northwest Research Station, Olympia, Washington 98512 USA DANIEL E. VARLAND Rayonier, Hoquiam, Washington 98550 USA Abstract.ÐWe conducted a four-year study on the Olympic Peninsula of Washington to assess the relationship between corvid (Gray Jay [Perisoreus canadensis], Steller's Jay [Cyanocitta stelleri], American Crow [Corvus brachyrhynchos] and Common Raven [Corvus corax]) abun- dance and the risk of nest predation. We assessed risk of predation through the use of arti®cial mid-canopy nests and assessed corvid abundance using a variety of techniques in- cluding point-count surveys, transect surveys, and the broadcast of corvid territorial and predator attraction calls. Point counts of corvid abundance had the strongest correlation with predation on arti®cial nests containing eggs. The relationship between nest predation rate and corvid abundance was strongest when study plots were used as replicated measures of landscape conditions rather than as independent samples. We suggest using the maximum value for each corvid species attained from several temporally replicated point-count surveys in each study plot. Corvid point-counts should be conducted on days with light winds (,20 kph) and no more than light precipitation. Use of attraction calls is important for gaining a meaningful measure of corvid abundance. Their use may overrepresent corvids at the local plot scale but is important in assessing the landscape scale presence of wide-ranging (Amer- ican Crows) and often non-vocal (Gray Jays) corvids. TEÂ CNICAS DE ENCUESTAS DE COÂ RBIDOS Y LA RELACIOÂ N ENTRE ABUNDANCIA RELATIVA DE ESTOS Y DEPREDACIOÂ N DE NIDOS Sinopsis.ÐDurante cuatro anÄos llevamos a cabo un estudio en la PenõÂnsula OlõÂmpica de Wash- ington para determinar la relacioÂn entre la abundancia de cuervos (Perisoreus canadensis), (Cya- nocitta stelleri), (Corvus brachyrhynchos), (Corvus corax) y el riesgo de la depredacioÂn de nidos. Evaluamos el riesgo de depredacioÂn a traveÂs del uso de nidos arti®ciales colocados en la estrata media de arboladas. AdemaÂs, de terminamos la abundancia de coÂrbidos utilizando teÂcnicas va- riadas que incluyeron conteos de puntos, transectos y el uso de grabaciones de la llamada terri- torial de coÂrbidos y las llamadas de atraccioÂn de depredadores. Los conteos de puntos de abun- dancia de coÂrbidos tuvieron la mayor correlacioÂn de depredacioÂn de nidos arti®ciales con huevos. La relacioÂn entre la tasa de depredacioÂn de nidos y la abundacia de coÂrbidos tuvo mayor fortaleza cuando las aÂreas de estudios fueron utilizadas como reÂplicas de medidas de las condiciones del paisaje en vez de muestras independientes. Sugerimos el uso del valor maÂximo de cada especie de coÂrbido, obtenido de varias reÂplicas de puntos de conteo en cada estacioÂn de estudio. Los puntos de conteo de coÂrbidos se deben conducir en dõÂas con poco viento (menor a 20 kph y no maÂs de una precipitacioÂn liviana. El uso de grabaciones de llamadas es de gran signi®cado para determinar la abundancia de los coÂrbidos. Su uso pudiera sobre representar los coÂrbidos en la escala de un punto particular, pero es importante para ®jar, en la escala de paisaje, la presencia de especies de amplia distribucioÂn o de poca vocalizacioÂn. Nest predation is pervasive and arguably the most important factor limiting avian productivity (Nice 1957; Ricklefs 1969; Wilcove 1985; Mar- 556 Vol. 72, No. 4 Counting Corvids [557 tin 1993a,b). Corvids are ef®cient nest predators (Angelstam 1986; Marz- luff and Balda 1992) whose populations have increased dramatically in the western United States (Marzluff et al. 1994) and in urban areas world- wide (Fraissinet 1989; Konstantinov 1996) over the last century. This in- crease may have important implications for the conservation of open- nesting birds, because many studies have shown a positive correlation between various indices of corvid abundance and predation rates on near- by nests (Angelstam 1986; Johnson et al. 1989; Andren 1992). The poten- tial for corvids to limit other bird populations poses a need for a consis- tent method to assess corvid numbers and index the risk of nest preda- tion. Such a method would allow managers to assess the risk of corvid predation simply by surveying corvids. Development of this method requires a thorough understanding of the biotic and abiotic factors that affect our ability to detect corvids, whether measures of corvid abundance correlate with nest predation, and the spatial scale at which any correlation exists. To this end, we conducted a four-year study (1995±1998) to assess the relationship between corvid (Gray Jay [Perisoreus canadensis], Steller's Jay [Cyanocitta stelleri], Ameri- can Crow [Corvus brachyrhynchos] and Common Raven [Corvus corax]) abundance and corvid nest predation. We examined a variety of survey protocols to determine what factors affect corvid surveys and how well corvid abundance predicts the rate of nest predation at simulated Mar- bled Murrelet (Brachyramphus marmoratus) nests. Marbled Murrelets are federally threatened birds that nest in the middle to upper canopy of mature coastal forests of the Paci®c Northwest (Ralph et al. 1995b). Mar- bled Murrelets suffer high nest predation rates, and corvids (Common Ravens and Steller's Jays) are suspected to have caused the majority of known nest failures (Nelson and Hamer 1995a; Miller et al. 1997). METHODS Study area.ÐThe study area was located on the western side of the Olympic Peninsula of Washington State, north and south of Forks, Wash- ington (478569N, 1248239W) in the Hoh, Soleduck, Quinault, and Queets River drainages. Study plots consisted of mixed-conifer forest ranging in age from 80±250 yr and in size from 37±106 ha. Plot elevations varied between 45 and 600 m. The study area is adjacent to the major concen- tration of murrelets in Washington (Varoujean and Williams 1995) and is in a landscape used substantially by nesting murrelets (Horton and Harrison 1997). We conducted experiments and surveys in a total of 49 plots in 12 landscape categories. Landscape categories consisted of all possible com- binations of two classes of forest fragmentation, three classes of forest structure, and two classes of proximity to human-use areas. Fragmentation classes were fragmented (plot .75% surrounded by clear-cuts) and con- tinuous (plot .75% surrounded by mature forest). Levels of plot frag- mentation (and plot size) varied somewhat due to the limited number of suitable forest patches occuring ``naturally'' throughout the study area. 558] J. M. Luginbuhl et al. J. Field Ornithol. Autumn 2001 TABLE 1. Summary of experimental design. Numbers in the table refer to study plots in each treatment. Forest structure Very complex old Simple mature Complex mature growth (80±120 yr) (80±120 yr) (.200 yr) Distance from human use areas ,1km .5km ,1km .5km ,1km .5km Surrounding forest landscape Fragmented 5 4 5 4 3 5 Continuous 3 4 3 3 5 5 Fragmented plots were delineated using existing forest edges, while con- tinuous plots were more arbitrarily delineated from within larger, more continuous forest landscapes. Structure classes were simple mature (80± 120 yr, canopy single storied with few gaps), complex mature (80±120 yr, canopy 1±2 storied with many gaps), and very complex old growth (.200 yr, canopy multi-storied with many gaps). The two proximity to human- use area classes were near (,1 km from plot edge) and far (.5 km from plot edge). We de®ned ``human-use'' as any human development that could potentially attract corvids (i.e., towns, farms, campgrounds, high- ways, and dumps; Table 1). Each plot (spatial replicate) was examined in 2±4 subsequent years (temporal replicates). Arti®cial nest experiments.ÐWe climbed trees and placed nests using techniques that minimize disturbances that might cue predators to the nest's location, such as damage to the bark from spurs or human scent trails left from touching the bole or limbs. We followed Perry (1978) to place ®shing line, then 4-mm cord, and ®nally an 11-mm static climbing rope, into a likely nest tree. We avoided contact with the tree by climbing with ascenders and rappelling to the ground. We wore latex or vinyl gloves while taking measurements and preparing the nest. We marked nest trees on the ground with white plastic ¯agging hung in a random direction approximately 3 m from the tree. The entire process of climbing, placing nests, and rappelling took approximately 90 min. Murrelet nests are easy to mimic because eggs are laid in simple de- pressions on moss-covered branches, eggs are sometimes left unattended for several hours during incubation, and nestlings are left alone for much of the day after they reach3dofage(Nelson and Hamer 1995b; Manley 1999). We simulated nests at typical heights and locations for murrelets on moss-covered branches with diameter .11 cm, within the live crown, .15 m above the ground, well covered from above (xÅ% overhead cover 5 83.0%, SE 5 0.46, n 5 671) and close to the trunk (xÅ distance to bole 5 40.2 cm, SE 5 0.93, n 5 671; Marshall 1988; Singer et al. 1991; Hamer and Nelson 1995). We placed a total of 905 arti®cial murrelet nests (Table 2). Nests were placed in separate trees at least 50 m apart, and only six nests were placed Vol. 72, No. 4 Counting Corvids [559 TABLE 2. Summary of nest distribution within experimental design. Numbers in table refer to total nests and, in parentheses, nests with eggs in each treatment. Forest structure Simple mature Complex mature Very complex old growth (80±120 yr) (80±120 yr) (.200 yr) Distance from human use areas ,1km .5km ,1km .5km ,1km .5km Surrounding forest landscape Fragmented 90 68 88 69 64 109 (45) (35) (42) (35) (31) (55) Continuous 36 98 47 36 100 100 (18) (48) (24) (18) (50) (50) in a given plot at any one time.