Nest Site Characteristics Research Articles

COMPETITIVE DISPLACEMENT? AN EXPERIMENTAL ASSESSMENT OF NEST SITE PREFERENCES OF CLIFF-NESTING GULLS

Red-legged Kittiwakes (Rissa brevirostris) are colonially breeding seabirds that are endemic to the Bering Sea, where they are sympatric with the more widely distributed Black-legged Kittiwake (R. tridactyla). Moreover, these two species breed interspersed on cliffs and potentially compete for nest sites on St. George Island, Alaska, which is the largest of only four known breeding locations of the Red-legged Kittiwake. On St. George Island, nesting ledges used by Red-legged Kittiwakes are narrower than those of Black-legged Kittiwakes, are more often located beneath overhangs, and are concentrated on high-elevation cliffs. To determine whether the nesting distribution of Red-legged Kittiwakes on St. George Island is shaped by competitive displacement by larger bodied Black-legged Kittiwakes, I compared realized niches of the two species (determined by measuring physical characteristics of natural nest sites) with fundamental niches of the two species (determined by observing nest site preferences of kittiwakes on artificial nesting ledges that differed in width, overhang cover, and elevation). The displacement hypothesis predicted that interspecific overlap in physical characteristics of nest sites would be greater for artificial than for natural nesting ledges (fundamental niches would overlap to a greater degree than realized niches). In contrast, nest site preferences of kittiwakes on artificial ledges differed markedly between species and mirrored the characteristics of natural nest sites: Red-legged Kittiwakes preferred narrower ledges, Black-legged Kittiwakes preferred wider ledges, and both species preferred ledges located within high-density clusters of conspecifics. Thus, I rejected the competitive displacement hypothesis and concluded that interspecific competition does not currently shape the breeding distributions of kittiwakes on St. George Island. Although interspecific competition in the past may have shaped current nest site preferences of kittiwakes, the concordance of habitat preferences with interspecific differences in body size, clutch size, and nest size and structure indicate that Red-legged Kittiwakes are uniquely adapted to exploit small ledges.

Differences in the intensity of nest predation in the curlew Numenius arquata: A consequence of land use and predator densities?

We quantified the intensity of nest predation in the curlew in two arable farmland areas in western Finland, as well as nest-site characteristics associated with nest predation using data on real nests (1995-1997) and artificial nests (1996). Predation on real curlew nests was consistently higher in the area of mixed farm/woodland than in the area of continuous farmland (64% versus 5% of nests depredated, respectively). There was no evidence that any of the measured nest-site characteristics was associated with high risk of nest predation and, therefore, area differences in predation on curlew nests probably were due to differences in predator density and landscape structure between the two areas. Predation on artificial nests was far higher than on real nests (79% versus 31% of nests depredated, respectively), but there was no difference in nest losses between areas. In both areas, the likelihood of predation on artificial nests was highest in the vicinity of forest edges. The majority of identified predators on real nests were mammals, whereas those on artificial nests were birds. As a result, we were unable to use artificial nests to estimate the threat of predation or to reliably identify factors related to predation risk on real nests in this study.

Nest Site Characteristics of the Grey-faced Buzzard Butastur indicus

The habitat of 52 nest sites of the Grey-faced Buzzard Butastur indicus was examined in a low mountain region of central Japan. Most (90-100%)of nest trees were within 600 m of a stream and paddy field on the valley bottom. Therefore, the availability of wet areas which offer food (e.g.frogs and snakes) may be an important factor affecting the nest site selection. Eighty-eight percent of nest trees were on the middle or lower portions of steep slopes, irrespective of the direction of the slope aspect.Sixty-nine percent of nest trees were either trees at the forest edge or prominent trees within the forest. So, the accessibility for birds approaching the nest may also be an important factor.

Reproductive success is a misleading indicator of nest-site preferences in the Red-winged Blackbird

Several methods are used to detect nest-site preferences of birds. One method postulates that individuals will prefer those nest-site features that increase their reproductive success. To determine which features affect reproductive success, and hence should be preferred, the method examines correlations between reproductive success and various nest-site features. When a statistically significant correlation is found, it is concluded that individuals exhibit a preference for that feature. In the absence of a correlation, it is concluded that individuals are indifferent to the feature. Here, I argue that this method may lead to misleading conclusions. If good sites are abundant, all individuals should choose them. Consequently, there will be little variation in the features of the chosen sites. This limited variation, when related to reproductive success, may fail to yield a statistically significant correlation and thus lead to the erroneous conclusion that individuals have no preference. An alternative method of detecting nest-site preferences is to compare features of chosen sites with features of available sites. In this study, I used both methods to investigate nest-site selection in the Red-winged Blackbird (Agelaius phoeniceus). I compared vegetation features of nest sites with those of randomly selected sampling sites and found significant differences. The differences imply that Red-winged Blackbirds prefer vegetation that grows in small patches and is tall and sparse, and the plants have thick stems. I found no relationship between these vegetation features and reproductive success. These results suggest that Red-winged Blackbirds have strong preferences for certain vegetation features, but these preferences cannot be detected from correlations with reproductive success.

Effects of Time and Nest-Site Characteristics on Concealment of Songbird Nests

We studied the effect of time and nest-site characteristics on nest concealment measurements and analyzed differences in concealment between parasitized, nonparasitized, depredated, and fledged nests. Mean concealment at nests of three old-field bird species was best explained by bird species, nest plant, and height of the nest. Nests lost concealment over time, particularly those placed high in shrubs or roses (Rosa spp.). Mean and minimum concealment did not explain occurrence of predation or brood parasitism for any of the three bird species, and concealment at parasitized versus unparasitized nests and depredated versus fledged nests did not change differently over time. A literature review showed that most studies of real passerine nests using visual nest concealment have taken measurements after nest termination, and few studies indicated that concealment was important in explaining nest predation or brood parasitism. Late concealment measurements may be an additional source of error in nesting studies, especially if predation or parasitism is more likely to occur at nests sharing similar vegetation characteristics.

Costs and benefits of nest cover for ptarmigan: changes within and between years

Selecting an ‘optimal’ nest site is believed to be an important component of fitness for birds, but paradoxically there is often considerable intraspecific variation in nest sites. Few studies have tested whether nest-site selection by birds changes after experience with nest predators. Using data collected over 9 years, we examined within- and between-year variation in 331 nests of white-tailed ptarmigan,Lagopus leucurus. We found the first direct evidence for a fitness trade-off with amount of cover at the nest; concealed sites were less often detected by predators but were more risky for incubating hens. Small ptarmigan and those in good body condition used sites with less cover, while large ptarmigan nested at higher elevations. Nesting success was not related to age or spring body condition. Responses to experience with predators were less clear. Successful females tended to use less cover the following year, whereas unsuccessful females tended to change cover types, but not the amount of cover, in subsequent years. Overall, characteristics of nest sites showed strong seasonal patterns, suggesting that microclimate may be a more important selective pressure than predation.

Effect of predation by skuas on breeding success of the Cape petrel Daption capense at Nelson Island, Antarctica

Breeding success of Cape petrels at Nelson Island (South Shetland Islands) in 1991/1992 averaged 29%. Predation by skuas accounted for 76% of all nesting failures. Direct attacks prevailed over opportunistic predation. Intensity of observer disturbance alone could not account for the inter-colony variation in predation rate, which tended to increase with colony size, presence of nearby breeding skuas and local availability of other prey species (penguins). Predation was significantly lower on nests that could be attacked only, or more easily, from the air and nests with close neighbours. Selective predation on nests in which relatively narrower eggs were laid was independent of nest-site characteristics and laying date, suggesting an effect of parental age/experience on breeding success through antipredator behaviour. Development of oil-spitting behaviour in growing chicks balanced the parental attendance and effort in nest defence, which declined as chick age increased. Increased predation on lighter but not smaller chicks may reflect the lower ability of such chicks to defend themselves by spitting stomach oil. Seasonal variations of predation rate on Cape petrel nests did not result from predator swamping by numbers of simultaneously active nests, but rather reflected temporal availability of alternative prey. Success of all phases of nesting declined with later laying date.

Antipredator benefits of nest-site selection in Black-legged Kittiwakes

We investigated the relationship between nest-site characteristics and breeding performance of Black-legged Kittiwakes (Rissa tridactyla) on Great Island, Newfoundland, during 2 years of extreme predation pressure. The nest-site feature most consistently associated with egg and chick survival was rock overhang above the nest. The presence of rock projections beside nests was positively correlated with success in 1993, and rock protrusions below the nest were not associated with breeding success in either year. Central position in the colony was positively correlated and nest density negatively correlated with success in 1992. Consistent differences in chick production among colony regions reflected differences in large-scale cliff structure and predator accessibility. This study implicates large- and fine-scale aspects of nest-site selection by Black-legged Kittiwakes, with large-scale cliff structure being the most important.

Nest-Site Characteristics and Reproductive Success in Magellanic Penguins (Spheniscus magellanicus)

We used cross-sectional, longitudinal, and experimental data to investigate the effects of habitat at the smallest spatial scale-the nest site-on reproductive success of Magellanic Penguins (Spheniscus magellanicus). Over an eight-year period, the amount of nest cover was positively correlated with fledging success. The same pairs tended to be more successful when they had more nest cover, and experimental increases and decreases in cov- er significantly affected survival of nest contents. Other characteristics of nest sites, such as nest type and type of vegetation over the nest, did not affect success. The positive effect of cover resulted mainly from reduced exposure of nest contents to predators during incuba- tion and to high temperatures when chicks were young. Roof cover was positively correlated with fledging success in nests from all areas. Cover on the sides of the nest giving the most protection from the sun was positively correlated with fledging success in warmer sites and with survival of young chicks in all areas. Young chicks at nests with less cover were more likely to move from their nests and to die on hot days. Experimental results indicated that the likelihood of egg detection by predators decreased with increasing nest cover. Height of nest entrance was a significant predictor of egg loss, suggesting that accessibility of nest contents to predators was an important component of predation risk. Thermal properties of nests and risk of predation were related; predation of nest contents was more likely when adults were absent, and during hot weather adults were more often absent from nests with little cover. Although the effect of cover on success was small relative to the large yearly variation in success due to food conditions, cover is likely to influence lifetime reproductive success substantially. Large and long-term data sets and experimental approaches may be necessary to identify subtle but biologically important factors among long-lived organisms that inhabit variable environments. Received 18 November 1996, accepted 29 May 1997.

Variation in Spotted Owl Nest Site Characteristics within the Eastern Cascade Mountains Province in Washington

Variation in Spotted Owl Nest Site Characteristics within the Eastern Cascade Mountains Province in Washington

Potential Competition for Nest Sites Between Feral European Honeybees (Apis mellifera) and Common Brushtail Possums (Trichosurus vulpecula).

The potential for feral Honeybees (Apis mellifera) to competitively exclude Common Brushtail Possums (Trichosurus vulpecula) from tree hollows was examined in the You Yangs Regional Park, Victoria. The characteristics and occupancy of 77 hollow-bearing trees and 250 hollows were recorded in six 2 ha sites and used to compare the extent of overlap in nest site selection between bees and posssums. Colonies of feral A. mellifera occupied 25 % of all hollow-bearing trees and 8 % of useable hollows, yielding a density of 1.66 colonies per ha, the highest recorded so far in Australia. Trichosurus vulpecula utilised 74 % of hollow-bearing trees and 48 % of useable hollows. Nest site characteristics of bees and possums overlapped in several dimensions, especially in the size of tree and height of nest. Relatively few vacant hollows were suitable for T. vulpecula, whereas many were available to Honeybees. Only 35% of bee nests were in hollows unsuitable for possums, indicating a relatively high potential for competition.

The importance of using the proper methodology and spatial scale in the study of habitat selection by birds

Nest-site preferences of birds have traditionally been inferred from correlations between reproductive success and various habitat attributes. Because there are several problems associated with this method, we examined preferences by comparing characteristics of actual nest sites with those of sites available to females at the time of their settlement. We examined these preferences on two spatial scales using the marsh-nesting Red-winged Blackbird (Agelaius phoeniceus). On the broad spatial scale, univariate analyses established that female settlement was nonrandom with respect to four of five habitat characteristics. However, a multivariate analysis revealed that females settled nonrandomly only with respect to water depth (they preferred areas located in deep water). The other characteristics, including cattail density, were not important. On the narrow spatial scale, females preferred sites located in dense cattail clumps that were surrounded by unusually sparse cattail vegetation. The preference of female redwings for deep-water areas was not detected in an earlier study conducted in the same marsh using the traditional method. In addition, if we were to have examined female preferences on the broad scale only, we would have failed to recognize the importance of cattail density in nest-site selection. The results of this study demonstrate the importance of examining nest-site selection using an appropriate method and multiple spatial scales.

Geographical Trends in Clutch Size: A Range-Wide Relationship with Laying Date in American Pipits

ODUM, E. P. 1941. Annual cycle of the Black-capped Chickadee. 2. Auk 58:518-535. OJANEN, M., AND M. ORELL. 1985. Changes in the breeding parameters of the Crested Tit Parus cristatus. Ornis Fennica 62:161-167. PETERSON, B., AND G. GAUTHIER. 1985. Nest site use by cavity-nesting birds of the Caribou Parkland, British Columbia. Wilson Bulletin 97:319-331. RAPHAEL, M. G., AND M. WHITE. 1984. Use of snags by cavity-nesting birds in the Sierra Nevada. Wildlife Monographs No. 86. RENCHER, A. C. 1995. Methods of multivariate analysis. John Wiley and Sons, New York. RICKLEFS, R. E. 1969. An analysis of nesting mortality in birds. Smithsonian Contributions to Zoology 9:1-48. RUNDE, D. E., AND D. E. CAPEN. 1987. Characteristics of northern hardwood trees used by cavitynesting birds. Journal of Wildlife Management 51:217-223. SEDGWICK, J. A., AND F. L. KNOPF. 1990. Habitat relationships and nest site characteristics of cavity-nesting birds in cottonwood floodplains. Journal of Wildlife Management 54:112-124. SMITH, S. M. 1991. The Black-capped Chickadee. Cornell University Press, Ithaca, New York. SONERUD, G. A. 1989. Reduced predation by pine martens on nests of Tengmalm's Owl in relocated boxes. Animal Behaviour 37:332-334. SONERUD, G. A. 1993. Reduced predation by nest box relocation: Differential effect on Tengmalm's Owl nests and artificial nests. Ornis Scandinavica 24:249-253. STAUFFER, D. F., AND L. B. BEST. 1982. Nest site selection by cavity-nesting birds of riparian habitats in Iowa. Wilson Bulletin 94:329-337. SWALLOW, S. K., R. J. GUTIARREZ, AND R. A. HOWARD, JR. 1986. Primary cavity-site selection by birds. Journal of Wildlife Management 50:576-583.

Nest site characteristics and male reproductive success in the upland bully, Gobiomorphus breviceps (Eleotridae)

Abstract— The characteristics of male upland bullies (Gobiomorphus breviceps) and their nest sites were examined in two New Zealand streams as possible determinants of male reproductive success. There was no association between male size, nest site characteristics (depth, distance to shore, river velocity, nest rock size), and male reproductive success. Similarly, the male's parasite load had no detectable influence on reproductive success. Multiple regression analysis revealed that no combination of factors could explain the wide variation in male reproductive success. It may be that the unpredictable nature of the stream habitats of bullies has favored reduced choosiness in females, which could lead to an absence of correlations between male and nest characteristics and reproductive success.

Variation in Life-History Traits and Nest-Site Selection Affects Risk of Nest Predation in the California Gnatcatcher

-I monitored nests and reproduction of California Gnatcatchers (Polioptila californica californica) at Miramar Naval Air Station, California, in 1994 and 1995 to determine correlates of nest placement, life-history traits, and nest predation. One in four nests survived to fledging, with the majority of losses caused by predation. Predation rate over the entire nesting cycle did not change seasonally and was lower in nests with full clutch sizes than in those with submodal clutch sizes. When the nest sample was divided into thirds according to nest height and then according to rate of disturbance by investigators, the upper and lower thirds suffered greater rates of predation than the middle third, and nests disturbed most often by investigators suffered greater predation rates than those disturbed less frequently. Finally, the probability of incubation lasting the full period (at least 14 days) dropped seasonally from about 0.9 for nests initiated in mid-March to less than 0.2 for nest initiated in early July, despite there being no apparent relationship between incubation length and mean daily high or low temperatures over the laying and incubation stages. These results indicate that reproductive success is related to variation in nest-site selection and lifehistory traits, as well as to variation in environmental variables and investigator activity. Received 2 May 1996, accepted 6 December 1996. NEST PREDATION iS the primary cause of reproductive failure in land birds (Ricklefs 1969). Many investigators have shown that life-history traits and nest-site selection affect the risk of nest predation (Nice 1957; Nolan 1978; Longcore and Jones 1969; Roseberry and Klimstra 1970; Best 1978; Best and Stauffer 1980; Martin 1988, 1993; Martin and Roper 1988; Hanski and Laurila 1993; Morton et al. 1993; Seitz and Zegers 1993; Hartley and Shepard 1994), indicating that optimal values for these traits exist in some species. The California Gnatcatcher (Polioptila californica californica) is an open-nesting, nonmigratory passerine that lives in coastal sage scrub in southern California. Breeding adults are sexually dichromatic, and males and females participate in all aspects of parental care. Nests of California Gnatcatchers in San Diego County increase seasonally in height and relative height (nest height relative to height of nest plant ), and nest height increases with height of the nest plant (Sockman unpubl. data). Nest concealment, however, apparently does not in' Present address: Department of Zoology, Washington State University, Pullman, Washington 99164, USA. E-mail: ksockman@wsu.edu crease seasonally despite the fact that foliage density increases seasonally. I attribute these findings to seasonal differences in nest placement (increasing nest height) that offset changes in overall vegetation density and serve to maintain an optimum level of concealment based on microclimate requirements and the need to minimize nest predation. Support for this hypothesis relies on data showing changes in risk of nest predation with respect to nest placement. Here, I provide these data in addition to data on basic life-history traits and how these traits affect reproductive success. Concern for the California Gnatcatcher, which is listed as a threatened species, has resulted in studies aimed at understanding its requirements. Yet, published accounts of their life history are scant. I examined fecundity, reproductive success, and duration of the incubation and nestling stages for seasonal variation and for correlations among these traits. Using these results and data on nest placement, I tested whether rate of nest predation: (1) differs seasonally and according to nest-plant species (hereafter substrate), nest-site characteristics, and disturbance by investigators (see Major 1990); (2) increases with increasing clutch size and brood size (because nestlings and parental

Breeding density and nest site selection in a Goshawk Accipiter gentilis population of the Central Apennines (Abruzzo, Italy)

Information on forest vegetation structure immediately around a Goshawk's nest can indicate the minimum forest stand surface necessary to be conserved or created to favour Goshawk settlement and help solve potential conflicts between timber harvest and maintenance of a viable Goshawk population. In order to gain greater insight into this species and to contribute to a correct policy for managing forests, Goshawk nest site characteristics were examined at three levels of scale (nest/nest trees level, nest site level, landscape level). The breeding density recorded in the study area is 5.03 sites/100 km2. The nests were found on north- and east-facing slopes and in the central part of the tree canopy. The average values of crown height and surface diameter at breast height (dbh) were higher than the corresponding values recorded in nest plots. The average values of dbh, tree height, height of trunk without branches, crown height, crown length, crown breadth, crown basal area, crown volume, distance between trunks and plot centre canopy cover were significantly higher in nest plots than in control plots. Some characteristics of the nest site were consistent among nest sites even though analysis of the landscape surrounding the nest site showed a great variety of habitat composition and structure.

Factors Affecting the Success of Colony Departure by Thick-Billed Murre Chicks

Young Thick-billed Murres leave their natal colony while still incapable of flight, jumping directly to the sea. The brief period of transition from cliff ledge to open ocean results in significant mortality. At Coats Island, Northwest Territories, Canada, we investigated the effects of predation by gulls, murre nest site characteristics, and adult murre aggression on the likelihood that chicks departed to sea. Approximately 20% of chicks that jumped from the cliff failed to depart successfully to sea. Live chicks were rarely taken by predators. Most mortality was caused by drowning, as a result of aggression by adult murres. However, chicks accompanied by defending parents usually were unaffected by aggression from other adult murres. As a result, the most important cause of chick mortality at colony departure was the separation of chick and parent before reaching the sea. Chicks landed alone because either they or their parent were knocked from the nest ledge during fights with neighbors as they moved to the jump site, or because chicks struck cliff ledges during the descent to the water. Apparently, the nearer the nest site was to the edge of the ledge, and the fewer obstructions below the jump site, the greater the departure success of chicks. Site characteristics that determine success during departure may differ from those associated with success in incubation and chick-rearing. Hence the effect of breeding site on reproduction may be incorrectly assessed if departure success is not taken into account.

Breeding ecology of the FulmarFulmarus glacialisand the KittiwakeRissa tridactylain high‐arctic northeastern Greenland, 1993

The breeding ecology of the FulmarFulmarus glacialisand the KittiwakeRissa tridactylain the high Arctic was studied in relation to the occurrence of the northeast water polynya in northeasternmost Greenland (80d̀N). Mean laying dates were 31 May in the Fulmar and 18 June in the Kittiwake; the total nesting season for the Fulmar just matched the time window of the polynya opening period. Fulmar colony attendance fluctuated within a period of 11.6 days because of variation in nonbreeding prospectors but showed no clear diurnal variation. Fulmar incubation shifts, on average, lasted 6.1 days (range 1–13 days), which is significantly longer than elsewhere, and the average chick‐guard period of 10.9 days (range 1–17 days) was significantly shorter than in other studies. Egg neglect occurred in 18% of Fulmar nests or 0.7% of nests per day. Overall breeding success (chicks fledged per egg laid) was 0.56 in the Fulmar and 0.67 in the Kittiwake; the latter produced 1.4 young per active nest or 1.2 per completed nest. Mean Kittiwake clutch size was 2.03; larger clutches were laid early. Nest site characteristics (presumably reflecting nest predation risk) and breeding behaviour affected breeding success. in the Fulmar, hatching success was negatively correlated with laying date and the proportion of egg neglect, while overall breeding success was correlated negatively with distance to nearest neighbouring site and positively with the length of the chick‐guard period. Kittiwake breeding success was negatively correlated with laying date. Using seabirds as indicators of marine food supply, breeding success in both species suggested moderate to good food supply in the northeast water polynya in 1993, although at least in the Fulmar the high reproductive output appeared partly maintained by behavioural buffering; long incubation shifts, egg neglect and short chick‐guard periods were symptoms of foraging constraints.

Effects of murre nest site characteristics and wind conditions on predation by glaucous gulls

Glaucous gulls, Larus hyperboreus, are the primary predator of thick-billed murre, Uria lomvia, eggs at arctic breeding colonies. This study used eggs placed experimentally on murre nesting ledges to test the effects on glaucous gull foraging success of (i) murre nesting density; (ii) murre group defence; (iii) nesting ledge width; (iv) timing of murre laying; and (v) wind speed. Defensive responses by murres increased egg survival times, and gull foraging success was reduced by high murre nesting density. The timing of murre egg laying and ledge width also influenced the effectiveness of murre defence within the colony. Eggs placed following the peak of murre egg laying survived longer than eggs placed on the same nest sites prior to the peak. Murres without eggs early in the season often abandoned the group when attacked, leaving experimental eggs and brooding murres vulnerable to gulls. Gulls had difficulty reaching exposed eggs on narrow ledges when foraging on foot. At wind speeds greater than 15 km/h, however, egg survival was independent of ledge width because wind increased the maneuverability of gulls in flight and enabled them to reach narrow ledges. Murre defence at high nesting densities was also less effective on narrow ledges. Apparently, murres had difficulty rotating on their nest sites to face attacking gulls without dislodging their eggs. We conclude that moderate wind speeds enhanced the ability of gulls to reach narrow ledges and avoid contact with defending murres while stealing eggs. Consequently, the vulnerability of murre nest sites depended on wind conditions at the colony.

Nest-Site Selection and Reproductive Success in Common Ravens

Studies of habitat selection by animals can be categorized in two ways: behavioral and evolutionary (Krebs 1994). Most studies of habitat selection consider the behavioral perspective whereby biotic and/ or abiotic components of the environment that appear to influence habitat use are identified, generally by comparing used sites with random or available sites (e.g. Mosher et al. 1986, Seamans and Gutierrez 1995). The evolutionary approach examines the effects of selecting particular habitats on an index of fitness, e.g. survivorship or reproduction (see Martin and Roper 1988, Petit and Petit 1996). If nest-site selection is a heritable trait, then natural selection should favor individuals that choose nest sites that confer greater reproductive success. Common Ravens (Corvus corax) are suitable subjects for examination of reproduction in relation to habitat characteristics owing to their variable clutch size, which ranges from three to seven eggs (Dunk et al. unpubl. data). Thus, in any one year, a relatively large range in number of young fledged is possible within a population, and it may be possible to detect a cline in reproductive success relative to nest-site characteristics. Common Ravens are widely distributed throughout North America and Europe. In western North America, ravens have been characterized as pests that easily adapt to human-modified landscapes (Butchko 1990, Boarman 1993, Marzluff et al. 1994). During the past 25 years, raven numbers have increased in many areas of the western United States (Boarman 1993, Dunk et al. 1994, Marzluff et al. 1994). Despite their ubiquitous nature, large numbers, and broad geographic range, little is known about many aspects of the breeding biology of ravens. In particular, very little has been published on raven nest-site selection. Ravens nest on many substrates, including cliffs (Ratcliffe 1962, White and Cade 1971, Hooper 1977, Skarphedinsson et al. 1990), highway overpasses and billboards (White and Tanner-White 1988), churches (Heinrich 1989), power poles (Knight and Kawashima 1993), and trees (Dorn 1972, this study), but nest-site characteristics have been quantified only for cliff nests (White and Cade 1971, Hooper 1977).