• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

A Possible Role for Red Squirrels in Structuring Breeding Bird Communities in Lodgepole Pine Forests

Posted on: Sunday, 19 February 2006, 03:03 CST

By Siepielski, Adam M

Abstract.

Nest predation is thought to play an important role in structuring certain breeding bird communities. One potential consequence of nest predation is lower recruitment in breeding birds, which may be manifested as lower breeding bird abundance. Lodgepole pine (Pinus contorta ssp. latifolia) forests east and west of the Rocky Mountains became isolated following glacial retreat 12 000 years ago and differ in whether or not red squirrels (Tamiasciurus hudsonicus), which are a key nest predator, are present. Breeding bird abundance in lodgepole pine forests was compared between four ranges with red squirrels and four ranges without red squirrels. Species grouped into canopy and understory nesting guilds were, on average, two and three times more abundant, respectively, in forest ranges without red squirrels than in ranges with red squirrels; no statistically significant differences were found for midstory, ground, or cavity nesters. These results suggest that geographic variation in the presence or absence of red squirrels is likely important in structuring breeding bird communities in lodgepole pine forests across the landscape.

Key words: bird abundance, breeding birds, community structure, nest predators, Pinus contorta ssp. latifolia, Rocky Mountains, Tamiasciurus hudsonicus.

Posible Papel de las Ardillas Tamiasciurus hudsonicus en la Estructuracin de las Comunidades de Aves Nidificantes en Bosques Dominados por el Pino Pinus contorta latifolia

Resumen. Se cree que la depredacin de nidos juega un papel importante en la estructuracin de algunas comunidades de aves nidificantes. Una posible consecuencia de la depredacin de nidos es un menor reclutamiento, lo que podra manifestarse en trminos de una menor abundancia de aves reproductivas. Los bosques dominados por el pino Pinus contorta ssp. latifolia que se encuentran al este y al oeste de las Montaas Rocallosas se aislaron luego del desplazamiento de los glaciares hace 12 000 aos, y difieren en cuanto a la presencia de la ardilla Tamiasciurus hudsonicus, un depredador de nidos clave. Se compar la abundancia de aves nidificantes entre cuatro reas de bosque de pino con ardillas y cuatro reas sin ardillas. Las especies pertenecientes a gremios que nidifican en el dosel y el sotobosque fueron en promedio dos y tres veces ms abundantes, respectivamente, en los bosques sin ardillas que en los bosques con ardillas. No existieron diferencias significativas en la abundancia de las especies que nidifican en los estratos medios de la vegetacin, en el suelo o en cavidades. Estos resultados sugieren que la variation geogrfica en la presencia o ausencia de T. hudsonicus probablemente es importante en la estructuracin a escala de paisaje de las comunidades de aves nidificantes de los bosques de pino.

Predation is widely recognized as an important process organizing a diverse array of communities from deep-sea thermal vents (Micheli et al. 2004) to desert rodents (Kotler et al. 1994) to breeding birds (Martin 1988a, 1988b, 1996, Willson and Gende 2000). For breeding birds, nest predation is an important ecological and evolutionary force because it may account for over 50% of nest failures (Ricklefs 1969, Krebs 1970), and thus affect breeding success and possibly population abundance (George 1987). Only recently, however, have researchers given much consideration to the role that nest predation may play in structuring breeding bird communities. For example, Martin (1988a, 1988b, 1993a, 1993b, 1996) suggests that nest predation can structure breeding bird communities by causing the vertical partitioning of nesting heights to minimize density-dependent nest predation (see also Reitsma and Whelan 2000). Additional evidence has shown that nest predators sometimes specialize on one or more nest types (Martin 1988a), such that individual nest predators may be especially important to certain species within a community.

Red squirrels (Tamiasciurus hudsonicus) are a key nest predator throughout coniferous and mixed forests in North America (Boag et al. 1984, Martin 1988b, 1993a, 1993b, Sieving and Willson 1998, Martin and Joron 2003). Several studies have compared nesting success (usually with artificial nests) between areas with and without red squirrels. Although details differ among authors, these studies show that nesting success is generally lower in some forests where red squirrels are present than in forests where red squirrels are absent (Sieving and Willson 1998, Bayne and Hobson 2000, Martin and Joron 2003, Willson et al. 2003; but see Reitsma et al. 1990). Moreover, densities of nests for some understory birds are lower in some areas with red squirrels in comparison to areas without red squirrels (Willson et al. 2003). These studies suggest that red squirrels could be an important agent structuring breeding bird communities by limiting nesting success.

Here, I use an observational approach to test the hypothesis that the presence or absence of red squirrels influences the community structure of breeding birds. In western North America, lodgepole pine (Pinus conforta ssp. latifolid) forest islands east and west of the northern Rocky Mountains became isolated as a result of glacial retreat 12 000 years ago (Thompson and Kuijt 1976), and differ in whether or not red squirrels are present (Benkman 1999). The absence of red squirrels is related to historical factors (e.g., glacial retreat), not contemporary ecological or landscape-level factors that would make the areas unsuitable for colonization by red squirrels (Benkman 1999, Benkman et al. 2001). For instance, red squirrels were historically absent in one study area (the Cypress Hills, Alberta) but were introduced there in 1950 (Newsome and Dix 1968), and now occur at densities almost four times higher than in comparable habitat in the Rocky Mountains, suggesting that no intrinsic differences exist in the habitat that make it unsuitable for red squirrels (Benkman 1999). Most lodgepole pines in the study areas have serotinous cones (Benkman and Siepielski 2004), which implies similar fire histories (Muir and Lotan 1985) and stand characteristics (Brown 1975). Because of the overall similarity of study sites, densities of red squirrels should be roughly similar (Sullivan and Moses 1986). In forests with Rocky Mountain lodgepole pine, red squirrel densities average about one per hectare (Wheatley et al. 2002). Other important potential nest predators such as small mammals (Burt and Grossenheider 1980), hawks, and most corvids (Sibley 2000) have similar geographic distributions in areas with and without red squirrels, and should not differ in a consistent manner that could confound comparisons between these areas. The one exception is Steller's Jays (Cyanocitta stelleri), which are largely absent from areas without red squirrels, but occur infrequently in areas with red squirrels (Gough et al. 1998; C. Benkman and AMS, pers. obs.). These unique features of lodgepole pine forests thus provide an excellent system in which to examine the influence that a single, but important, nest predator may have on community structure for breeding birds. Lodgepole pine forests are also ecologically of great importance to breeding birds as they dominate over 20 million hectares of forest in North America (Critchfield 1980).

One consequence of higher levels of nest predation is lower recruitment, which could result in the lower abundance of individuals that are more susceptible to nest predation within communities. Indeed, lower levels of recruitment are generally associated with lower measures of bird abundance (Bock and Jones 2004). Thus, I predicted that the abundance of birds that were more likely to suffer nest predation by red squirrels would be lower in areas where red squirrels were present in comparison to areas where red squirrels were absent. Unfortunately, few data have been published regarding differential rates of nest predation in relation to natural nest locations. However, Reitsma and Whelan (2000) showed that nesting success tended to decrease with increasing nest height, and Martin and Li (1992) showed that open-cup nesting species tended to have lower nesting success. Consequently, I predicted that birds nesting above the ground in open-cup nests would have lower abundances than would ground and cavity nesters. In testing this prediction I was able to evaluate the potential role of geographic variation in the presence and absence of an important nest predator, red squirrels, in structuring breeding bird communities in lodgepole pine forests.

METHODS

Eight study sites were selected in the north-central United States and southern Canada (Benkman 1999: fig. 2). The four mountain ranges with red squirrels were the Cypress Hills in Alberta, and the Highwood, Little Belt, and Judith Mountains in Montana. The four ranges without red squirrels were the South Hills in Idaho, and the Little Rocky and Bears Paw Mountains, and the Sweetgrass Hills in Montana. The landscape was composed of islands of lodgepole pine forest surrounded by grassland steppe. Study sites were separated by a minimum of 100 km of grassland steppe to maximize independence of abundance estimates. Grassland steppe prevents re\d squirrel immigration because red squirrels are unlikely to disperse through open areas (Wauters et al. 1994).

Although vegetation was not quantified, study sites were located in mountain ranges dominated almost entirely by lodgepole pine (Benkman 1999, Benkman and Siepielski 2004, Siepielski and Benkman 2005), and all point count stations were located in mature lodgepole pine stands. Most sites were at similar latitudes and elevation, with the exception of the South Hills, Idaho site, which was located further south than the other study areas. However, other major vegetation types (i.e., canopy-forming trees, understory and midstory vegetation), topographic relief, and other habitat features (e.g., water, forest openings) were likely similar among sites. Livestock grazing, which could potentially affect nesting success of ground or understory nesting birds (Walsberg 2005) occurred in all ranges, and thus was likely not a confounding factor. There was undoubtedly some variation in local environments among sites (Benkman and Siepielski 2004), however this variation was likely greatest among sites, not between areas with and without squirrels. Although most habitat features were similar, total area of lodgepole pine habitat differed widely among sites, ranging from 4 km^sup 2^ to >500 km^sup 2^ (Siepielski and Benkman 2005).

To estimate the abundance of breeding birds in each range, I conducted 30 10-min point counts in early May-June in 2001 and 2002. The order in which both sites and individual point count locations were visited was reversed the second year to minimize potential differences associated with the timing of counts. I assumed that detection probabilities of birds were comparable among ranges because of the overall similarity of sites and because only one observer conducted all counts (Siepielski and Benkman 2005). Only birds heard or seen within 50 m over the course of the 10-min period were used in analyses. Bird abundance estimates were averaged over all counts within a site to provide one estimate of abundance for each species per site per year. Point count locations were separated by a minimum of 250 m to maintain independence of observations (Pendleton 1995), and all point counts were located at least 100 m from the forest edge to minimize edge effects. Surveys were conducted between 05:45 and 10:45 (Mountain Standard Time) on days without strong wind or rain.

Because I was interested in broad patterns, and there were relatively few replicates (mountain ranges) between areas with and without red squirrels, data were analyzed by nesting guild rather than individual species. For comparative purposes, I also present mean nesting guild abundances separately for the Cypress Hills, Alberta, where red squirrels were historically absent but introduced in 1950 (Newsome and Dix 1968). Species were assigned to a nesting guild following Ehrlich et al. (1988; Table 1). For species nesting above ground level, the mean nest height calculated from the ranges presented in Ehrlich et al. (1988) was used to assign species to one of three nest-height guilds: understory (≤, m), midstory (3-6 m), or canopy (>6 m). To calculate mean abundance for each nesting guild, individual species means within a guild were summed and divided by the number of species within a guild.

TABLE 1. Comparison of mean abundance (mean SE) of nesting guilds between four mountain ranges with and four mountain ranges without red squirrels in the north-central United States and southern Canada. The numbers in parentheses in the first column are the number of species within a guild. Means are the combined means of 30 point counts per mountain range conducted during May and June of 2001 and 2002 averaged over all species within a guild and across years. P-values are from one-way Wilcoxon rank sum tests. The "red squirrel introduced" column refers to the Cypress Hills, Alberta, where red squirrels were historically absent but were introduced in 1950. The Cypress Hills were included in the statistical comparison as a range with squirrels.

Two species (Red Crossbill [Loxia curvirostm] and Pine Siskin [Carduelis pinus}) which were more abundant in ranges without red squirrels were excluded from analyses, because differences in the abundance of these species between areas with and without red squirrels can be attributed in part to competition with red squirrels for pine seeds (Benkman 1999, Benkman et al. 2001, 2003, Siepielski and Benkman 2005; C. Benkman and AMS, pers. obs.).

STATISTICAL ANALYSES

Because variation in breeding bird abundance could be explained by habitat area, I used linear regression analysis to examine whether abundance within each nesting guild was associated with area of lodgepole pine forest using lodgepole pine habitat area estimates from Siepielski and Benkman (2005). Area was not a significant predictor of variation in breeding bird abundance (all regression models P > 0.05), and is not discussed further.

Wilcoxon rank sum tests were used to determine if there was an effect of year on nesting guild abundance. No effects of year were found (all P > 0.05), thus data were pooled across years. Wilcoxon rank sum tests were also used to compare nesting guild abundances between areas with and without red squirrels. All tests were one- way comparisons evaluating the prediction that abundance was greater in the absence of red squirrels. Lastly, in lieu of individual comparisons, the non-parametric sign test was used to examine if there was a trend for greater abundances of all open-cup and cavity nesting species in the absence of red squirrels. Significance levels were set at α = 0.05.

RESULTS

Species grouped into canopy and understory nesting guilds were, on average, almost two and three times more abundant, respectively, in ranges without red squirrels than in ranges with red squirrels (Table 1). No statistically significant differences in mean abundances of the other nesting guilds were found (Table 1). There was a significant trend for abundances of all open-cup nesting species to be greater in ranges without red squirrels (13 of 13 species were greater in ranges without red squirrels, sign test P < 0.001). This trend was not found for cavity-nesting species (4 of 9 were greater in ranges without red squirrels, sign test P > 0.05).

DISCUSSION

The lower abundances of open-cup nesting birds in the forest canopy and understory in areas where red squirrels are present suggests that geographic variation in the presence or absence of this nest predator contributes to structuring avian community assemblages in lodgepole pine forests. In combination with other studies that have shown lower nesting success in areas where red squirrels are an important nest predator (Sieving and Willson 1998, Bayne and Hobson 2000, Martin and Joron 2003, Willson et al. 2003; but see Reitsma et al. 1990), a plausible mechanism for the observed pattern is that nest predation results in lower recruitment, which is reflected by lower abundances of some nesting guilds. Indeed, a review of over 100 studies found that measures of bird abundance are often lower in areas where reproductive success is lower (Bock and Jones 2004).

Although the results of this study are consistent with others, it cannot be stated that there is an "effect of squirrels" because this study was observational, rather than experimental. It is possible that red squirrel presence could have other impacts on breeding birds, or that other factors that covary with the presence and absence of red squirrels produced the observed pattern. For example, red squirrels are known to occasionally kill adult passerine birds (Sullivan 1991). However, several experimental studies have compared nesting success of birds between areas where red squirrels have been removed and control plots where they have not. Bayne and Hobson (2000) found higher levels of artificial nest success in boreal forest habitats where red squirrels were removed, and Martin and Joron (2003) found lower nest success for birds on islands where mature coniferous forests dominated and where red squirrels had been introduced, than on islands without squirrel introductions. In contrast, Reitsma et al. (1990) found no evidence that nesting success increased with removal of red squirrels in northern hardwood forests, possibly because of compensatory predation from other predators. However, several experimental studies generally confirm the idea that where red squirrels are present, nesting success is lower.

Red squirrels were historically absent from one of the study areas (Cypress Hills, Alberta) but introduced there in 1950 (Newsome and Dix 1968). Unfortunately, there is no information available on what breeding bird abundances were before the introduction of squirrels. However, it is not unreasonable to assume that nesting guild abundances there prior to the introduction of squirrels were comparable to the other ranges without squirrels, and thus the introduction of squirrels there provides an unplanned experiment. Nesting guild abundances in the Cypress Hills now are comparable to abundance estimates for the other ranges with squirrels, which further suggests that the differences observed in nesting guild abundance between areas with and without squirrels are likely a consequence of the effects of nest predation by squirrels, and that some other factor associated with the original absence of squirrels is an unlikely explanation. These results also suggest the potential impact that squirrels may have on structuring breeding bird communities acts rather quickly (circa 50 years), however this may also be a consequence of greater squirrel densities in the Cypress Hills.

The observed patterns in breeding bird abundance suggest that the importance of potential nest predation by red squirrels was, in part, dependent on nest placement. Birds nesting in the forest canopy and understory had lo\wer abundances in ranges where red squirrels were present. The lower abundance of some understory- nesting birds in the presence of red squirrels is consistent with the findings of Sieving and Willson (1998), who showed that nest densities of some understory birds are lower in some areas with red squirrels in comparison to areas without red squirrels. In contrast, no differences in the abundances of cavity, ground, or midstory nesting birds were found. Such a pattern is perhaps expected, given the arboreal nature of red squirrels; however, red squirrels are not restricted to upper levels as they frequently move across all forest strata as they travel among trees. Why there was no statistically significant difference in the abundance of midstory-nesting birds is unclear. However, there was a trend for greater abundances of individual midstory-nesting species in the absence of red squirrels (3 of 3 were greater). Lack of a difference in the abundance of cavity-nesting species is consistent with the finding that cavity- nesting birds generally tend to have higher nesting success than open-cup nesting birds (Martin and Li 1992, Wilson and Gende 2000). For example, Martin and Li (1992) showed that cavity-nesting birds (in particular, cavity-excavating species) had the greatest nesting success in mixed forest-breeding bird communities where red squirrels were important nest predators. Moreover, Walters and Miller (2001) found no evidence that red squirrels preyed on nests of some cavity-nesting birds. In fact, they found that red squirrels and some cavity-nesting species coexisted in the same tree without nest predation by squirrels. Although I found no difference in the abundance of the one ground-nesting bird (Dark-eyed Junco [Junco hyemalis]) between areas with and without red squirrels, it is worth noting an interesting shift in their nesting habits in at least one range without squirrels. Juncos usually nest on the ground or in banks (Ehrlich et al. 1988), however, in the South Hills, Idaho, (a range with no squirrels) where observers have spent thousands of hours over the past six years, no junco nests have ever been found on the ground. The few nests located were found on branches 1.59m up in lodgepole pines (C. Benkman, pers. comm.). Such potential shifts in nesting location between areas with and without squirrels deserve further study.

An alternative explanation for the lower abundances of some nesting species in areas with red squirrels is that these changes reflect an adaptive shift by birds to avoid areas where certain nest predators are present, rather than individual species experiencing lower nesting success and thus reduced recruitment (Sieving and Willson 1998, Willson and Gende 2000, Willson et al. 2003). Choosing areas to minimize the risk of predation is expected so that birds can maximize fitness. Such a mechanism could conceivably organize bird communities at the landscape level if certain species prefer areas where nest predators such as red squirrels are absent. In support of the idea that birds may choose areas where nesting success is higher, Haas (1998) showed that birds experiencing greater nest success had a higher probability of returning to those same nest sites in subsequent years (see also Paton and Edwards 1996, Hoover 2003). Although the results found in Haas's (1998) study were within a given locale, the mechanism could be extended to larger landscape levels, encompassing areas where key predators or other factors limiting nest success may vary across the landscape. Because nest predators in general, and red squirrels in particular (Martin and Joron 2003), tend to vary both spatially and temporally within a given location, habitat selection at the landscape level (i.e., among mountain ranges) may be more important (Marini 1997). Although this mechanism is contingent on the distributions of other predators (Willson et al. 2003), areas with red squirrels may be a sink habitat (Pulliam 1988) for some species, and thus are avoided.

I am especially grateful to C. Benkman for thoughtful advice, encouragement, and financial support. I thank D. Dobkin, E. Mezquida, T. Parchman, L. Reitsma, A. Rodewald, L. Santisteban, J. Smith, L. Snowberg, and M. Willson for helpful comments that improved this paper; and landowners, the Bear's Paw Tribal Council, the U.S. Forest Service, and the Cypress Hills Provincial Park for help in facilitating this research. This research was funded by a National Geographic Society Grant (Award 6820-00) awarded to C. Benkman and additional support from the Biology Department at New Mexico State University awarded to AMS.

LITERATURE CITED

BAYNE, E. M., AND K. A. HOBSON. 2000. Effects of red squirrel (Tamiasciurus hudsonicus) removal on survival of artificial songbird nests in boreal forest fragments. American Midland Naturalist 147:72- 79.

BENKMAN, C. W. 1999. The selection mosaic and diversifying coevolution between crossbills and lodgepole pine. American Naturalist 153:875-891.

BENKMAN, C. W., W. C. HOLIMON, AND J. W. SMITH. 2001. The influence of a competitor on the geographic mosaic of coevolution between crossbills and lodgepole pine. Evolution 55:282-294.

BENKMAN, C. W., T. L. PARCHMAN, A. FAVIS, AND A. M. SIEPIELSKI. 2003. Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine. American Naturalist 162:182-194.

BENKMAN, C. W., AND A. M. SIEPIELSKI. 2004. A keystone selective agent? Pine squirrels and the frequency of serotiny in lodgepole pine. Ecology 58:2082-2087.

BOAG, D. A., S. G. REEBS, AND M. A. SCHROEDER. 1984. Egg loss among Spruce Grouse inhabiting lodgepole pine forests. Canadian Journal of Zoology 62:1034-1037.

BOCK, C. E., AND Z. F. JONES. 2004. Avian habitat evaluation: should counting birds count? Frontiers in Ecology and the Environment 8:403-410.

BROWN, J. K. 1975. Fire cycles and community dynamics in lodgepole pine forests, p. 429-456. In D. M. Baumgartner [ED.], Management of lodgepole pine ecosystems symposium proceedings. Washington State University, Pullman, WA.

BURT, W. H., AND R. P. GROSSENHEIDER. 1980. The Peterson field guide to mammals. Houghton Mifflin, New York.

CRITCHFIELD, W. B. 1980. Genetics of lodgepole pine. USDA Forest Service Research Paper WO-37.

EHRLICH, P. R., D. S. DOBKIN, AND D. WHEYE. 1988. The birders' handbook: a field guide to the natural history of North American birds. Simon and Schuster, New York.

GEORGE, T. L. 1987. Greater land bird densities on island vs. mainland: relation to nest predation level. Ecology 68:1393-1400.

GOUGH, G. A., J. R. SAUER, AND M. ILIFF [ONLINE]. 1998. Patuxent bird identification infocenter. Version 97.1. Patuxent Wildlife Research Center, Laurel, MD. (8 September 2005).

HAAS, C. A. 1998. Effects of prior nesting success on site fidelity and breeding dispersal: an experimental approach. Auk 115:929-936.

HOOVER, J. P. 2003. Decision rules for site fidelity in a migratory bird, the Prothonotary Warbler. Ecology 84:416-430.

KOTLER, B. P., J. S. BROWN, AND W. A. MITCHELL. 1994. The role of predation in shaping the behavior, morphology, and community organiof desert rodents. Australian Journal of Zoology 42:449-466.

KREBS, J. R. 1970. Regulation of numbers in the Great Tit (Aves: Passeriformes). Journal of Zoology, London 162:317-333.

MARINI, M. A. 1997. Predation-mediated bird nest diversity: an experimental test. Canadian Journal of Zoology 75:317-323.

MARTIN, J. L., AND J. JORON. 2003. Nest predation in forest birds: influence of predator type and predator's habitat quality. Oikos 102:641-653.

MARTIN, T. E. 1988a. Processes organizing open-nesting bird assemblages: competition or nest predation? Evolutionary Ecology 2:37-50.

MARTIN, T. E. 1988b. On the advantage of being different: nest predation and the coexistence of bird species. Proceedings of the National Academy of Sciences USA 85:2196-2199.

MARTIN, T. E. 1993a. Nest predation, nest sites, and birds: new perspectives on old patterns. Bioscience 43:523-532.

MARTIN, T. E. 1993b. Nest predation among vegetation layers and habitat types: revising the dogmas. American Naturalist 141:897- 913.

MARTIN, T. E. 1996. Fitness costs of resource overlap among coexisting bird species. Nature 380:338-340.

MARTIN, T. E., AND P. LI. 1992. Life history traits of open vs. cavity-nesting birds. Ecology 73:579-592.

MICHELI, F., C. H. PETERSON, L. S. MULLINEAUX, C. R. FISHER, S. W. MILLS, G. SANCHO, G. A. JOHNSON, AND H. S. LENIHAN. 2004. Predation structures communities at deep-sea hydrothermal vents. Ecological Monographs 72:365-382.

MUIR, P. S., AND J. E. LOTAN. 1985. Disturbance history and serotiny of Pinus contorta in western Montana. Ecology 66:1658- 1668.

NEWSOME, R. D., AND R. L. Dix. 1968. The forests of the Cypress Hills, Alberta and Saskatchewan, Canada. American Midland Naturalist 80: 118-185.

PATON, P. W. C., AND T. C. EDWARDS JR. 1996. Factors affecting interannual movements of Snowy Plovers. Auk 113:534-543.

PENDLETON, G. W. 1995. Effects of sampling strategy, detection probability, and independence of counts on the use of point counts, p. 131-134. In C. J. Ralph, J. R. Sauer, and S. Droege [EDS.], Monitoring bird populations by point counts. USDA Forest Service General Technical Report PSW-GTR-149.

PULLIAM, H. R. 1988. Sources, sinks and population regulation. American Naturalist 132:652-661.

REITSMA, L. R., R. T. HOLMES, AND T. W. SHERRY. 1990. Effects of removal of red squirrels, Tamiasciurus hudsonicus, and eastern chipmunks, Tamias striatus, on nest predation in a northern hardwood forest: an artificial nest experiment. Oikos 57:375-380.

REITSMA, L. R., AND C. J. WHELAN. 2000. Does vertical partitioning of nest sites decrease nest predation? Auk 117:409- 415.

RICKLEFS, R. E. 1969. Analysis of nesting mortality in birds. Smithsonian Contributions to Zoology 9:1-48.

SIBLEY, D. A. 2000. The Sibley guide t\o birds. Alfred A. Knopf, New York.

SIEPIELSKI, A. M., AND C. W. BENKMAN. 2005. A role for habitat area in the geographic mosaic of coevolution between Red Crossbills and lodgepole pine. Journal of Evolutionary Biology 18:1042-1050.

SIEVING, K. E., AND M. F. WILLSON. 1998. Nest predation and avian species diversity in northwestern forest understory. Ecology 79:2391- 2402.

SULLIVAN, T. P. 1991. Additional vertebrate prey items of the red squirrel, Tamiasciurus hudsonicus. Canadian Field Naturalist 105:398- 399.

SULLIVAN, T. P., AND R. A. MOSES. 1986. Red squirrel populations in natural and managed stands of lodgepole pine. Journal of Wildlife Management 50:595-601.

THOMPSON, L. S., AND J. KUIJT. 1976. Montane and subalpine plants of the Sweetgrass Hills, Montana, and their relation to early post- glacial environments of the northern Great Plains. Canadian Field Naturalist 90:432-448.

WALSBERG, G. E. 2005. Cattle grazing in a national forest greatly reduces nesting success in a ground-nesting sparrow. Condor 107:714- 716.

WALTERS, E. L., AND E. H. MILLER. 2001. Predation on woodpeckers in British Columbia. Canadian Field Naturalist 115:413-419.

WAUTERS, L. A., P. CASALE, AND A. A. DHONDT. 1994. Space use and dispersal of red squirrels in fragmented habitat. Oikos 69:140-146.

WHEATLEY, M., K. W. LARSEN, AND S. BOUTIN. 2002. Does density reflect habitat quality for North American red squirrels during a sprucecone failure? Journal of Mammalogy 83:716-727.

WILLSON, M. F., T. L. DE SANTO, AND K. E. SIEVING. 2003. Red squirrels and nest predation risks to bird nests in northern forests. Canadian Journal of Zoology 81:1202-1208.

WILLSON, M. F., AND S. M. GENDE. 2000. Nesting success of forest birds in southeast Alaska and adjacent Canada. Condor 102:314-325.

ADAM M. SIEPIELSKI1

Department of Biology, New Mexico State University, Las Cruces, NM 88003

Manuscript received 23 November 2004; accepted 21 September 2005.

1 Present address: Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071. E-mail: asiepiel@uwyo.edu

Copyright Cooper Ornithological Society Feb 2006

Source: Condor, The

More News in this Category