Nelson and Hamer 



Chapter 8 



Nest Success and Effects of Predation 



Great Horned Owls are increasing in numbers throughout 

 the western United States, especially in response to increases 

 in habitat fragmentation and human disturbance (Johnson 

 1993; Marzluff 1994; Marzluff and Balda 1992; Robbins 

 and others 1986; Rosenberg and Raphael 1986; Yahner and 

 Scott 1988). In contrast, numerous neotropical migrant species 

 are declining in numbers because they are unable to adjust to 

 fragmentation and rapidly changing habitat conditions (Hagan 

 and Johnson 1992; Hansen and others 1991, Hejl 1992, 

 Martin 1992, Morrison and others 1992, Rosenberg and 

 Raphael 1986). The Marbled Murrelet was listed as a 

 threatened species in 1992 as changes in the forested landscape 

 appear to be negatively impacting their populations (U.S. 

 Fish and Wildlife Service 1992). 



Although the relationship between predator numbers, 

 habitat fragmentation, and predation on Marbled Murrelet 

 nests has not been specifically studied, we believe, based on 

 the following data, that changes in their habitat, such as 

 increased amounts of edge, may significantly affect their 

 nesting success. First, evidence from murrelet nests in this 

 study suggests that distance to edge, stand size, canopy 

 closure, percent cover above the nest cup (nest concealment), 

 and distance of the nest from the tree trunk may be affecting 

 predation rates (table 5). In a comparison of these habitat 

 characteristics between successful nests (n = 9) and nests 

 that failed because of predation (n = 8, excluding Alaska), 

 we determined that successful nests were located significantly 

 farther from edges (U = 2.9, n = 16 trees, P < 0.05) (table 5). 

 All successful nests were located >55 m ( x = 166.3, n = 8 

 trees, s.e. = 82.3) from an edge (road or clearcut), with the 

 exception of the Nemah nest in Washington, which was 

 located within 10 m of an old road near the center of a 142 ha 

 forest. In contrast, all nests that failed because of predation 

 were located within 64 m ( x = 21 A, s.e. = 6.0) of an edge. In 

 a review of numerous artificial nest predation studies, Paton 

 (1994) found evidence that predation of bird nests is higher 

 within 50 m of edges. This result supports our hypothesis 

 that murrelet nests near edges may be more vulnerable to 

 predation than those located in the stand interior. In addition, 

 nest concealment was significantly greater at successful nests 

 ( x = 87.2 percent, s.e. = 3.9) compared with nests that failed 

 because of predation ( x = 67.5 percent, s.e. = 8.2) (U = 2.3, 

 n = 17, P < 0.05) (table 5). Nest concealment has been 

 shown to decrease predation rates (Chasko and Gates 1982; 

 Marzluff and Balda 1992; Martin and Roper 1988). Stand 

 size (532.0 versus 407.4 ha, n = 11 stands) and canopy 

 closure near nests (63.6 versus 47.0 percent, n = 16 plots) 

 were higher and nests located closer to the trunk (46.5 versus 

 163.3 cm) at successful sites, but were not significantly 

 different from nests that failed because of predation. 



Second, it has been suggested that changes in forests 

 where boundaries are contiguous with secondary succession 

 may not create the same predation problems as those observed 

 in static, simple forests in urban and agricultural areas that 

 are defined by distinct boundaries (Rosenberg and Raphael 

 1986; Rudnicky and Hunter 1993). However, numerous 



studies in the eastern United States provide empirical evidence 

 that edge effects in a forest dominated landscape (forest/ 

 clearcut edge) are similar to those in forest/urban or 

 agricultural settings. For example, in studies of eastern 

 neotropical migrants, predation was lower in the forest interior 

 (>50 m from the edge) compared with edge habitat (Chasko 

 and Gates 1982; Yahner and Scott 1988). Predation was also 

 lower in areas with high vegetative heterogeneity and 

 concealing cover (Chasko and Gates 1982). 



Evidence from artificial nest studies in forests of the 

 Pacific Northwest also suggests that predation of birds' nests 

 may be affected by habitat fragmentation and forest 

 management. On Vancouver Island, British Columbia, Bryant 

 (1994) demonstrated that artificial ground and shrub nests 

 located along forest/clearcut edges (within 100 m) were subject 

 to higher predation rates than those in the forest interior 

 (100-550 m from the edge). In the Oregon Coast Range, 

 predation on artificial shrub nests was higher in clearcuts and 

 shelterwood (20-30 green tress >53 cm d.b.h./ha) stands than 

 in stands with group selection cuts (1/3 volume removed in 

 0.2 ha openings) and unmanaged (control) stands (Chambers, 

 pers. comm.). Additionally, in the Oregon Cascades, Vega 

 (1994) found that predation on ground nests was significantly 

 greater in clearcuts compared with retention stands ( 1 2 trees/ 

 ha and 7.5 snags/ha), and predation on shrub nests was highest 

 in retention stands compared to the other treatment types 

 (clearcuts and mature stands). Steller's Jays, the suspected 

 predator of the shrub nests, were more abundant in the retention 

 stands, where they probably used the remnant trees for perching 

 (see Wilcove 1985; Yahner and Wright 1985). 



Third, despite differences in results among nest predation 

 studies (e.g., Rudnicky and Hunter 1993 versus Yahner and 

 Scott 1988), existing evidence strongly indicates that avian 

 nesting success declines near edges (Paton 1994). In addition, 

 regardless of the type of edge, fragmentation of forests often 

 reduces structural complexity and heterogeneity of stands, 

 and exposes remnant patches to edge effects (Hansen and 

 others 1991; Harris 1984; Lehmkuhl and Ruggerio 1991). 

 Because of increases in the amount of edge, productivity of 

 interior forest species is generally impacted (Lehmkuhl and 

 Ruggerio 1991; Reese and Ratti 1988; Yahner and others 

 1989), and generalist species, which benefit from the ecotone, 

 usually increase in numbers (Yahner and Scott 1988). In 

 addition, as vegetative complexity and canopy volume are 

 reduced through fragmentation, bird nests (especially those 

 located in shrubs or trees) may be more conspicuous and 

 easier for avian predators to locate (Rudnicky and Hunter 

 1993; Vega 1994; Wilcove 1985; Yahner and Cypher 1987; 

 Yahner and others 1989; Yahner and Scott 1988). 



The rates of predation on Marbled Murrelet nests in this 

 study appear higher than for many seabirds and forest birds. 

 If the observed predation rates are representative of predation 

 rates throughout the murrelet' s range, then the impacts of 

 predation on murrelet nesting success is significant and of 

 concern (Wilcove 1985). Even if these high predation rates 

 are localized to certain states or areas within states, the 



96 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



