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(e.g..Robinson et al. 1995), but to the best of our know!- nearest 15 cm for nests :53 m above the ground and%o cage these two processes have never been causalh, the nearest 0.5 n-, for rugher nests. We pooled data Linked. If nest predation is higher in exotic species. this across years but anaivzed DMRs of robins and thrushes ma'- provide one potential mechanism for the popuia- separately with the program CONTRAST(Hines&Sauer tion declines seen in some songbird species occupying 1989). Nests placed in dead trees or shrubs,Rosa muli this region. For instance, both Mills et al. (1989) and flora, and Celastrus spp..were excluded from the analy� Germaine et al. (1998) found that native bird species di- ses due to loan sample sizes. versin- and density were positively correlated with the volume of native vegetation but were negatively corre- lated or uncorrelated with the voiurne of exotic vegeta- Results tion. The link between invasion of exotic shrubs and population declines, if it exists, will take much research We performed pairwise comparisons of daily nest mortal- to uncover. We examined the connection between ex- in•rate (DMR)between nests built in Lonicera versus na- otic species and nest predation rates. tive species (including Crataegus and Viburnum as well ' We document nest predation on two songbird spe- as native tree species, such as Acer, Ostrya, and Prunus) ' Gies, American Robins (Turdus migratorius) and Wood and built in Rhamnus Versus native species. Nests built Thrushes (Hvlocichla mustelina). over 6 years in an ur- in Lonicera had significantly higher DMRs than nests in ban reserve outside of Chicago. Both Lonicera and natives for both robins (X2 = 12.31, df = 1,p < 0.001; Rhamnus (predominantINI R. cathartica) are abundant Fig. 1) and thrushes (X` = 4.0, df = 1,p < 0.05;Fig. 1), # at the site where they have replaced native species of whereas nests built in Rhamnus did not have higher Viburnum and Crataegus. Rhamnus lack the sharp DMRs than natives for either species (p> 0.25;Fig. 1). thorns that characterize Crataegus, whereas Lonicera As a potential explanation for these results, we tested has sturdy branches and reduced basal cover relative to for an effect of nest height in robins (thrushes showed Viburnum (K.A.S., personal observations). We hypothe- too little variability in nest height for an analysis) with size that these features may increase nest predation di- regression analysis. Daily mortality rate, calculated from redly through predator facilitation. Furthermore, early a minimum of five nest attempts per height category, leaf flush and expansion in Lonicera maackii (Trisel & was inversely related to nest height (r2 = 0.496,p = Gorchov 1994) may make them a focus for nest build- 0.001; Fig. 2). Moreover, the proportion (aresin-square- ing. Concentrating nest densities by nesting in a re- root-transformed) of depredated nests(minimum of four stricted subset of plant types available may in turn at- depred.arion_ pzr "eight category) ascribed to large tract generalist predators to these aggregations of nests (Martin & Roper 1988; Schmidt &Whelan 1998). 0.07 72 o thrush T ■ robin Methods (D 0.06 ,a From 1992 to 1997, we studied the nesting success of 0.05 American Robins and Wood Thrushes in the approsi- �oz T53 mately 200 ha of deciduous woodlands at The :'Morton0.04Arboretum 05 km west of Chicago, Illinois, U.S.A.) and ,o the abutting 150-ha Hidden Lake Forest Preserve(for site p 21140 0.03 T description see Schmidt and Whelan [19981). We lo- E 24 cated and monitored nests throughout the breeding sea- >., son by visual inspection every 2-5 days. We classified 0.02 successful nests as those that fledged at least one host 0 young, determined by inspection of the nest and sur- 0.01 roundings and by the timing of the disappearance of nestlings. Unsuccessful nests often failed due to preda- tion (>95,"0). We ascribed predation to large mammals U5 ea `�e2 �J5 (e.g., raccoons) when nests were physically and vio- Gta�ae9 �o lently disturbed (e.g., tipped over or knocked out of a tree) or when tracks, fur, scratch marks on bark, and Plant species broken branches or stems were conspicuous. We calcu- lated the daily nest mortality rate using Mayfield's (1975) Figure 1. Nest daily mortality rate (DMR t 1 SE) by method. "X'e also recorded the nest shrub or tree species nest substrate for_American Robin and Wood Thrush. for all nests and visually estimated nest height to the Sample sires are given above bars. Conscrvvion Biology Voiume 13,No.6,December 1999