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Viburnum ('rig. _a;. SamDie size considerations Co l aq sL-ai.ned the firs analysis to robins and the second an,-d sis to thrushes. we tested these comparisons for desig differences. such as the absence of long, sharp thorns i Rhamnus. and architecrara differences. such as stung �rataeoLis branches or reduced basal cover in Lonicera, relative t those of native shrubs of comparable height and starun We performed separate analyses using either Dllgts ca Rhamnus cuiated from all predation events or only those pred: uon events ascribed to large mammals because we h, j� pothesized that the influence of thorns and substr-dt Vibumum architecture would have the largest effect on this grout of predators. For robins DMRs were higher in Rhamnu all predation events when considering(X` = 3.94,df= 1 i-♦-f Lonicera p < 0.05; Fig. 1). Furthermore, this difference was mon pronounced when predation by only large rnarnm.IL, i was considered (Rhamnus: DMR = 0.0272 { 0.0054 Crataegus: no mammalian predation; X' = 7.26, df = 0 5 10 15 1. p < 0.01). For thrushes, DMRs were not signin. candy different for either comparison (p > 0.30). Nest height (m) Despite experiencing higher predation when nesting in Lonicera, robins dramatically increased their use 01 0.15-- Lonicera over the stud-,- period (r'- = 0.912,p < 0.01; • Fig 320 from 51,o in 1992 to 33% in 1997, with a concom• itant decline in the use of native trees. Thrushes showed no overall trend (p > 0.25), but Lonicera was more fre- quently used in the latter half of the study(Fig. 3a). This >, 0.10 increased overlap comes at a cost to thrushes. The DMR ._ • of thrush nests built in Lonicera was positively related to the annual number of robins nesting in Lonicera(r' _ p • 0.462,p < 0.07, 1-tailed test; Fig 3b)and not simply the • • cumulative number of nests, as previously determined >' 0.05 •• (Schmidt &Whelan 1998). • ' Discussion Q.0 1Our data show that exotic Lonicera and Rhamnus af- 0 5 10 15 fected songbird nest success in two ways. First, exotic shrubs directly enhanced nest predation (primarily by Nest height (m) large mammals) in American Robins, perhaps through a combination of lower nest height, the absence of sharp Figure 2. (a)Mean (_ I SD)height of American thorns. and a branch architecture that may facilitate Robin nests within the different nest plant sDecies: (-b) predator movement. Despite higher predation, robins regression of the daily morzalitt r:te of robin nests increased their use of Lonicera during our study (Fig. against nest height (m). The data fit a logarithmic re- 3a). The cause of the increase is unknown but appears gression substantially better 0.496) than a linear related to Lonicera's earl- leaf flush and expansion -egression (r' = 0,37C). (Tnsel & Gorchov 1994)_ Robins nest in Lonicera most often earl- in the season (K.A.S., unpublished data), al- .houen it is unknown whether this is because early leaf mammals was also iltverseiv related to nest height (r= = flush attracts eariy-season nesting activity or whether 1291,p — 0.02). robins choose aitemative substrates after attempts in To control for the effects of nest height in subsequent Lonicera have failed. The former hypothesis suggests an ariah•ses, we used pair wise comparisons berween sub- ecological trap (Gates & Gvsel 19'8), whereas the latter t ates with strongly oyeri'apping nest height distribu- suggests an adaptive response. although the two behav- tions: Rhamnus versus Crataegus and Lonicera versus fors are no: mutually exclusive. Other tem,t)oral correla- In K!vlt llln if Mir lry'