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_� _ .�.,........•..w.«... ram:-7ESi:::u:. - .;CD77iSCt v Nce� a Viburnum (Fig. Za;. SamDie size considerations cor. strained the first anaivsrs to robins and the second fee sis to thrushes. We tested these comparisons ford differences, such as the absence of long, sharp thorns is Rhamnus. and architectural differences. such as sturd. i • ^. .. n r ..r azaeZ LS branches or reduced basai cover in Lonicera, relative t, those of native shrubs of comparable height and stature We performed separate analyses using either DMRs cal culated from all predation events or only those preda Lion events ascribed to large mammals because we hv. pothesized that the influence of thorns and substrate vlbf.11T]= architecture would have the largest effect on this grour of predators. For robins DMRs were higher in Rbamnus all predation events when considering(Xz = 3.94,df= 1 -� Lonicera p < 0.05; Fig. 1). Furthermore, this difference was more pronounced when predation by only large mammals was considered (Rhamnus: DMR = 0.02-2 ± 0.0054; Crataegus: no mammalian predation; X' = 7.26, df = 0 5 10 15 1, p < 0.01). For thrushes, DMRs were not signifi- cantly different for either comparison(p > 0.30). Nest height (m) Despite experiencing higher predation when nesting in Lonicera, robins dramatically increased their use of 0.15 Lonicera over the stud} period (r2 = 0.912,p < 0.01; • b Fig 3a) from 5% in 1992 to 33% in 1997,with a concom- itant decline in the use of native trees.Thrushes showed C 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 • 0 of thrush nests built in Lonicera was positively related C6 to the annual number of robins nesting in Lonicera(r2 = 40 0.462,p < 0.07, 1-tailed test; Fig 3b)and not simply the • cumulative number of nests, as previously determined 0.05 0 •• (Schmidt &Whelan 1998). • • • Discussion • 0.0 Our data show that exotic Lonicera and Rbamnus 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 ofAmerican thorns. and a branch architecture that may facilitate Robin nests within the different nest plant species: rn) predator movement. Despite higher predation, robins regression of the daily mortality rate 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(r2 =0.496) than a linear related to Lonicera's early leaf flush and expansion regression (r` = 0.3-6). (Trisei & Gorchov 1994). Robins nest in Lonicera most often early in the season (K.A.S., unpublished data), al- thoueh it is unknown whether this is because early leaf mammals was also inverseiv related to nest height (r' = flush attracts eariv-season nesting activity or whether 0 291,p = 0.02). robins choose alternative substrates after attempts in • To control for the effects of nest height in subsequent Lonicera have failed. The former hypothesis suggests an analyses, we used pairw•ise comparisons between sub- ecological trap (Gates &Gvsel 197 8), whereas the latter strates with strongly overlapping nest height distnbu- suggests an adaptive response. although the two behav- tions: Rhamnus versus Crataegus and Lonicera versus ions are not mutually exclusive. Other temporal correia- ^on5Ctv8fum fiinlrrt+v