Oikos 120: 1139–1150, 2011 doi: 10.1111/j.1600-0706.2010.18835.x © 2011 Th e Authors. Oikos © 2011 Nordic Society Oikos Subject Editor: Rob Robinson. Accepted 23 November 2010 An ‘ ecological trap ’ for yellow warbler nest microhabitat selection Quresh S. Latif , Sacha K. Heath and John T. Rotenberry Q. S. Latif ([email protected]), J. T. Rotenberry, Dept of Biology, Univ. of California, Riverside, 900 University Ave., Riverside, CA 92521, USA. Present address for QSL and JTR: Center for Conservation Biology, 1303 Webber Hall, 900 University Ave., Riverside, CA 92521, USA. – S. K. Heath, PRBO Conservation Science, 3820 Cypress Drive 11, Petaluma, CA 94954, USA. Present address for SKH: Dept of Wildlife, Humboldt State Univ., 1 Harpst St., Arcata, CA 95521, USA. Contrary to assumptions of habitat selection theory, fi eld studies frequently detect ‘ ecological traps ’ , where animals prefer habitats conferring lower fi tness than available alternatives. Evidence for traps includes cases where birds prefer breeding habitats associated with relatively high nest predation rates despite the importance of nest survival to avian fi tness. Because birds select breeding habitat at multiple spatial scales, the processes underlying traps for birds are likely scale-dependent. We studied a potential ecological trap for a population of yellow warblers Dendroica petechia while paying specifi c attention to spatial scale. We quantifi ed nest microhabitat preference by comparing nest- versus random-site microhabitat structure and related preferred microhabitat features with nest survival. Over a nine-year study period and three study sites, we found a consistently negative relationship between preferred microhabitat patches and nest survival rates. Data from experimental nests described a similar relationship, corroborating the apparent positive relationship between preferred microhabitat and nest predation. As do other songbirds, yellow warblers select breeding habitat in at least two steps at two spatial scales; (1) they select territories at a coarser spatial scale and (2) nest microhabitats at a fi ner scale from within individual territories. By comparing nest versus random sites within territories, we showed that maladaptive nest microhabitat preferences arose during within-territory nest site selection (step 2). Furthermore, nest predation rates varied at a fi ne enough scale to pro- vide individual yellow warblers with lower-predation alternatives to preferred microhabitats. Given these results, tradeoff s between nest survival and other fi tness components are unlikely since fi tness components other than nest survival are prob- ably more relevant to territory-scale habitat selection. Instead, exchanges of individuals among populations facing diff erent predation regimes, the recent proliferation of the parasitic brown-headed cowbird Molothrus ater , and/or anthropogenic changes to riparian vegetation structure are more likely explanations. Habitat selection is the behavioral process by which animals predation rates and reduced nest survival (Misenhelter and choose where to live and breed. Th eory assumes that animals Rotenberry 2000; additional studies reviewed by Robertson will prefer habitats that maximize their fi tness (Fretwell and and Hutto 2006). Th ese purported traps may have arisen Lucas 1970). However, fi eld studies providing evidence for because anthropogenic disturbance has disrupted nest preda- ‘ ecological traps ’ challenge this basic theoretical assumption. tor behavior, distributions, or community composition in ways Members of several populations across a variety of animal that are not recognized by birds selecting nest sites. Alterna- taxa appear to favor poor-quality habitats despite the avail- tively, despite the obvious importance of nest predation, other ability of better-quality habitats (reviewed by Battin 2004). selective agents acting upon other fi tness components could Ecological traps have the potential to threaten population still balance out costs incurred by utilizing high-predation habi- persistence particularly when a substantial fraction of popu- tats. Th us, why some birds continue to prefer high-predation lation members select poor quality habitats (Kristan 2003). breeding habitats is not well understood. Th us, the presence of traps raises an important question Since birds select habitat at multiple spatial scales (Wiens for conservation and evolutionary ecologists: why do the et al. 1987) and since habitat-specifi c predation patterns arise members of some populations favor habitats that confer at multiple scales (Th ompson 2007), the processes underly- lower fi tness than available alternatives? ing ecological traps for birds are likely scale-dependent. At Studies of birds have provided much of the empirical a relatively coarse scale, birds select breeding territories to evidence for ecological traps. Nest survival is an important accommodate multiple activities, including foraging, roost- component of avian fi tness (Clark and Martin 2007) and ing, and nesting. At a fi ner scale, birds select microhabitats predation is typically the main cause of nest failure (Martin from within their territories for particular activities, includ- 1993). Given the importance of predation, ornithologists ing microhabitats for nest establishment. With respect to expect birds to select breeding habitats that minimize preda- predation risk, habitat at coarse scales can infl uence predator tion risk (Martin 1998). However, several studies document numbers and/or distributions, whereas at fi ner scales, micro- preferences for breeding habitat associated with elevated habitat can infl uence predator hunting strategies and/or nest 1139 detection rates (Th ompson 2007). Coarse-scale predation predators of nestlings in our study system, although cor- patterns would have a greater bearing on territory selection, vids and mice have also been observed taking chicks. Cow- but territory selection may also be shaped by other factors, birds depredate nests in conjunction with brood parasitism such as food availability (Shochat et al. 2005, Chalfoun and (49% of yellow warbler nests were parasitized in our study Martin 2007). By contrast, fi ne-scale, microhabitat-related area; Heath 2008). patterns should strongly infl uence birds during nest site Our study spanned 2000 – 2008. From 2000 – 2005, we selection, especially since other agents of natural selection collected data as part of an all-species riparian bird moni- are less likely to have any bearing on this phase of habitat toring program along the lower reaches of four tributaries selection. Indeed, most purported traps for birds (reviewed to Mono Lake, Mono County, California, USA: Rush, Lee by Robertson and Hutto 2006) are observed at coarser scales, Vining, Mill, and Wilson creeks (1972 – 2020 m, 37 o 56′N, so most populations purportedly ‘ trapped ’ by predation 119 o 04′W – 38 o 04′N, 119 o 09′W). Two study plots along may in reality be favoring territories for their food resources, each creek (n ϭ 8) encompassed a total of 39, 30, 15 and the benefi ts of which may or may not outweigh apparent 15 ha of riparian fl oodplain vegetation respectively (Heath predation costs. et al. 2006). From 2006 – 2008 we studied the processes con- Here, we studied the processes underlying an apparent tributing to nest habitat preference and predation patterns at ecological trap for a population of yellow warblers Dend- one Rush Creek study plot (n ϭ 1; 20 ha). A strong xeric roica petechia . First, we compared microhabitat use versus – mesic gradient characterized fl oodplain vegetation, with availability to quantify nest microhabitat preferences, and mixed willow (Salix spp.) and black cottonwood Populus bal- we demonstrated that nests in preferred sites suff er higher samifera ssp. trichocarpa at the mesic extreme, non-riparian predation rates than nests in less preferred sites. Next, since shrubs big sagebrush/ bitterbrush/rabbitbrush (Artemisia we expect birds to be most responsive to predation risk dur- tridentata/ Purshia tridentata / Chrysothamnus spp.) at the ing nest site selection, we compared use versus availability xeric extreme, and Woods ’ rose Rosa woodsii in between within individual territories to determine if apparent mal- (McBain and Trush 2003). Th is gradient has been infl u- adaptive preferences arose during nest site selection. Finally, enced by decades of anthropogenic habitat alterations. Water we analyzed data from both natural and experimental nests diversions and livestock grazing (Stine et al. 1984), followed to examine the mechanistic underpinnings of microhabitat – by grazing cessation and stream re-watering (Kauff man et al. predation relationships. Th is information allowed us to 2000), have caused substantial fl uctuations in the extent of consider both the relative predator-avoidance benefi ts asso- riparian vegetation within the fl oodplain. Sagebrush-asso- ciated with alternative microhabitat choices and when dur- ciated vegetation remained a substantial component of the ing the habitat selection process yellow warblers chose nest fl oodplain shrub layer during our study (McBain and Trush microhabitats. In light of our fi ndings, we considered vari- 2003). Yellow warbler densities ranged from 2.2 pairs haϪ 1 on ous alternative explanations for apparently maladaptive nest Rush to 0.4 pairs ha Ϫ1 on Mill creeks (Heath et al. 2006). microhabitat preferences, including tradeoff s between nest survival and other fi tness components. Yellow warbler nest microhabitat use, availability