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Landscapes of Fear for Adult Birds Reduces Nestling Condition Across an Urban Gradient 1, 2 3 AARON M

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Landscapes of Fear for Adult Birds Reduces Nestling Condition Across an Urban Gradient 1, 2 3 AARON M Perilous choices: landscapes of fear for adult birds reduces nestling condition across an urban gradient 1, 2 3 AARON M. GRADE , SUSANNAH B. LERMAN , AND PAIGE S. WARREN 1Program in Organismic and Evolutionary Biology, University of Massachusetts, 160 Holdsworth Way, Amherst, Massachusetts 01003 USA 2USDA Forest Service Northern Research Station, 160 Holdsworth Way, Amherst, Massachusetts 01003 USA 3Department of Environmental Conservation, University of Massachusetts, 160 Holdsworth Way, Amherst, Massachusetts 01003 USA Citation: Grade, A. M., S. B. Lerman, and P. S. Warren. 2021. Perilous choices: landscapes of fear for adult birds reduces nestling condition across an urban gradient. Ecosphere 12(7):e03665. 10.1002/ecs2.3665 Abstract. Predator fear effects influence reproductive outcomes in many species. In non-urban systems, passerines often respond to predator cues by reducing parental investment, resulting in smaller and lighter nestlings. Since trophic interactions in urban areas are highly altered, it is unclear how passerines respond to fear effects in human-altered landscapes. Nestlings of passerines in urban areas also tend to be smaller and lighter than their rural counterparts and are often exposed to high densities of potential predators yet experience lower per capita predation—the predation paradox. We suggest fear effects in urban habitats could be a significant mechanism influencing nestling condition in birds, despite lowered predation rates. We manipulated exposure of nesting birds to adult-consuming predator risk in residential yards across a gradient of urbanization to determine the relative influence of urbanization and fear on nestling condition. We found nestlings had reduced mass in nests exposed to predator playbacks as well as in more urban areas. Despite lower per capita predation rates in urban areas, fear effects from increased predator densi- ties may influence passerine fitness through reduced nestling condition. As urban development expands, biodiversity conservation hinges on a deeper mechanistic understanding of how urbanization affects reproductive outcomes. Key words: behavior; birds; fear effects; house wren; non-lethal effects; Troglodytes aedon; urban gradient; urbanization. Received 16 September 2020; revised 22 January 2021; accepted 1 April 2021; final version received 20 May 2021. Corre- sponding Editor: Brooke Maslo. Copyright: © 2021 The Authors. This article has been contributed to by US Government employees and their work is in the public domain in the USA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. E-mail: [email protected] INTRODUCTION effects could be influencing animal demography across human-altered landscapes (e.g., suburbs, Fear of predators is a significant ecological cities), which are characterized by high densities mechanism that has consequences at the individ- of potential predators yet low per capita preda- ual (Werner et al. 1983, Creel et al. 2005), popula- tion rates (Fischer et al. 2012). To maximize life- tion (Sinclair and Arcese 1995, Creel et al. 2005, time reproductive success, breeding adults Zanette et al. 2011), and community levels (Wer- respond to perceived risk by adjusting investment ner and Anholt 1996, Hua et al. 2013). Fear effects, in current reproduction (e.g., provisioning of also called non-lethal or non-consumptive effects young) vs. self-maintenance and vigilance, and of predation, are manifested primarily through this trade-off may vary across gradients of human behavioral changes in response to cues of per- alteration as a function of levels of fear (Fig. 1; ceived predation risk (Cresswell 2008). Thus, fear also see Lima 1993, Ghalambor and Martin 2000, v www.esajournals.org 1 July 2021 v Volume 12(7) v Article e03665 GRADE ET AL. Fig. 1. We experimentally added fear effects of adult-consuming predators (red rectangle, top left) to an urban gradient system in which urbanization effects were already present (green rectangle, top right). Across the gradi- ent of urbanization, habitat, food resources, nest predators, and adult-consuming predators all influence parental investment (behavioral plasticity) of breeding birds (blue balancing scale). The investment prioritizes either investment in self-maintenance and vigilance, which increases the adult probability of survival (blue rectangle, bottom right), or parental provisioning, which increases nestling condition and nestling probability of survival (blue rectangle, bottom left). The balance of these investments is fine-tuned over evolutionary time to maximize lifetime reproductive fitness (blue rectangle, bottom center) by responding to environmental cues. By adding fear effects into a system with cues already present (i.e., cue-added study), we were able to detect which direction and to what degree the added cues push the parental investment of breeding birds. Fontaine and Martin 2006). However, given the (Cresswell 2008). In a given breeding season, mismatch between predator densities and preda- birds presented with nest predator cues respond tion rates in urban settings, the question remains rapidly by reducing provisioning rates—a proxi- as to whether animal responses to fear effects sig- mate mechanism that can result in reduced nest- nificantly influence reproductive outcomes. ling mass and condition (Martin 2011, Zanette Passerines are adept at detecting (Lohr et al. et al. 2011, Hua et al. 2014). This strategy reduces 2003), communicating (Courter and Ritchison current reproductive investment when the 2010), and modifying behavior (Huang et al. chance of nest failure is high to increase likeli- 2012) in response to even slight changes to land- hood of adult survival and future reproductive scapes of fear. Experiments conducted on passer- opportunities (Ghalambor and Martin 2000, Fon- ines in intact natural systems have shown that in taine and Martin 2006). Alternatively, nestlings the presence of nest predator cues, breeding themselves may respond to predator cues with birds reduce their reproductive investment by hormonal regulation (Tilgar et al. 2010) and mor- changes in behavior (Huang et al. 2012, Hua phometric growth trade-offs (Cheng and Martin et al. 2014) or clutch size (Martin 2011, Zanette 2012) to maximize their chance of survival when et al. 2011). Over time, these individual behav- they leave the nest. iors could have population-level effects on The effects of fear were the subject of a recent demography (Pangle et al. 2007) and evolution critique in which the authors argued that previous v www.esajournals.org 2 July 2021 v Volume 12(7) v Article e03665 GRADE ET AL. studies have overstated the influence of fear on In areas of increased urbanization, clutch sizes prey demography (Peers et al. 2018). Peers et al. are typically smaller and nestlings in poorer con- (2018) rightly suggest that this ecological mecha- dition (Newhouse et al. 2008, Chamberlain et al. nism requires further exploration with carefully 2009, Evans et al. 2011). Despite relaxed preda- designed in situ experiments. Additionally, despite tion in urban systems, passerine reproductive the preponderance of nest predator fear effects outcomes shift in a direction consistent with an studies in passerines, few studies have experimen- increased risk of predation (Fig. 1; also see tally tested the effect of adult-consuming (i.e., Chamberlain et al. 2009). predators known to consume breeding-age birds) In these systems, fear effects may misrepresent predator cues on reproductive investment (but see actual predation risk and may serve as a signifi- Ghalambor and Martin 2000, Hua et al. 2014, Mal- cant ecological mechanism in urban habitats one et al. 2017). Adult-consuming predators, such (Shochat et al. 2004, Bonnington et al. 2013). as hawks, are often present in higher densities in Although studies have tested the effects of fear in urban areas (Rullman and Marzluff 2014). A small urban vs. rural greenspace (Malone et al. 2017), number of previous studies present conflicting no study to our knowledge has assessed how responses to this type of threat (Ghalambor and fear affects nestling condition across urban gradi- Martin 2000, Malone et al. 2017). In fact, Malone ents in response to fear of adult-consuming (vs. et al. (2017) argue that shifting patterns of fear nest) predators. In addition, behavioral response effects (nestling vs. adult mortality) may yield dif- to fear is understudied in residential lands. Resi- ferent reproductive outcomes. Passerines behav- dential lands are a prominent and growing land- iorally respond to predator cues in short time use type comprising almost half of urban green scales (Lima 1993). Therefore, it is likely that adult- spaces and have highly altered trophic dynamics consuming predator cues indirectly influence and widespread conservation implications (Ler- reproductive outcomes through changes in paren- man and Warren 2011). tal behavior, though it is unclear whether the We designed an experimental cue-addition exposure to these cues typically results in playback study to test how fear of adult- increased or decreased reproductive investment consuming predators affects nestling condition. (Ghalambor and Martin 2000, Hua et al. 2013, We hypothesized that introduction of adult- Malone et al. 2017). It is also unclear to what extent consuming predator cues would significantly nestling response to predator cues influences nest-
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