Global Change Biology Global Change Biology (2014), doi: 10.1111/gcb.12761

Greater shrub dominance alters breeding habitat and food resources for migratory songbirds in Alaskan arctic tundra NATALIE T. BOELMAN1, LAURA GOUGH2, JOHN WINGFIELD3, SCOTT GOETZ4, ASHLEY ASMUS2*, HELEN E. CHMURA3*, JESSE S. KRAUSE3*, JONATHAN H. PEREZ3*, S H A N N A N K . S W E E T 1 * and K E V I N C . G U A Y 4 1 Lamont-Doherty Earth Observatory, and Department of Earth and Environmental Sciences, Columbia University, Palisades, NY 10964, USA, 2Department of Biology, University of Texas at Arlington, Arlington, TX 76019, USA, 3Department of Neurobiology, Physiology and Behavior, University of California at Davis, Davis, CA 95616, USA, 4Woods Hole Research Center, 149 Woods Hole Road, Falmouth, MA 02540, USA

Abstract Climate warming is affecting the Arctic in multiple ways, including via increased dominance of deciduous shrubs. Although many studies have focused on how this vegetation shift is altering nutrient cycling and energy balance, few have explicitly considered effects on tundra fauna, such as the millions of migratory songbirds that breed in northern regions every year. To understand how increasing deciduous shrub dominance may alter breeding songbird habitat, we quantified vegetation and arthropod community characteristics in both graminoid and shrub dominated tundra. We combined measurements of preferred nest site characteristics for Lapland longspurs (Calcarius lapponicus) and Gambel’s White-crowned sparrows (Zonotrichia leucophrys gambelii) with modeled predictions for the distribution of plant community types in the Alaskan arctic foothills region for the year 2050. Lapland longspur nests were found in sedge-dominated tussock tundra where shrub height does not exceed 20 cm, whereas White-crowned sparrows nested only under shrubs between 20 cm and 1 m in height, with no preference for shrub species. Shrub canopies had higher canopy-dwelling arthropod availability (i.e. small flies and spiders) but lower ground-dwelling arthropod availability (i.e. large spiders and beetles). Since flies are the birds’ preferred prey, increasing shrubs may result in a net enhancement in preferred prey availability. Acknowledging the coarse resolution of existing tundra vegetation models, we predict that by 2050 there will be a northward shift in current White-crowned sparrow habitat range and a 20–60% increase in their preferred habitat extent, while Lapland longspur habitat extent will be equivalently reduced. Our findings can be used to make first approximations of future habitat change for species with similar nesting requirements. However, we contend that as exemplified by this study’s findings, existing tundra modeling tools cannot yet simulate the fine-scale habitat characteristics that are critical to accurately predicting future habitat extent for many wildlife species. Keywords: arthropods, climate change, Gambel’s White-crowned sparrow (Zonotrichia leucophrys gambelii), habitat, Lapland longspur (Calcarius lapponicus), migratory songbirds, shrubs Received 25 March 2014 and accepted 30 September 2014

Introduction Since 1960, arctic regions have been warming at a rate two to three times higher than the global average (Anisimov et al., 2007), and the physical and biological responses are proving acute (Callaghan et al., 2004). The main documented responses include increasing vegetation productivity (e.g. Goetz et al., 2007; Epstein et al., 2012), lengthened growing seasons (Zeng et al., Correspondence: Natalie T. Boelman, tel. (845) 365-8480, fax (845) 365-8150, e-mail: [email protected] *Indicates that these authors contributed equally to the study.

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2011), enhanced fire regimes (Jones et al., 2009; BretHarte et al., 2013), thawing permafrost, and increasing active layer depth and temperature (Anisimov et al., 2007). In addition, woody deciduous shrubs have become increasingly dominant in many arctic regions, including the North Slope of Alaska, over the past 50 years (Sturm et al., 2001; Tape et al., 2006), and several experimental and modeling studies show that this trend is expected to continue over the next several decades (Pearson et al., 2013; Zhang et al., 2013). As highlighted in a recent review of impacts of ‘shrubification’ (Myers-Smith et al., 2011), multiple studies have focused on how shrub expansion on the tundra alters 1

2 N . T . B O E L M A N et al. nutrient cycling and energy balance, however, fewer have explicitly considered the role that shrub expansion may have on tundra fauna (Z€ ockler, 2005; Henden et al., 2011; Ehrich et al., 2012; Sokolov et al., 2012). Millions of migratory songbirds migrate to the arctic tundra biome every year (Pielou, 1994) due in part to the abundant summer food resources, long day length, and fewer predators and parasites (in most years) relative to more southern ecosystems (e.g. Richardson et al., 1995; Piersma, 1997; McKinnon et al., 2010). Songbirds are important prey for many tundra predators, and also provide essential ecosystem services – such as seed dispersal and insect control (Sekercioglu, 2006) – not only in the Arctic, but in the more southern and often disparate ecosystems they transit during migration and inhabit over winter months. However, as deciduous shrubs become more dominant in the arctic tundra, songbird breeding habitats are being altered in a variety of ways that have currently unknown consequences for birds. Previous work has shown that deciduous tundra shrubs [willow, birch and alder (Salix, Betula and Alnus spp., respectively)] have greater dominance at the expense of understory plant species such as mosses, graminoids, and evergreens (e.g. Cornelissen et al., 2001), thereby not only shifting plant community composition but also creating a taller and structurally more complex canopy (Walker et al., 2005; Macias-Fauria et al., 2012). Increasing shrub dominance in tundra regions is occurring in three main ways: infilling of existing shrub patches, increases in vertical and lateral growth of existing shrubs, and advancement of shrubline (Myers-Smith et al., 2011). These changes could significantly alter availability and variety of suitable nesting and feeding habitats utilized by different bird species, as well as availability of shelter from inclement weather and predation. Songbirds that breed in northern Alaska every year include both shrub nesting passerine species [e.g. Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii), American tree sparrows (Spizella arborea), and American robins (Turdus migratorius)] and open tundra nesters [e.g. Lapland longspurs (Calcarius lapponicus) and Smiths longspurs (Calcarius pictus)] (Poole, 2005). This broad distinction in nesting habitat characteristics suggests that as arctic tundra regions become increasingly shrubby, nest habitat quality will increase or decrease depending on individual species preferences (Sokolov et al., 2012; Henden et al., 2013). However, the potential importance of more nuanced habitat characteristics associated with increasing shrub dominance – such as shrub height and species dominance – also require consideration. While a few studies have examined how some tundra nesting species such as Lapland longspurs and White-crowned sparrows discriminate among microtopographical fea-

tures or the presence of water (Rodrigues, 1994; Wingfield et al., 2004; Boal & Andersen, 2005), most studies have only broadly characterized nest site preferences based on hill slope position and vegetation community type cover (e.g. Blanchard Oakeson, 1954). However, Norment (1993) conducted a study in a forest-tundra ecotone that explicitly showed the importance of forest structural characteristics to nesting sparrows, suggesting that changes to the tundra’s physical structure that accompany increasing deciduous shrub dominance require consideration. Further, to our knowledge, no tundra studies have yet considered whether shrub-nesting songbirds exhibit nesting preferences for or against any particular deciduous shrub species (i.e. willow, birch, and alder). In addition to vegetation height and species composition, shrub-dominated communities likely differ relative to graminoid-dominated tundra in the quality and quantity of nutritional resources – such as berries, inflorescences, green foliage, and arthropods – that are available to songbirds. How increasing shrub dominance will impact food resource availability for tundra breeding songbirds has not been explored. In a previous study (Rich et al., 2013), we found greater arthropod availability in graminoid compared to shrub-dominated tundra communities, but the scope was limited to ground-dwelling arthropods (i.e. spiders and beetles), and excluded the contribution of canopydwelling communities (i.e. flies). As arthropods are a high-protein food resource required in abundance by chicks (Visser et al., 1998), landscape-level changes in arthropod availability that may accompany increasing shrub dominance could play an essential role in determining songbird reproductive success in arctic tundra. The overall goal of this study was to understand and predict how increasing deciduous shrub dominance may alter nesting habitat extent and food availability in northern Alaskan tundra for two migratory songbird species – Gambel’s White-crowned sparrows (a largely boreal species) and Lapland longspurs (an arctic specialist). Because these species represent two broad groups of arctic tundra breeding songbirds with contrasting habitat requirements – shrub nesting vs. open tundra nesting, respectively – they serve as indicator species that inform how songbirds with similar nest site preferences may experience shifts in habitat as warming continues. Our specific objectives were to: (i) determine how vegetation height, plant species presence/ absence, and plant community type influence nest site selection for Gamble’s White-crowned sparrows and Lapland longpsurs; (ii) quantify and characterize food resources (arthropods, berries, and inflorescences) available to nesting songbirds in several shrub and graminoid-dominated communities; (iii) obtain a first

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

SHRUB COVER CHANGE ALTERS SONGBIRD HABITAT 3 approximation of future songbird nesting habitat availability in our study region by combining existing knowledge and our own field-based characterization of nest site vegetation with previously modeled predictions for the distribution of arctic tundra vegetation community types by the year 2050 (Pearson et al., 2013). To our knowledge, this is the first attempt to use a published vegetation model to quantifiably predict changes in nesting habitat of songbirds in the Arctic.

Materials and methods

Study area The study area includes tundra in the vicinity of the Arctic Long Term Ecological Research (ARC LTER) site at Toolik Field Station in the arctic foothills region of Alaska (68°380 N, 149°340 W, elevation 760 m) (Fig. 1). The four research sites include Roche Mountonee (ROMO), Toolik Lake Field Station (TLFS), Imnavait Creek (IMVT), and Sag River-Department of Transportation Camp (SDOT). Sites were chosen in May 2010 to represent the most common shrub tundra types in the northern foothills of the Brooks Range. Each of the four sites includes two 20 000 m2 plots: one graminoid-dominated plot (Open plot) and one woody deciduous shrub-dominated plot (Shrub plot). These two plot types were selected based on a priori information of the general tundra vegetation community types preferred by the two study songbird species – White-

crowned sparrows (shrub tundra nesters) and Lapland longspurs (open tundra nesters) – so that our plot vegetation and arthropod food resource availability measured within each plot can be directly related to the nearby habitats in which study nests were found. Within each Open and Shrub plot, two permanent 100-m transects were established, for a total of eight Open and eight Shrub transects. Along each transect, ten 1-m2 permanent vegetation quadrats were established at 10-m intervals for repeated, nondestructive vegetation measurements. Open plots are made up of short tussock tundra vegetation, comprised of sedges (e.g. Eriophorum spp. and Carex spp.), forbs (e.g. Polygonum spp. and Pedicularis spp.), dwarf evergreen shrubs (e.g. Vaccinium vitis-idaea), occasional dwarf deciduous shrubs (e.g. Betula nana, Salix pulchra), mosses, and lichens (see Table S1). Shrub plots are made up of either dwarf or erect-deciduous shrub vegetation, dominated by a mixture of Salix pulchra, Salix glauca, Salix alaxensis, Salix richardsonii, Betula nana, and Vaccinium uliginosum, with a mixture of sedges, evergreens, and other herbaceous perennials (IMVT and TLFS), or, riparian shrub tundra vegetation dominated by taller shrubs, mainly Salix alaxensis and Betula nana (ROMO and SDOT) (see Table S1). The height of the tallest deciduous shrub in each quadrat was measured in each Open and Shrub plot, and the mean of these values was used to characterize the average maximum height of shrubs in each of the four Open and Shrub plots: 16  1 cm/22  2 cm (IMVT Open/Shrub), 23  2 cm/35  4.5 cm (TLFS Open/Shrub), 13  2 cm/86  11 cm (ROMO Open/Shrub), and 28  2 cm/84  20 cm (SDOT Open/Shrub).

Fig. 1 Map of Alaska (inset) and map of the North Slope of Alaska showing the location of the four field sites used in this study (cross). The dashed polygon indicates the approximate area within which nest searching was conducted. © 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

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Plant species Fig. 3 The percentage of Lapland longspur (LALO) (a) and Gambel’s White-crowned sparrow (GWCS) (b) nests placed directly under or within a specific plant species. Values represent the means of all studied nests for each songbird species (n = 57 for Lapland longspurs, n = 48 for White-crowned sparrows.

with Shrub plots (F1,155 = 11.5, P = 0.0009), while abundance did not differ between vegetation types (P > 0.05, Fig. 4c and d). Berries and inflorescence abundance. There were no significant differences in either berry or inflorescence abundance in Shrub relative to Open plots (P > 0.05). Berry abundance across the four sites averaged 308 m2 (164 SEM) in Open and 380 m2 (174 SEM) in Shrub plots, while inflorescence abundance averaged 847 m2 (347 SEM) in Open and 677 m2 (373 SEM) in Shrub plots.

Future songbird habitat availability We found that relative to the present day (Fig. 5a), White-crowned sparrow habitat extent is predicted to increase by 22% under the 5 km (Fig. 5b) and by 58% under the unlimited seed dispersal (Fig. 5c) scenarios. Lapland longspur habitat is predicted to be reduced between 18% under the 5-km dispersal scenario (Fig. 5b) and 56% under the unlimited dispersal scenario (Fig. 5c).

Our findings strongly suggest that increasing shrub dominance in northern Alaska will enhance Whitecrowned sparrow habitat quality and extent and diminish that of Lapland longspurs, but also reveal more nuanced preferences for nest site characteristics, particularly for White-crowned sparrows. Our 2 years of nest site data suggest that White-crowned sparrows prefer to nest under shrubs of moderate height but not under shrubs taller than 1 m. However, due to the inherent difficulty in locating nests under large compared to smaller shrubs, we may have slightly underestimated the use of shrubs taller than 1 m as nesting habitat by White-crowned sparrows. As shrubs become more abundant and expand their range, taller, denser shrubs may actually constrain habitat availability for this species. Although it is possible that arctic White-crowned sparrows breeding north of the Brooks Range may adapt by nesting under taller shrubs more than they do at present, this seems unlikely to occur as even populations breeding in forested ecosystems do not nest directly in or under trees, instead preferring the shelter of moderate height shrubs (Norment, 1993; Chilton et al., 1995). White-crowned sparrows do utilize tall shrubs as perches for singing, territory defense and predator lookouts, suggesting that even though tall shrubs may be unsuitable for nesting under, they likely serve an important secondary role in nest site selection (J.C. Wingfield, personal observation). In our study, we documented that White-crowned sparrow nests were only found on the edges of dense shrub thickets (where tall shrubs formed a near-complete canopy; also see Chilton et al., 1995; Norment, 1993), and rarely, if ever, located within them, suggesting that documented increases in shrub density by existing shrubs (Myers-Smith et al., 2011; Tape et al., 2012) may actually reduce White-crowned sparrow nesting habitat (J.C. Wingfield, personal observation). In contrast to the strong influence of structural characteristics on nest site selection, we found that White-crowned sparrows show no preference for nesting among the four most common tall birch and willow species present in the arctic foothills region of Alaska. This provides new insight into current and future White-crowned sparrow habitat availability given there is significant heterogeneity in the dominant deciduous shrub species among various locations in northern Alaska (Sturm et al., 2001; Tape

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S H R U B C O V E R C H A N G E A L T E R S S O N G B I R D H A B I T A T 13 Wingfield JC, Owen-Ashley N, Benowitz-Fred-Ericks ZM et al. (2004) Arctic spring: the arrival biology of migrant birds. Acta Zoologica Sinica, 50, 948–960. Zeng H, Jia G, Epstein H (2011) Recent changes in phenology over the northern high latitudes detected from multi-satellite data. Environmental Research Letters, 6, 045508. Zhang W, Miller PA, Smith B, Wania R, Koenigk T, D€ oscher R (2013) Tundra shrubification and tree-line advance amplify arctic climate warming: results from an individual-based dynamic vegetation model. Environmental Research Letters, 8, 034023. doi:10.1088/1748-9326/8/3/034023. Z€ ockler C (1998) Patterns in Biodiversity in Arctic Birds. WCMC, Cambridge, UK. Z€ ockler C (2005) Migratory bird species as indicators for the state of the environment. Biodiversity, 6, 7–13. Z€ ockler C, Lysenko I (2000) Water birds on the edge. First circumpolar assessment of climate change impact on Arctic breeding water birds. WCMC Biodiversity Series

Supporting Information Additional Supporting Information may be found in the online version of this article: Table S1. Mean relative abundance (percent cover) of plant growth forms and woody stems in each plot based on ground area of each quadrat. Values represent mean quadrat values across the two transects within each plot type (n = 20). Error estimates are shown in small font and represent 1 SEM.

No. 11, Cambridge, UK.

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

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% of nests

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Eri vag tussock

Car spp.

Moss

Other

Plant species 100

(b)

% of nests

80 60 40 20 0

Betula nana Salix pulchra Salix alaxensis Salix glauca

Other

Plant species Fig. 3 The percentage of Lapland longspur (LALO) (a) and Gambel’s White-crowned sparrow (GWCS) (b) nests placed directly under or within a specific plant species. Values represent the means of all studied nests for each songbird species (n = 57 for Lapland longspurs, n = 48 for White-crowned sparrows.

with Shrub plots (F1,155 = 11.5, P = 0.0009), while abundance did not differ between vegetation types (P > 0.05, Fig. 4c and d). Berries and inflorescence abundance. There were no significant differences in either berry or inflorescence abundance in Shrub relative to Open plots (P > 0.05). Berry abundance across the four sites averaged 308 m2 (164 SEM) in Open and 380 m2 (174 SEM) in Shrub plots, while inflorescence abundance averaged 847 m2 (347 SEM) in Open and 677 m2 (373 SEM) in Shrub plots.

Future songbird habitat availability We found that relative to the present day (Fig. 5a), White-crowned sparrow habitat extent is predicted to increase by 22% under the 5 km (Fig. 5b) and by 58% under the unlimited seed dispersal (Fig. 5c) scenarios. Lapland longspur habitat is predicted to be reduced between 18% under the 5-km dispersal scenario (Fig. 5b) and 56% under the unlimited dispersal scenario (Fig. 5c).

Our findings strongly suggest that increasing shrub dominance in northern Alaska will enhance Whitecrowned sparrow habitat quality and extent and diminish that of Lapland longspurs, but also reveal more nuanced preferences for nest site characteristics, particularly for White-crowned sparrows. Our 2 years of nest site data suggest that White-crowned sparrows prefer to nest under shrubs of moderate height but not under shrubs taller than 1 m. However, due to the inherent difficulty in locating nests under large compared to smaller shrubs, we may have slightly underestimated the use of shrubs taller than 1 m as nesting habitat by White-crowned sparrows. As shrubs become more abundant and expand their range, taller, denser shrubs may actually constrain habitat availability for this species. Although it is possible that arctic White-crowned sparrows breeding north of the Brooks Range may adapt by nesting under taller shrubs more than they do at present, this seems unlikely to occur as even populations breeding in forested ecosystems do not nest directly in or under trees, instead preferring the shelter of moderate height shrubs (Norment, 1993; Chilton et al., 1995). White-crowned sparrows do utilize tall shrubs as perches for singing, territory defense and predator lookouts, suggesting that even though tall shrubs may be unsuitable for nesting under, they likely serve an important secondary role in nest site selection (J.C. Wingfield, personal observation). In our study, we documented that White-crowned sparrow nests were only found on the edges of dense shrub thickets (where tall shrubs formed a near-complete canopy; also see Chilton et al., 1995; Norment, 1993), and rarely, if ever, located within them, suggesting that documented increases in shrub density by existing shrubs (Myers-Smith et al., 2011; Tape et al., 2012) may actually reduce White-crowned sparrow nesting habitat (J.C. Wingfield, personal observation). In contrast to the strong influence of structural characteristics on nest site selection, we found that White-crowned sparrows show no preference for nesting among the four most common tall birch and willow species present in the arctic foothills region of Alaska. This provides new insight into current and future White-crowned sparrow habitat availability given there is significant heterogeneity in the dominant deciduous shrub species among various locations in northern Alaska (Sturm et al., 2001; Tape

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

SHRUB COVER CHANGE ALTERS SONGBIRD HABITAT 7

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Fig. 4 Mean aggregated abundance (a and c) and biomass (b and d) for canopy dwelling (sweep net sampling, a and b) and ground dwelling (pitfall sampling, c and d) arthropods for Shrub and Open plots. Means are sums of weekly samples collected from June 6 to July 19, 2012, and averaged across all four field sites. Error bars represent 1 SEM. For each figure, statistically significant differences between Open and Shrub plot means are shown with an asterisk (P < 0.05). Arthropod abundance and biomass values were averaged among the two transects within each plot type (Open or Shrub) at each of the four sites, and values presented represent the means of each plot type across the four field sites (n = 4).

et al., 2006, 2010). Few studies have explored the relative rates at which different species of deciduous shrubs become more dominant as shrubification of tundra regions continues, but there is some evidence of variability in growth rates among species as a result of both direct and indirect effects of warming (Bret-Harte et al., 2002). Although alder is much less common in our study region than birch or willow, White-crowned sparrows do nest in alder communities in central Alaska (Blanchard Oakeson & Erickson, 1949). Our findings suggest that nesting habitat availability for White-crowned sparrows may be unrelated to variation in which species of shrub is most dominant. Instead, White-crowned sparrow nesting habitat will be largely determined by changes in shrub height and their distribution across the landscape relative to other plant growth forms. In sharp contrast to White-crowned sparrows, Lapland longspurs nest exclusively in graminoid-dominated communities where deciduous shrubs are sparsely distributed and do not exceed 20 cm in height. Further, nesting Lapland longspurs exhibit very strong plant species specificity, placing their nests primarily into the sides of tussocks that are created by a single sedge species, E. vaginatum. This species specificity is presumably associated with the biophysical structure of the tussock, which likely offers protection from predators, overheating caused by direct solar radiation, and insulation from low temperatures and inclement

weather (Williamson, 1968; Boal & Andersen, 2005). By building on what is already known regarding Lapland longspur nest site preferences (Hunt et al., 1995; Hussell & Montgomerie, 2002), our findings provide confirmation that preferred nesting habitat of Lapland longspurs in the arctic foothills region of Alaska is likely to decline as deciduous shrub dominance increases at the expense of graminoid and moss species in low arctic, wet tundra types (e.g. Cornelissen et al., 2001; Wahren et al., 2005; Walker et al., 2006; Elmendorf et al., 2012).

Food resources differ between graminoid- and shrubdominated communities In addition to vegetation compositional and structural characteristics that determine the availability and location of songbird nesting habitat, food resource availability for both adults and young is of critical importance to their reproductive success (Lack, 1968; Visser et al., 1998). Our quantitative measurements of food resource availability show that canopy-dwelling arthropod biomass (i.e. flies and web-building spiders) was higher in shrub tundra relative to open tundra; however, ground-dwelling arthropod biomass (i.e. ground spiders and beetles) was lower in shrub tundra likely due to the greater number of large spiders and beetles active in more open habitat (Rich et al., 2013).

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

8 N . T . B O E L M A N et al. (a)

(b)

(c)

Fig. 5 Predicted present day (a) and future (b and c) nesting habitat distributions for Gambel’s White-crowned sparrows (GWCS) and Lapland longspurs (LALO) in the arctic foothills regions of Alaska by the year 2050 based on two different modeling scenarios. Percentage values shown in (b) and (c) indicate the direction and percentage of habitat extent change relative to present day (a) predicted for each species.

Although White-crowned sparrows and Lapland longspurs have highly specialized and distinct nesting requirements, like most passerines they are opportunistic feeders whose summer diet includes plant-based resources and arthropods (Custer & Pitelka, 1977; Seastedt & Maclean, 1979; Chilton et al., 1995; Hussell & Montgomerie, 2002). The diet of arctic breeding populations of both species is determined by seasonal and geographic variation in availability of these food sources rather than food selection in northern Alaska, Canada, and in Greenland (Chilton et al., 1995; Hussell & Montgomerie, 2002). Stomach content and observational data have both shown that during the nesting period – when arthropod availability is high – adult diet is comprised primarily of arthropods and plant matter (mainly seeds, but also leaf material), while nestlings are fed arthropods almost exclusively (Seastedt & Maclean, 1979; Chilton et al., 1995; Hussell &

Montgomerie, 2002). Arthropod consumption by both songbird species is dominated by crane flies (Tipula carinifrons), sawflies (Tenthredinidae), midges (Chironomidae), and muscoid flies (Muscidae), but also includes beetles (Coleoptera), spiders (Araneida), caterpillars (Lepidoptera), true bugs (Hemiptera), and various other insects (Poole, 2005). Analysis of our own limited collection of fecal samples from adult and nestling of both species confirms this diet (unpublished data). Our own observations confirm those in older literature that suggest arctic breeding Lapland longspurs forage primarily in short vegetation and remain on the ground, picking insects and spiders off low lying tundra plants or snatching them from the air (Drury, 1961; Williamson, 1968; Watson, 1957), while White-crowned sparrows have a much broader foraging niche that also includes picking from taller shrub canopies (Morton, 1967; Hussell & Montgomerie, 2002; J.C. Wingfield, personal observation). Further, during the nesting period, both species limit the spatial extent of their foraging to be closer to their nests such that Lapland longspurs foraged primary in nearby open tundra, while White-crowned sparrows forage mainly in nearby shrub dominated tundra (Blanchard Oakeson & Erickson, 1949; Chilton et al., 1995, J.C. Wingfield, personal observation). Due to these differences in foraging habitat, future changes in arthropod availability are likely to affect these two species differently. We hypothesize that arthropod community composition will likely shift towards more abundant small flies and spiders, and fewer large spiders and beetles as shrubs become more dominant. This prediction conservatively assumes that arthropod niches will remain similar as climate continues to warm, although arthropod populations may respond in somewhat unpredictable ways. Given that both White-crowned sparrows and Lapland longspurs consume primarily flies during the nesting period, their preferred prey availability may be enhanced. This supports the hypothesis by Ims & Henden (2012) that willow thickets harbor an abundant and rich community of arthropods that may benefit all songbird species, although the longspurs will only benefit if they expand their foraging habitat during nesting to include taller, shrub dominated communities. Currently, longspurs mostly do not venture into shrub habitats to forage so such a behavioral change seems unlikely. It is also important to consider that temporal changes in availability and quality of this critical, high quality food source may have more of an impact on songbird body condition and reproductive success (Durant et al., 2007; Both et al., 2009; Bolduc et al., 2013) than the spatial differences explored here, as the timing of arthropod emergence is predicted to

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

SHRUB COVER CHANGE ALTERS SONGBIRD HABITAT 9 become mismatched with the timing of songbird nesting in the Arctic (Tulp & Schekkerman, 2008; Both et al., 2009).

Greater shrub dominance will have contrasting effects on future habitat extent Using the results presented above with the vegetation model, we predict a general reduction in Lapland longspur and concurrent expansion in White-crowned sparrow nest habitat availability as tundra vegetation shifts from graminoid dominated to erect dwarf- and low-shrub tundra, or tree cover mosaic (Pearson et al., 2013). Our findings support Sokolov et al.’s (2012) expectation that shrub specialist songbirds in Eurasian arctic tundra will become more abundant as tundra shrubs become increasingly dominant, largely due to boreal species expanding northward into tundra at the expense of arctic tundra specialists. Similarly, our model predictions at least partially support findings by Henden et al. (2013) that suggest that the abundance of shrub nesting songbird species will increase while open nesting songbirds will be unaffected. Our predictions are both complementary to these studies and novel. While previous research surveyed the abundance of songbirds in relation to vegetation community characteristics, we focused explicitly on nest site selection to allow us to understand a crucial factor in reproductive success. In addition, this study was conducted in northern Alaska, which complements previous studies focused on Eurasian arctic tundra, thereby contributing towards a pan-Arctic perspective on changing songbird habitat. While White-crowned sparrows are likely to benefit from increasing shrub dominance in the Arctic, unless Lapland longspurs are able to adapt to the ongoing compositional and structural changes in large portions of their current nesting habitat, a decline in their breeding populations is a likely outcome and may be further amplified by ongoing changes in their wintering habitats (Newton, 2004; Holmes, 2007). Further, unlike more southern species, Lapland longspurs have very limited opportunity to disperse northward as the tundra gives way only to the polar marine environment. Alterations of nesting habitat will determine future reproductive success and population viability in the long term, although changes to these species’ wintering grounds can also affect their populations (Norris et al., 2004). This study focuses on Lapland longspurs and Whitecrowned sparrows, but our first approximations of future spatial extent of both habitat types are relevant to habitat change for species with similar nesting requirements. For example, American tree sparrows

and American robins are obligate shrub nesters, while Smith’s longspurs nest in graminoid dominated tundra communities (Naugler, 1993; Hunt et al., 1995; Sallabanks & James, 1999). However, given our field-based findings suggest nuanced preferences for specific nest site characteristics of White-crowned sparrows (e.g. moderate height shrubs), accurate species level predictions require closer examination of species-specific nest site preferences and more refined modeling tools than currently exist.

Current vegetation model constraints limit habitat predictions Relative to other ecosystems, there are very few predictive studies of the spatial distribution of vegetated habitat for tundra wildlife (Pearce et al., 2012; Hope et al., 2013; Gustine et al., 2014; Stralberg et al., 2014). This is despite overwhelming evidence from existing literature that tundra vegetation communities are changing significantly (ACIA, 2004), and that such changes affect tundra wildlife by altering food and shelter availability (Klein et al., 2005; Post & Forchhammer, 2008; Joly et al., 2009; Willerslev et al., 2014). We suspect that this gap exists largely because, unlike forested habitats where gap dynamics models are widely available and mature (as reviewed by Bugmann, 2001; and Busing & Daniel, 2004), existing tundra modeling tools cannot yet simulate the finescale habitat characteristics that are critical to many wildlife species. For example, willows, but not dwarf birch nor alder, are the strongly preferred forage species for caribou, moose, ptarmigan, and other wildlife species (Viereck & Little, 2007; den Herder et al., 2008; Tape et al., 2010), but current models do not resolve species dominance in deciduous shrub communities. In addition, occurrence of communities dominated by woody shrubs is spatially heterogeneous and dependent on the fine-scale topography of the landscape. Similarly, while tundra breeding caribou preferentially forage on willow shrubs during summer months, they tend to avoid tall, dense shrub canopies that are attractive to both moose and wolves (Briand et al., 2009). These habitat preferences highlight the need for habitat models to include high spatial resolution predictions of the physical structure of tundra vegetation. As one example, Pearson et al.’s (2013) ecological niche modeling of tundra vegetation communities (i.e. CAVM classes) was spatially explicit at 4.5 km (~20 km2) resolution, and had broad spatial coverage (i.e. the pan-Arctic domain). Despite being the most advanced predictions of future tundra vegetation communities to date, Pearson et al.’s (2013) effort has

© 2014 John Wiley & Sons Ltd, Global Change Biology, doi: 10.1111/gcb.12761

10 N . T . B O E L M A N et al. two major limitations that make it a first approximation of the potential extent and spatial pattern of future habitat change in northern Alaska. First, the fine-scale spatial heterogeneity of vegetation types in the arctic foothills region is not captured at 4.5 km spatial resolution. This limitation in spatial scale of the modeling was determined by the baseline CAVM and GLC2000 reference maps used in the predictions. Second, the broad characterization of vegetation communities by the CAVM vegetation classes did not provide information on the more nuanced vegetation community characteristics that are often critical to habitat quality and species preferences. Thus, the predictions made by Pearson et al. (2013) enable only a coarse estimation of the future of the contrasting habitat types used by nesting White-crown sparrows vs. Lapland longspurs. Not only does the spatial resolution mask the fine-scale patchy nature of the vegetation communities important to the birds but also physical properties of the vegetation are unaccounted for. For example, the presence and size of shrub thickets cannot be considered because shrub density and height is not known (although can be roughly approximated by the CAVM classes with height and structure definitions). For example, the CAVM G4 vegetation class includes dwarf shrubs up to 40-cm tall, whereas our field observations of nest site height (see Results) indicate Lapland longspurs nest exclusively in tussock tundra of less than 20-cm height while White-crowned sparrows nest in shrub tundra greater than 20-cm tall. This mismatch between model resolution and the scale of habitat characteristics important to songbirds allows for somewhat limited predictions of future habitat extents used by shrub or graminoid nesting songbirds, and this limitation likely extends to other wildlife species. Refining habitat predictions for habitat specialists would require tundra vegetation models (i) be based on fine spatial resolution (

Greater shrub dominance alters breeding habitat and food resources for migratory songbirds in Alaskan arctic tundra.

Climate warming is affecting the Arctic in multiple ways, including via increased dominance of deciduous shrubs. Although many studies have focused on...
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