Movements of Black-Capped Chickadees (Poecile Atricapillus) in a Highly Fragmented Urban Environment

Movements of black-capped chickadees (Poecile atricapillus) in a highly fragmented urban environment

Author: Robyn N. Perkins Mentor: D. L. (Dee) Patriquin

University of Alberta, Augustana Campus

Introduction Habitat fragmentation, urbanization, and functional connectivity Studies on the effects of fragmentation on arboreal bird species in Quebec1 and Banff National Park2 indicate that arboreal birds are increasingly reluctant to cross open areas as gap size increases. Consequently, pockets of habitat fragmented by human development become isolated and support smaller populations that are left with fewer opportunities to avoid predation, to forage, and to disperse or find mates.3,4 As a result, populations’ health and adaptability to environmental changes are compromised.1,5 Although the effects of fragmentation in natural and semi-natural areas have been well documented, findings of landscape-scale studies in areas with low degrees of fragmentation may not be applicable in highly fragmented environments, such as already urbanized landscapes.6 Urbanization leaves discrete patches of native vegetation in the urban matrix and has been associated with changes in animal behaviour, morphology, and genetics, and may lead to extinction.7 Furthermore, noise associated with roads may interfere with avian species’ predator avoidance, as well as mating and fledging communications.8 However, relatively little is understood about the mechanisms by which bird behaviour is altered in urban environments and how their movements are restricted.9 A compensator for habitat fragmentation is functional connectivity, which describes the degree that animal movement between suitable habitat patches is facilitated by landscape features, such as corridors and stepping-stones.3,4,10,11 Functional connectivity is important for several ecosystem functions including range expansion and recolonization, population persistence, and the maintenance of biodiversity by enabling movement between isolated populations.3,11,12 Connectivity is enhanced and protected against future landscape change when multiple pathways are available between habitats.13 For arboreal birds, continuous forest and interspersed trees improve functional connectivity.1,2 Black-capped chickadees: a synanthropic generalist species Black-capped chickadees (Poecile atricapillus) are a common year-round resident of both urban and rural centres in Alberta. Chickadees prefer deciduous-dominated forests with interspersed snags in which to carve out their cavity nests.14 If there is sufficient foliage for feeding, chickadees may also be found within urban environments.14,15 In recent years, the relative abundance of chickadees has decreased in the Parkland region of Alberta, likely a result of aspen-dominated forests and snags being cleared for agriculture and urban expansion.14 Studies by Desrochers and Fortin in agricultural areas near Edmonton suggest that chickadees strongly prefer traveling in forest cover, parallel to forest boundaries.16 Gaps in the forest are a significant barrier to chickadee movement due to the associated increase in predation risk and exposure to adverse weather conditions.12,17 Consequently, chickadees tend to venture no farther than 25 m from the canopy and exhibit strong resistance to crossing gaps as narrow as 50 m.18,19 Although relatively few studies have considered barriers to chickadee mobility in urban settings, evidence suggests that bridges and roads form barriers in all seasons with resistance increasing as road width and traffic levels increase.12 Additionally, there are possible cumulative effects of multiple roads across large areas.20,21 Due to their narrow width, railroads present a smaller obstacle.12 Man-made and natural water bodies create significant natural barriers, perhaps due to their role as territory boundaries or as an evolutionary response to predation risk.12,19,21 Despite the multitude of factors that can hinder mobility, even sparsely distributed trees can act as stepping-stones and increase functional connectivity.20 Due to the abundance, the

relatively large home range, and the vagility of chickadees, they are an ideal species with which to study habitat connectivity for arboreal birds within an urban setting. Circuitscape: a new tool for landscape ecology Landscape corridors have traditionally been mapped using a simplistic binary habitat- matrix paradigm and have been assumed to fulfill ecosystem processes with little post- implementation evaluation.3,22 However, different matrix compositions inhibit organism movements in different ways. Therefore, considerations of only patch size and isolation are not enough when the matrix is heterogeneous, such as in an urban setting.22,23 Moreover, traditional corridors typically neglect processes of habitat selection and movement for target organisms. Landscape corridors are species-specific and their effectiveness depends on perceptual ranges and behavioural responses to landscape structure for a given target species.3 Nonetheless, the importance of behaviour has been widely overlooked in wildlife corridor selection.9 Taylor et al. suggest that landscape managers require more explicit use of behaviour-driven movement to create effective corridors.10 Electric circuit theory combines behavior-driven movement rules and habitat modeling across a landscape to quantify connectivity as resistance and current.3 Circuitscape is a relatively new tool that uses a combination of electric circuit theory and random walk analysis to give an estimation of all possible pathways available to moving individuals13 and is better supported by analytic theory than traditional least-cost-path analysis.24 This combination allows measures of current (flow of individuals) and resistance (opposition to individual movement) between nodes to be interpreted in terms of the movement probabilities of individuals across a resistance raster.4,25 As a result, Circuitscape removes potential sources of bias in corridor selection and generates an intuitive output map of connectivity.3,13 Although Circuitscape has been used to study wildlife corridors for a variety of mammals and plants, it has not commonly been used for arboreal bird species.4,26,27 However, arboreal birds are a suitable subject for Circuitscape models since they can only perceive their close surroundings,2 therefore their movements may be similar to the virtual random walkers assumed in Circuitscape. Study purpose Although urbanization and habitat fragmentation associated with human development is a serious threat to global biodiversity, ecologists have traditionally focused on the effects of disturbance in wilderness areas.28 However, with human development continually pushing outward, we need to better understand how ecosystems function within these highly fragmented environments. Chickadees can provide a good model to test urban connectivity for arboreal birds, and enhance our understanding about the impacts of urbanization and the synergistic effects of barriers on chickadee mobility. Using Circuitscape to model habitat connectivity and principles of island biogeography and metapopulation dynamics, we hypothesized that more connected neighbourhoods will have higher chickadee densities than isolated neighbourhoods. By better understanding the cumulative effects of features that facilitate or inhibit chickadee mobility, trends may be generalized and used to improve overall habitat connectivity. This may help foster healthy bird populations at a time with one-third of all North American bird species are experiencing significant population declines.29

Methods Study area This study took place within the city limits of Camrose, Alberta, Canada (53°01’N, 112°50’W). Camrose is located in the Aspen Parkland region, and has a population of approximately 18,000 residents.30 The city is committed to sustainable development31 and its moderate size offered an easily defined, heterogeneous landscape through which arboreal birds travel. Ringed by agricultural fields, dispersal into the city from surrounding areas may be restricted. Other possible barriers to wildlife movement include a four-lane highway (Highway 13), smaller residential roads, and sparsely vegetated recreational, commercial, and industrial areas. Major corridors may be provided by the riparian buffer of the central water feature (Camrose Creek) and interspersed trees that may act as ‘stepping-stones’.

GIS and Circuitscape mapping Circuitscape requires an input ‘resistance’ raster that identifies relative resistances of landscape features. The raster was built from relevant GIS layers, including some data layers provided by the City of Camrose GIS department: a high resolution spring 2016 orthophoto, city boundary, land-use zoning, roads, railways, and partial layers for water bodies and tree cover. The remainder of the water body and tree cover layers were digitized on-screen using the orthophoto and ArcGIS 10.3.1. A buffer spanning 20% of the city limits was classified as cropland to decrease the bias in Circuitscape associated with artificial edges in input features.4,32 Line features (e.g. roads, railroads) and point features (e.g., individual trees) were buffered with average widths estimated from the orthophoto. A 25 m and a 50 m buffer was added to the tree cover layer to reflect distances chickadees are willing to travel from vegetated cover.19 We selected a 10 m x 10 m raster cell size, a distance that chickadees can perceive, and a match for available data quality.13,25 All relevant layers were converted into rasters and reclassified based on the degree that they facilitate or inhibit chickadee movements, using a classification scheme adapted from a previous model created for the city of Edmonton.33 Continuous tree cover was rated as +10, the 25 m buffer as +8 and the 50 m buffer as +5.19 Water features and major roads were given a score of -10.12,19,21 Arterial roads (-8), secondary roads (-6), and residential roads (-2) were similarly scored based on relative traffic levels and road widths. Disturbances related to land use were also estimated using zoning parcels: agriculture (-1), residential (-2), commercial and industrial (-3). These classified raster layers were then added together into a resistance raster. To clean the raster for use in Circuitscape, values lower than -1 were considered impenetrable to chickadee movements and were set to “NoData”. Several source and ground nodes were selected around the outside of the study area and a sample output was obtained in Circuitscape. Nodes were then added to the interiors of isolated neighbourhoods, based on personal knowledge of chickadee presence. This produced a more accurate map of habitat connectivity that included all neighbourhoods within the city.

Local chickadee densities To investigate the impacts of habitat fragmentation on chickadee populations, we surveyed winter chickadee abundance at ten points selected near the centre of habitat patches on either side of several mapped barriers to chickadee movement. Since chickadees form flocks with larger home ranges in the winter, an estimation of densities within a relatively large area may be obtained.12,34 Point count locations were recorded with a Garmin 60cx GPS unit to an error of 4 m. In the morning hours on two days in November, bird activity within a 25 m radius

was observed for an initial period of 5 minutes, noting where and how chickadees were moving. Afterwards, a chickadee mobbing call was played for 2 minutes from a speaker placed on the ground, followed by 3 minutes of observation. Chickadees are very responsive to mobbing calls from their own species and will travel some distance to investigate,12 which allowed a more accurate estimate of flock size in each area. Point counts were performed twice, with two days between tests to reduce habituation to the mobbing call and to account for movements within a chickadee’s home range. Point counts were not performed in high winds or precipitation since these weather patterns diminish bird activity.19 Chickadee densities were calculated using the average abundance at each site. Resulting densities were compared to the Circuitscape model output, to compare densities in more isolated ‘islands’ and connected areas.

Results Circuitscape output The resistance raster was entered into Circuitscape to produce a current/flow map estimating habitat connectivity in Camrose for chickadees (Figure 1). Large, open areas (agricultural fields) on the city’s outskirts resulted in low connectivity surrounding the city, except for a handful of narrow corridors created by shelterbelts and vegetated ditches by railroads. However, overall there was relatively high connectivity for chickadees within the city. The exceptions came from major roads (Highway 13 and 13A) and large parks, which blocked connectivity. Although some individuals may cross these barriers, they created isolated patches within neighbourhoods. Additionally, low connectivity existed on the western and eastern edges of town within the large commercial and industrial areas respectively. Local chickadee densities Using the mobbing call to lure chickadees to the centre of the point count area resulted in higher density calculations than the initial observation period (Table 1). The mobbing call did not appear to bring in multiple chickadee flocks or draw in chickadees from surrounding territories. No chickadees were observed at sites 6 or 10, and very few were observed at site 7. Sites 1 and 9 had very small flocks or only floaters (who do not belong to a flock, but drift between established flock territories34). Larger flocks were recorded at sites 2, 3, 4, 5, and 8 with between six to eight chickadees observed in each of these sites.

Table 1. Black-capped chickadee densities at point count sampling sites. Data was collected on November 20 and 23, 2016 with results represented as the average of the two days. Initial Mobbing Site Abundance Abundance Site Description 1a 4.0 0.5 Southern creek reaches, natural 2b 4.0 7.5 South of Hwy13A, natural 3b 3.0 7.0 North of Hwy13A, natural 4c 2.0 5.5 West of 50St, residential 5c 1.5 5.5 East of 50St, residential 6d 0.0 0.0 South of Hwy13, residential 7d 1.0 0.0 North of Hwy 13, commercial 8a 3.5 6.5 Northern creek reaches, natural 9e 1.0 3.5 West of Hwy13A, residential 10e 0.0 0.0 East of Hwy13A, ringed by agriculture, residential *Superscript letters indicate a pairing of sample sites across a barrier

Figure 1. Connectivity map of Camrose with roads, railroads, and water features overlaid for spatial reference and with insets of point counts locations estimating relative chickadee densities

Comparisons Overall, densities were higher inside residential areas, where functional connectivity for chickadees was high. More isolated sites had lower chickadee densities. Site 10 was isolated from the interior of the city by Highway 13A and from natural sites by cropland, and was uninhabited by chickadees. Similarly, sites 4, 5, 6, 7, and 9 also exhibited relatively low degrees of habitat connectivity and low chickadee densities. These sites had less dense tree cover than highly connected neighbourhoods (possibly also indicating less dense resources), and had close proximity to busy roads and large gaps in forest cover. Conversely, sites 2, 3, and 8 along the Camrose Creek had high chickadee densities and presented plentiful habitat and high connectivity to adjacent ranges. Site 1 did not conform to this pattern, perhaps due to poor site selection as it was surrounded with sparse aspen trees.

Discussion Implications of findings When combined with the density estimations (Table 1), this model (Figure 1) supported the hypothesis that more connected areas are able to support larger chickadee populations than those that are poorly connected. Other ecologists have speculated that the fragmentation caused by anthropogenic barriers in urban environments likely results in decreased bird densities.12,20,21 To our knowledge, this is the first study to confirm this hypothesis. Although sites 1, 4, and 7 did not conform to this pattern, these sites may not in fact be habitat patches, but rather corridors between patches. Since Circuitscape shows pinch points through which chickadees are funneled as ‘green’, corridors may be confused for habitat patches. However, corridors cannot support abundant bird populations themselves due to low resource availability, but are important for gene flow and population resilience.11 These results are important to consider when pursuing the goals of sustainability through best-practice development. While it is vital to maintain protected areas for species that cannot exist in an urban setting, we also need to allow our cities to foster greater biodiversity. One way we can do this is by providing higher degrees of connectivity across the urban landscape to increase the overall density of birds that can be supported. As an example for other urban settings, corridors within Camrose are provided by riparian buffers along natural water features, as well as by trees planted alongside residential roads that act as stepping-stones connecting one suitable habitat patch to another. Additional corridors could be added in the western commercial and eastern industrial developments, which currently exhibit very low degrees of connectivity. While the map shows an abundance of moderate connectivity (yellow) in these areas, these routes are probably only used by risk-takers. It is unlikely that most chickadees would perceive many genuine dispersal routes and would instead go around these areas using the few corridors (green) that are present, even if a larger distance must be traveled.1,2 Furthermore, a handful of shelterbelts, treed ditches along railroads (both active and retired), and riparian zones are the only corridors connecting the city to surrounding areas. More shelterbelts leading into the city would improve connectivity across the landscape for arboreal species. Additionally, some of these corridors may provide enough habitat to support breeding populations of chickadees,34 which may spillover into the interior of Camrose, rescuing fragile populations.11 Although corridors for one species may not necessarily be used by another,4 if efforts are taken to improve habitat connectivity for an umbrella species, such as black-capped chickadees, it is possible that other arboreal bird species will similarly benefit including nuthatches, woodpeckers, and warblers.12,19,20 However, many of these species appear to be more sensitive to gaps and should be expected to behave more conservatively.17,19,21,36 To make travel easier for

arboreal bird species, tree density along existing corridors could be increased wherever the abatement cost is not too high. However, it must be acknowledged that other wildlife species require unvegetated areas for their survival. For example, the purple martin (Progne subis), requires large open areas for foraging and breeding.14 Therefore we must refrain from converting every open area in the city into a forest as this would result in a potential loss of biodiversity. Other ways we can help avian species in cities is by preserving snags for cavity nesters37 and planting a variety of native trees and shrubs to help retain native bird species.8,38

Limitations The limitations to circuit theory analysis include potential error in the creation of landscape resistance rasters.17 This limitation could be better addressed by using LiDAR data to refine the tree cover and water body layers as well as to provide an estimation of parking lot sizes and building footprints. Through using real canopy sizes rather than an averaged buffer to point features, the benefits derived from stepping-stones would be more accurately assessed. Studies by Edgar and Kershaw in Edmonton suggest that community age affects avian densities due to the associated age of planted vegetation;39 therefore tree age should be better reflected in the model. Some of the boulevard trees within the city are still very young, with canopies smaller than the average buffer, thus potentially overestimating connectivity in more recently developed areas. Including features such an estimate of tree age, building footprints, and parking lots at a higher resolution would improve the precision of the input data and result in a more accurate output map. Another limitation is Circuitscape’s use of random walkers to estimate connectivity across a landscape. Since random steps are independent of all others, movement of random walkers is not entirely realistic for many species, because it removes memory and knowledge of the landscape as well as ignoring learned changes in behaviour.13 However, chickadees may not retain a complete knowledge of their territory throughout all seasons, since the hippocampal region of their brain (associated with spatial memory) shrinks by up to 30% in the spring, due to a decreased reliance on food caches.40 As a result, their movements may be similar to random walkers than other species due to seasonal limitations to their knowledge of the landscape.

Future research requirements More research is required to understand the effect on chickadee mobility of species, age, and canopy size of the stepping-stone trees alongside residential roads. As well there is poor understanding about the effects of non-tree vegetation on avian movement behaviour. For example, chickadees may be moving amongst the cattails along riparian zones, a possible, but undocumented use of habitat. Similarly, bird feeders bolster winter survivability41 and may influence bird movement behaviours in urban areas, although this has not been thoroughly examined. Furthermore, additional research is required to understand avian movement behaviour associated with large bridges. For example, although site 3 was on the northern side of Highway 13A, it exhibited high chickadee density perhaps due to a possible corridor formed by a stretch of trees under a nearby bridge. However, one chickadee (and one woodpecker) was observed flying over the bridge rather than through these trees, a behaviour observed in other studies as well.20 Although there are knowledge gaps of species-specific responses to certain landscape features, Circuitscape generates an enhanced model of landscape connectivity.

Acknowledgements This research was made possible by Christi Bratrud from the city of Camrose GIS department through her provision of the city’s GIS data. I would also like to the Ian Basford for his insightful comments on previous drafts of the Circuitscape output map as well as Dee Patriquin for her continual support and guidance throughout the semester. Endnotes 11. Hilty JA. 2006. The ecological framework. 1. Bélisle M, Desrochers A, Fortin MJ. 2001. Hilty JA, Lidicker WZ, Merenlender AM, Influence of forest cover on the movements of editors. In: Corridor ecology: the science and forest birds: a homing experiment. Ecology. practice of linking landscapes for biodiversity 82(7): 1893-1904. conservation. Washington (DC): Island Press. p. 118-120. 2. Bélisle M, St. Clair CC. 2001. Cumulative effects of barriers on the movements of forest 12. Tremblay MA, St. Clair CC. 2009. Factors birds. Conserv Ecol. 5(9): 1893-1904. affecting the permeability of transportation and riparian corridors to the movements of 3. Chetkiewicz CL, St. Clair CC, Boyce MS. 2006. songbirds in an urban landscape. J Appl Ecol. Corridors for conservation: integrating pattern 46: 1314-1322. and process. Annu. Rev. Ecol. Evol. Syst. 37: 317-342. 13. McRae BH, Dickson BG, Keitt TH, Shah VB. 2008. Using circuit theory to model 4. Koen EL, Bowman J, Sadowski C, Walpole connectivity in ecology, evolution, and AA. 2014. Landscape connectivity for wildlife: conservation. Ecol. 89(10): 2712-2724. development and validation of multispecies linkage maps. Methods Ecol Evol. 5: 626-633. 14. The Federation of Alberta Naturalists. 2007. The atlas of breeding birds of Alberta: a second 5. Couvet D. 2002. Deleterious effects of restricted look. Edmonton (AB): The Federation of gene flow in fragmented populations. Conserv Alberta Naturalists. Biol. 16(2): 369-376. 15. Smith SM. 1993. Black-capped Chickadee. In: 6. Betts MG, Forbes GJ, Diamond AW. 2007. Poole A, editor. The Birds of North America. Threshold in songbird occurrence in relation to Ithaca (NY): Cornell University Press. landscape structure. Conserv Biol. 21(4): 1046- 1058. 16. Desrochers A, Fortin MJ. 2000. Understanding avian responses to forest boundaries: a case 7. Shochat E, Warren PS, Faeth SH, McIntyre NE, study with chickadee winter flocks. Oikos. 91: Hope D. 2006. From patterns to emerging 376-384. processes in mechanistic urban ecology. Trends Ecol Evolut. 21(4): 186-191. 17. Bélisle M. 2005. Measuring landscape connectivity: the challenge of behavioural 8. Chace JF, Walsh JJ. 2006. Urban effects on landscape ecology. Ecology. 86(8): 1988-1995. native avifauna: a review. Landscape Urban Plan. 74: 46-69. 18. Lima SL. 1985. Maximizing feeding efficiency and minimizing time exposed to predators: a 9. Hansen AJ, Knight RL, Marzluff JM, Powell S, trade-off in the black-capped chickadee. Brown K, Guide PG, Jones K. 2005. Effects of Oecologia. 66: 60-67. exurban development on biodiversity: patterns, mechanisms, and research needs. Ecol Appl. 19. St. Clair CC, Bélisle M, Desrochers A, Hannon 15(6): 1893-1905. S. 1998. Winter responses of forest birds to habitat corridors and gaps. Conserv Ecol. 2(2): 10. Taylor PD, Fahrig L, Henein K, Merriam G. 13. 1993. Connectivity is a vital element of landscape structure. Oikos. 68(3): 571-573. 20. Tremblay MA, St. Clair CC. 2011. Permeability of a heterogeneous urban landscape to the movements of forest songbirds. J Appl Ecol. 48: 679-688.

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