117 Notornis, 2016, Vol. 63: 117-129 0029-4470 © The Ornithological Society of New Zealand Inc. Bird densities at recreational parks within the species’ native and introduced ranges JAMES A. J. MORTIMER 5 Flinders Road, Heathcote Valley, Christchurch 8022, New Zealand Abstract Many bird species have been successfully introduced beyond their natural range, some becoming more abundant in their new environment than in their country of origin. In this study, bird density was measured at 2 study areas comprising a total of 48 recreational parks in northern England and Canterbury, New Zealand, for 10 focal species (native to the former, introduced to the latter). Site characteristics and presence of other bird species were also recorded and investigated as potential explanatory factors for differences in density between the 2 study areas. Common redpoll, common starling, European greenfinch and house sparrow had significantly higher densities at the New Zealand sites. Analysis using generalised linear models revealed a negative relationship between common starling density and proportion cover of trees and shrubs, and a positive relationship between common redpoll, common starling and European greenfinch densities and site species richness. However, since there were no significant differences in site characteristics or site species richness between study areas, these relationships could not account for higher densities at the New Zealand sites. There was an apparent negative relationship between densities of common starling and house sparrow and foraging guild diversity, suggesting that interspecific competition may contribute to differences in density between study areas. The proportion of variation explained by the models was relatively low, suggesting that there may have been missing variables that influenced species density. More detailed study of a wider range of variables is required to investigate this further. Mortimer, J.A.J. 2016. Bird densities at recreational parks within the species' native and introduced ranges. Notornis 63 (3-4): 117-129. Keywords: species richness; foraging guild; habitat; interspecific competition INTRODUCTION many British birds. These were particularly Introduction of species beyond their natural range successful in heavily modified urban and agricultural is a global phenomenon and has been the subject habitats (MacLeod et al. 2009), whereas many birds of a number of studies (e.g., Case 1996; Davis native to New Zealand were unable to occupy 2003; Duncan et al. 2003; Cassey et al. 2004). The these post-settlement environments (Williams distribution of some species has expanded well 1969). Birds were introduced for biological pest beyond the natural geographical limits, as a result control (to reduce crop damage), as game birds or of intentional or accidental human assistance. as reminders of the colonists’ home countries (King In New Zealand, acclimatisation societies 1984; Wodzizki & Wright 1984; Wilson 2004). organised numerous introductions in the latter half Some species were so successful in their new of the 19th Century (Long 1981), resulting in wide- environment that they became more abundant than spread establishment of various species, including in their country of origin. MacLeod et al. (2009) demonstrated that a number of British bird species Received 27 April 2016; accepted 8 July 2016 had considerably higher densities at farmland sites Correspondence: [email protected] in New Zealand than at sites of equivalent habitat 118 Mortimer Fig. 1. Locations of survey sites in northern England: 1. Fig. 2. Locations of survey sites in Canterbury, New Alexandra Park, Cramlington; 2. Beacon Park, Pickering; Zealand: 1. Amberley Domain; 2. Argyle Park, Ashburton; 3. Burradon Community Recreation Ground; 4. Churchill 3. Ashburton Recreational Reserve; 4. Ashgrove Park, Playing Field, Whitley Bay; 5. Croft Park, Blythe; 6. Rangiora; 5. Barrington Park, Christchurch; 6. Beckenham Duncombe Park, Helmsley; 7. Filey Brigg; 8. Highfield Lane Park, Christchurch; 7. Bradford Park, Christchurch; Recreation Ground, Malton; 9. Hirst Park, Ashington; 10. 8. Brookside Park, Rolleston; 9. Cheviot Domain; 10. Kirkbymoorside Sports Field; 11. Lockey Park, Wideopen; Darfield Domain; 11. Dudley Park, Rangiora; 12. Hagley 12. Malton Cemetery; 13. Museum Gardens, York; 14. Park South, Christchurch; 13. Hansen Park, Christchurch; Peasholme Park, Scarborough; 15. Peoples Park, Ashington; 14. Hinemoa Park, Kaiapoi; 15. Kaiapoi Domain; 16. 16. Rillington Playing Field; 17. Norton Road Riverbank, Kaiapoi Park; 17. Matawai Nature Park, Rangiora; 18. Norton; 18. Snainton Recreation Ground; 19. South Bay, Murphy Park, Kaiapoi; 19. Rakaia Domain; 20. Somerfield Scarborough; 20. South Park, Darlington; 21. St. Peter’s Park, Christchurch; 21. Sydenham Park, Christchurch; 22. Cemetery, Norton; 22. Valley Park, Cramlington; 23. Valley Tinwald Domain; 23. Waltham Park, Christchurch; 24. Park, Scarborough; 24. Westfield Park, Cramlington. West Melton Domain in Britain. Their study investigated habitat and philomelos; Inger et al. 2014; Eaton et al. 2015). Studying climate as potential explanatory factors to predict the success of these and other similar species in places bird densities. Although the variables measured to which they have been introduced may provide could not explain differences in densities, it was increased understanding of their ecology and how suggested that in New Zealand specific habitats they interact with their environment. Identification may provide introduced species with higher quality of the relative importance of factors that regulate resources and birds may be subjected to fewer or density could in turn inform conservation action less extreme adverse weather events. within their natural range. In addition to habitat and climate, interactions In this study, measures of bird densities for with other organisms (e.g., predation, parasitism and a selection of focal species (native to Britain, competition) can also influence population density introduced to New Zealand) were compared at (Krebs 1985). The effects of interspecific competition recreational parks in 2 study areas: one in northern have been the subject of several studies, including England and the other in Canterbury, New Zealand. for example song sparrow (Melospiza melodia) in Site characteristics (size and proportion of habitat North America (Yeaton & Cody 1974) and forest cover) and species richness (indicating potential for birds of Hawaii (Mountainspring & Scott 1985), interspecific competition) were also compared, with the results of which indicated that interspecific the aim of assessing these as potential explanatory competition could negatively affect territory size factors for differences in species density between the and density, respectively. 2 study areas. For the latter, the guild concept was Several bird species introduced from Britain to applied (Krebs 1985; Simberloff & Dayan 1991). New Zealand are now declining in their country of origin and are consequently listed as birds of conservation concern, including common starling MATERIALS AND METHODS (Sturnus vulgaris), dunnock (Prunella modularis), house Twenty-four sites were selected in each of the 2 sparrow (Passer domesticus) and song thrush (Turdus study areas (Fig. 1 & 2). Timing constraints and other Bird densities within native and introduced ranges 119 Table 1. Candidate model descriptions for GLM analysis. D = density; fg = foraging guild diversity; spp = site species richness, area = site area; trees = mean cover of trees and shrubs; grass = mean cover of grassland; built = mean cover of built. Model Formula Description M1 D ~ fg + spp + area + trees + grass + built Species richness (foraging guild, site) & site characteristics M2 D ~ fg Foraging guild diversity only M3 D ~ spp Site species richness only M4 D ~ fg + spp Species richness (foraging guild, site) M5 D ~ fg + spp + area Species richness (foraging guild, site) & site characteristics (site area) M6 D ~ area + trees + grass + built Site characteristics M7 D ~ trees + grass + built Site characteristics (habitat types) M8 D ~ fg + area Species richness (foraging guild) & site characteristics (site area) M9 D ~ fg + trees Species richness (foraging guild) & site characteristics (trees & shrubs) M10 D ~ fg + grass Species richness (foraging guild) & site characteristics (grassland) M11 D ~ fg + built Species richness (foraging guild) & site characteristics (built) M12 D ~ fg + trees + built Species richness (foraging guild) & site characteristics (trees & shrubs, built) practicalities precluded random or systematic site For each count area, a habitat assessment was selection (which therefore limits inference beyond completed using aerial photographs (Google the sites studied). However, sites were chosen to Maps 2014), assigning habitats to 4 categories: represent a range of site characteristics (e.g., size, trees and shrubs (woodland, scattered trees, scrub, proportion cover of different habitat types). hedgerows); grassland (open areas with a continuous Differences in timing of the seasons between the grass sward); built (buildings, hard-standing, roads); northern and southern hemispheres necessitated a and other (any other habitat types). Each habitat type gap of ~6 months between field survey periods for was given a score of 0-10, to indicate the approximate the 2 study areas, to ensure surveys were conducted percentage cover within the count area (0 = 0%, 1 = in the same season. The England sites were surveyed