Studies in Avian Biology No. 10:105-l 18, 1987. HABITAT AND NEST-SITE SELECTION OF MEW AND GLAUCOUS-WINGED GULLS IN COASTAL BRITISH COLUMBIA KEESVERMEER' AND KEVIN DEVITO~ Ass-raAcr.-The types of lakes, islands and nest sites Mew Gulls (Larus canus)selected on Vancouver Island, British Columbia, were examined. Mew Gulls nested on lakes that had significantly more islands and rivers, were larger, had longer shore lines, were closer to the sea, and were at lower altitudes than lakes not used by gulls.The nestingpopulation of Mew Gulls on Vancouver Island was estimatedfrom a highly significantregression equation containing these lake variables. Mew Gulls significantlypreferred small islands without trees, with a substrateof bare rock or low herbaceous cover. Nests were strongly associatedwith island crests and with a flat moss-covered substrate, and were significantly higher and farther from the water than randomly chosen matched points. Mew Gulls also nested frequently on tops of poles, pilings or tree stumps emerging from the water. On Vancouver Island, Mew Gulls nested mostly (80%) as solitary pairs. Island and nest site selection by Mew Gulls was compared with that of Glaucous-winged Gulls (Larus gluucescens),a colonial nesterat southeasternVancouver Island. Ground-nestingGlaucous-winged Gulls selected islands that were significantly smaller, had a lower relief, were less forested and contained a higher percentage of bare rock or ground with low herbaceouscover than islands where gulls were absent. Nests of solitary pairs of Glaucous-wingedGulls, like Mew Gulls, were strongly associatedwith island crests,but unlike Mew Gulls, these had a grasssubstrate. Colonial Glaucous-winged Gulls showed no associationwith any island location, but were strongly associatedwith a bare rock substrateand, to a lesserextent, with grass.Matched points taken from both solitary and colonial nests occurred on significantly steeperslopes than nests and were also closer to shore. Intersnecific variation and nlasticitv in the nest site establishment of Mew and Glaucous-wingedgulls are explored and their implications discussed. The breeding range of Mew Gulls (Larus cun- cous-winged Gulls has been studied by Vermeer us) in Eurasia extends as a broad band from (1963) Ward (1973) and Hunt and Hunt (1976) northwestern Europe to Kamchatka in the east- but some aspects of its selection of islands and ern USSR, but is restricted to the northwestern nest sites will be dealt with for the first time in portion of North America (Voous 1960). Atlases this paper. on breeding distribution of birds do not show We also compare the nesting habitat of Mew the Mew Gull nesting in central and southern Gulls on Vancouver Island with that of this British Columbia (e.g., Voous 1960, Godfrey species in northern Europe. The comparison will 1966), although it is a widespread, albeit dis- be subjective, because although the habitat of the persed breeder there. Campbell (1970) made the species in Europe has been described (e.g., Bianki first attempt to compile information on the 1967, Haftom 197 1, Vaisanen and Jarvinen 1977, breeding status of Mew Gulls in British Colum- Giitmark 1982), statistical treatment of nest-site bia, but quantitative data were lacking at the selection similar to our method is lacking. time. Aspects ofthe nesting biology of Mew Gulls in British Columbia were described by Vermeer METHODS and Devito (1986). In this paper, we examine Surveysof nesting Mew Gulls on Vancouver Island which lakes, islands, and sites Mew Gulls se- were conducted during the last week of May and first lected for nesting on Vancouver Island, British half of June in 1984 and 1985 on 58 lakes accessible Columbia. by road (Table 1, Fig. 1). Lakeswere chosenfrom those present on a Canadian Hydrographic Service (CHS) Island and nest site selection by Mew Gulls is chart (Chart No. 4C 300 1, scale 1:525,000). The entire compared with that of Glaucous-winged Gulls shoreline of each lake was explored by boat and the (Lams ghucescens)in the Gulf Islands at south- position of each nest was mapped. The number of is- eastern Vancouver Island. These species are the lands present was noted in the field, while measure- only coastal nesting larids in British Columbia. ments of lake area, perimeter, altitude, and distanceto Glaucous-winged Gulls nest almost completely seawere made from 1:50,000and 1:80,000 scalemaps. in the marine habitat in British Columbia (Drent Mean lake depths and supporting morphometric data and Guiguet 196 1). The nesting biology of Glau- for 40 lakes were obtained from lake inventory files of the Fisheries Branch. Ministrv of Environment, Vic- toria, British Columbia. The number of rivers entering ’ ’ Canadian Wddlife Serwce, P.O. Box 340, Delta, British Columbia or leaving a lake was determined by counting the blue V4K 3Y3, Canada. lines indicating the water courseson the CHS chart. 2 Watershed Ecosystem Program, Trent University, Peterborough, On- The extent of human disturbanceat a lake was deter- tario K9J 7B8, Canada. mined by a method similar to that describedfor loons 105 106 STUDIES IN AVIAN BIOLOGY TABLE 1 LAKES ON VANCOUVER ISLAND SURVEYED FOR NESTING MEW GULL.S IN MAY AND JUNEOF 1984 AND 1985 Name of lake Name of lake 1. Georgie Lake 0 30. Mohun Lake 2 2. Nahwitti Lake 0 31. Morton Lake 0 3. Kains Lake 3 32. Amor Lake 4. Quatse Lake 0 33. Gray Lake 0 5. Alice Lake 4 34. Boot Lake 0 6. Victoria Lake 35. Goslin Lake 0 7. Benson Lake 0 36. John Hart Lake 0 8. Maynard Lake 0 37. Campbell Lake 2 9. Iron Lake 0 38. Beavertail Lake 0 10. Lac Truite 0 39. GooseneckLake 1 11. Three Isle Lake 4 40. Middle Quinsam Lake 0 12. Keogh Lake 0 41. Wokas Lake 0 13. Nimpkish Lake 4 42. Upper Quinsam Lake 0 14. Atluck Lake 0 43. Upper Camubell Lake 0 15. Wolfe Lake 0 44. Buttle Lake- 0 16. Mukwilla Lake 0 45. Wolf Lake 0 17. Woss Lake 0 46. Comox Lake 18. Klaklakama Lake 1 0 47. Elsie Lake 0 19. Klaklakama Lake 2 0 48. Great Central Lake 2 20. Schoen Lake 0 49. Sproat Lake 3 21. Vernon Lake 0 50. Home Lake 1 22. Muchalat Lake 0 51. Cameron Lake 0 23. McCreight Lake 0 52. Muriel Lake 3 24. Pye Lake 0 53. Kennedy Lake 24 (27)* 25. Lower Stella Lake 0 54. Maggie Lake 3 26. Stella Lake 1 55. Nanaimo Lake 1 0 27. Roberts Lake 56. Nanaimo Lake 2 0 28. Mud Lake 0 57. Cowichan Lake 11 29. Amor Lake 1 58. Nitinat Lake 0 * 24 nem in 1984 and 27 nem in 1985. by Vermeer (1973). Ten disturbance units were given June and July 1985. All islands were visited by boat, for each patrolled campsite and resort, five units for the number of nestinggulls was countedat each nesting each wildernesscampsite (camp area or a spot for 1 or island and the number and position of each colony or 2 campers), and one unit for each house and summer solitary nest site were mapped. For each island the cottage bordering a lake. For each nesting island, the percentbare ground (low herbaceouscover and/or bare surface area and the distance to the mainland shore rock) and extent of forestation (> 50 percent = forested and nearest island were determined from maps or ac- island) was noted. The surfacearea, distance to main- tual site measurements.Presence of trees (> 1.5 m tall) land (Vancouver Island) and maximum height of each and percent bare ground (low herbaceouscover and/ island were determined from the CHS chart No. 33 10 or bare rock) were noted. At Kennedy Lake, measure- (sheets 1 to 4) or at the site. ments were taken at each island for later comparison Observations similar to those made on Mew Gulls between islandswith and without nests.The dominant were made on the nest sitesof Glaucous-wingedGulls. vegetation, distanceto shore, nest height above water, Approximately 5% of the ground nests (up to a max- distance to nearest tree, and microslope were deter- imum of 10 nests) were selectedrandomly from each mined for nestsand for matched points 2 m away from colony. Matched points were selected2 m from solitary nests. Microslope was determined from the average nestsand the selectedcolonial nestingpairs. Since only maximum slope over a 30 cm distancecentered at the a small number of solitary Glaucous-wingedGull pairs nest or matched point. Matched points were selected were located on islands (there were more on beacons randomly from one to 10 compass points, similar to and cliffs), one island with three and another with two the method describedby Burger and Gochfeld (1981). pairs were included in the single pair category. Those The distanceof 2 m was chosenarbritrarily, but prob- two islands were large and the pairs nested far apart ably fell within the nesting territory as the distance to in a solitary manner. the nearestneighboring nest, where more than one pair For both lake and island parameters, an overall or of gulls nested on an island, ranged from 3.5 to 15 m. multivariate test was conducted to test the hypothesis Surveyson nestingGlaucous-winged Gulls were con- of equal population mean vectors for lakes and islands ducted in the Gulf Islands, from 49”09’N (including with and without nests (see Hummel and Sligo 1971). the Flat Top Islands)to 48”33’N (D’Arcy Island) during The Wilks Lambda statistic (X) was used. Following a HABITAT AND NEST-SITE SELECTION- Vermeer and Devito 107 BRITISH COLUMBIA ISLAND ;.;:.f... %.‘ 1......:.;._ FIGURE 1. Lakes surveyed and those with nesting Mew Gulls on Vancouver Island, 1984 and 1985. A = Tsusiat lake, B = Henderson Lake, C = Hesquiat Lake.