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Group formation is beneficial for many small endotherms, in part to create favourable conditions for developing offspring. However, for individuals occupying spatially limited structures, such as tree cavities, group formation may influence—and be influenced by—the range of structures available for use. This may be especially true of bats, which often form maternity groups that regularly change in size. We examined colonies of tree-roosting little brown bats (Myotis lucifugus (LeConte, 1831)) in northern Alberta to address two questions: (1) is group size associated with patterns of roost use, in particular the use of large-diameter trees, and (2) does group size change during reproduction and correspond to changes in roost use? We located roosts over 2 years using radiotelemetry and conducted exit counts at a subset of these roosts. Larger diameter trees had larger and more variable group sizes. Roost-tree diameter peaked near the start of parturition, the time when bats formed the largest groups. We suggest that large groups are especially beneficial at this time due to the thermoregulatory benefits to small, unfurred pups. Our results indicate that roost use and group size are interrelated and that large-diameter trees may be especially beneficial by supporting a wider range of group sizes.

The Brown Creeper (Certhia americana) is a small, bark-foraging songbird found throughout the forested regions from Central America north to south-central Canada and Alaska. Its avoidance of edges, cryptic plumage, and highpitched songs and calls contribute to a low detection rate for this species on most surveys (Hejl and others 2002; Alberta Sustainable Resource Development 2003). The abundance and distribution of the species at the northern edge of its range is poorly understood and is likely currently underestimated (Hejl and others 2002; Alberta Sustainable Resource Development 2003). Although creepers are known to occur along the Pacific coast to south-central Alaska, no confirmed breeding records have been reported above 606N latitude east of the continental divide (Hejl and others 2002). In this note, we document the 1st known occurrences of breeding Brown Creepers in the Northwest Territories, Canada. We observed breeding activity at 2 Brown Creeper territories within 100 m of point-count stations initiated for long-term forest songbird monitoring by the Canadian Wildlife Service (CWS) in the Liard Valley of the Northwest Territories. The study area for regional songbird monitoring spans over 700 km, and includes over 195 point-count stations and several line transects and census routes (Machtans and Latour 2003). The Liard Valley is in the far southwest corner of the Northwest Territories, just north of the British Columbia border and east of the Yukon border. Our observations were made in the low relief boreal forest region east of the Liard River. The Liard Valley typically has higher average temperatures and more precipitation than many regions of the northern boreal forest (Machtans and Latour 2003). Forests are typically old and, on average, taller than in other areas of the Northwest Territories. Vegetation and site characteristics for the Liard Valley are described in Machtans (2000) and Machtans and Latour (2003). From 15 June to 12 July 2008, several observations of Brown Creepers were made at 2 locations near Fort Liard, Northwest Territories (606139N, 1236229W) that confirmed breeding by this species in the area. On 15 June 2008, 2 Brown Creepers were heard and observed foraging during point-count surveys in the area (UTM Zone 10: 480493E 6697423N, NAD1983; hereafter location A). Upon further investigation, at least 3 juvenile creepers were flushed from the trunk of a tree, which provoked distraction displays from one or both parents. This display consisted of several aggressive flights to within 2 m of the observer’s head. On 21 June 2008, an audio recording of a singing Brown Creeper was obtained using a high quality hand held digital recorder (Edirol R-09, Roland Corporation) near the same location. On 6 July 2008, at least 4 Brown Creepers were foraging approximately 300 m from the original location. Several pictures were taken of the birds observed on 6 July (Fig. 1). On 12 July 2008, these birds were again observed and an inactive nest was found near the original sighting at location A (Fig. 2). The nest was 5.5 m above the ground in a 35-m-tall Trembling Aspen (Populus tremuloides) snag with a diameter at breast height (dbh) of 39 cm. On 15 June 2008, a Brown Creeper was heard and observed for about 10 min during a pointcount survey at a location approximately 4.6 km away from location A (UTM Zone 10: 480601E 6692793N, NAD1983; hereafter location B). On NORTHWESTERN NATURALIST 90:156–159 AUTUMN 2009 156 NORTHWESTERN NATURALIST 90(2)