Ibis (2019), 161, 192–197 doi: 10.1111/ibi.12631 Short communication than other substrates, indicating how vegetation restora- tion could aid the conservation of this species. The depth of Sooty Keywords: burrow dimensions, burrowing petrels, Falkland Islands, nest architecture, Puffinus griseus. Shearwater Ardenna grisea burrows varies with Many seabirds, including most small-medium Procellari- habitat and increases with iformes (hereafter ‘petrels’), as well as many alcids and some penguins, breed in burrows. Yet, burrows are ener- competition for space getically costly to build and can be subject to flooding, † TYLER J. CLARK,1, * ANNE-SOPHIE collapse and other disadvantages (Warham 1996). BONNET-LEBRUN,2,3 LETIZIA CAMPIONI,4 Therefore, the advantages of protection from predators PAULO CATRY4 & EWAN WAKEFIELD1 and climatic extremes must outweigh the costs of bur- 1Institute of Biodiversity, Animal Health, and row nesting. Burrow nesting by seabirds can have wider Comparative Medicine, University of Glasgow, Graham effects, such as modifying the structure and nutrient Kerr Building, Glasgow G12 8QQ, UK content of island soils, which can then cascade to other 2Department of Zoology, University of Cambridge, trophic levels (Bancroft et al. 2005). In other fossorial Downing Street, Cambridge CB2 3EJ, UK animals, the impact of these processes depends in part 3Centre d’Ecologie Fonctionnelle et Evolutive CEFE on the structure of the burrow systems (Laundre & Rey- UMR 5175, CNRS - Universite de Montpellier - nolds 1993). Some petrels, such as the White-chinned Universite Paul-Valery Montpellier – EPHE, 919 route Petrel Procellaria aequinoctialis or the Flesh-footed de Mende 34293, Montpellier Cedex 5, France Shearwater Ardenna carneipes construct relatively simple 4MARE – Marine and Environmental Sciences Centre, and straight burrows (Parker & Rexer-Huber 2015). ISPA - Instituto Universitario, Rua Jardim do Tabaco Conversely, the burrows of others, such as the Sooty 34, 1149-041 Lisbon, Portugal Shearwater Ardenna grisea, are longer and complex, sometimes with multiple entrances and bifurcations (Warham 1996, Hamilton 2000). However, little is known about the drivers of variation in seabird burrow architecture. The Sooty Shearwater Ardenna grisea, an abundant but One potential driver is competition for space; that is, declining petrel, is one of many seabird species that con- to obtain a breeding territory in a crowded area, birds struct breeding burrows, presumably because these con- may dig further underground (Warham 1996, Ramos fer protection from predators and the elements. Little is et al. 1997). In addition, it is likely that burrow charac- known about the causes of variation in Sooty Shearwater teristics vary with habitat, such as vegetation, slope, soil burrow architecture, which can differ markedly both structure or type, as these will affect both the energetic within and between breeding sites. We hypothesize that cost of excavation and the structural and hydrological burrow architecture varies in response to habitat type properties of the burrow (Ramos et al. 1997, Powell and competition for space. To address these hypotheses, et al. 2007). Neither of these hypotheses has been we recorded Sooty Shearwater burrow dimensions on tested. Kidney Island, the largest Sooty Shearwater colony in The Sooty Shearwater is an abundant but declining the Falkland Islands, South Atlantic, and modelled these burrowing petrel. It is currently listed as Near-threa- as functions of burrow density (a proxy for competition) tened, in part due to loss of breeding habitat (Scott et al. and habitat indices. Our models suggest that Sooty 2008). Sooty Shearwaters prefer to breed under dense Shearwaters burrow further underground in response to vegetation, including tussac grasses Poa spp., and small competition for breeding space, and that soil underlying trees, e.g. tree daisies Olearia spp. (Scott et al. 2009, dense tussac grass Poa flabellata is more easily excavated Geary et al. 2014, T.J. Clark et al. unpubl. data). It has been suggested that the loss of the tussac grass Poa fla- bellata caused a reduction in Sooty Shearwater breeding †Current address: Department of Ecosystem and Conservation populations in the Falkland Islands (T.J. Clark et al. Sciences, University of Montana, 32 Campus Drive, Missoula, unpubl. data). Sooty Shearwater burrows may extend to MT 59812, USA. 3 m in length, sometimes comprising labyrinthine sys- tems (Hamilton 2000). Burrow architecture can vary *Corresponding author. considerably between and within colonies. For example, Email: [email protected] burrows at Putauhinu Island, New Zealand, are on Twitter: @teejclark1873 © 2018 The Authors. Ibis published by John Wiley & Sons Ltd on behalf of British Ornithologists' Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Burrowing petrel architecture 193 average 0.5 m longer (McKechnie et al. 2007) than divisions marked on the flexible probe of a burrowscope those at Tıtı Island, New Zealand (Geary et al. 2014). It (Ridgid micro CA-300 Hand-held Inspection Camera, is not known whether this is due to differences in habi- Elyria, OH, USA). Due to the constraints of another tat or competition. study carried out at the same time, we only monitored Here, we test whether the architecture (i.e. burrow single-entrance burrows, which were determined by entrance width, height and burrow length) of Sooty hand. In addition, we calculated slope in QGIS 2.8.3 Shearwater burrows on Kidney Island, Falklands Islands, (QGIS Development Team 2016) using a digital eleva- varies with competition and habitat. Understanding the tion model (10 m resolution) retrieved from the Earth- drivers of variation in burrow architecture could inform data search portal, courtesy of NASA Land Processes habitat restoration projects as well as studies on compe- Distributed Active Archive Center (NASA JPL 2013) tition and nutrient cycling by seabirds. and provided by the South Atlantic Environmental Research Institute data centre. METHODS Tussac grass was harvested periodically on Kidney Island until the 1950s and was also damaged by a fire in We carried out fieldwork from 7 to 21 January 2017 on the 1940s (Kidney Island Group 2006). Subsequently, it Kidney Island (51.6238°S, 57.7520°W), which is has re-grown. We estimated the change in tussac cover approximately 0.32 km2 in area and is largely covered in over time by comparing a satellite image of Kidney dense tussac grass. Kidney Island has been designated as Island from 2017 (Google Earth Pro 2017) with an aer- an Important Bird Area and National Nature Reserve by ial image taken by the British Geological Survey in the Falkland Islands Government (Kidney Island Group 1956, provided by the Falkland Islands Department of 2006), principally because it is the main breeding site Mineral Resources. We estimated tussac grass coverage for Sooty Shearwaters in the South Atlantic, with a pop- for each photo by extracting the grey raster band values ulation size of approximately 140 000 breeding pairs for each sample plot in QGIS, with darker values corre- (T.J. Clark et al. unpubl. data). sponding to higher cover and lower values to lower We established 66 study plots by projecting a regular cover. We defined the relative change in cover as the 50 9 75 m grid over a polygon delimitating the vege- difference between the grey raster band values for the tated area of Kidney Island. At each grid node, we set 1956 and 2017 images, with smaller absolute values out a circular plot of 2.5 m radius (i.e. c.20m2). We indicating similar cover between the years in the images estimated burrow density, which we assume to be a and larger absolute values indicating different cover. proxy for competition for space, by counting the num- White-chinned Petrels and Magellanic Penguins ber of burrows in each plot, ignoring those < 40 cm Spheniscus magellanicus breed at low densities in burrows long, which Scott et al. (2008) suggested would not be on Kidney Island and therefore potentially compete with viable. Initial observations showed that burrows gener- Sooty Shearwaters for space. We recorded the presence ally tapered in the first few centimetres from the of these species in study plots, but found no White- entrance, before becoming more constant. In the inter- chinned Petrels and only four Magellanic Penguin bur- ests of repeatability, we defined the minimum entrance rows, so we did not consider their influence further. width and height as the minimum dimensions within We modelled variation in burrow entrance width and the first 5 cm of each burrow. Within each plot, we ran- height, and burrow length using generalized linear mod- domly measured to the nearest centimetre the minimum els (GLMs) fitted in R (R Core Team 2013), assuming entrance width and height of up to five burrow Gaussian errors and an identify link function. We con- entrances (some plots contained fewer than five bur- firmed model assumptions of normality and homoscedas- rows) using measuring tape. We then recorded the fol- ticity of residuals by examining normal quantile–quantile lowing habitat variables: soil moisture, mean tussac plots and residuals vs. fitted values, respectively. We height, percentage tussac cover and presence of protrud- simplified models by backwards selection using a step- ing rock substrate. We classified soil moisture on a wise procedure based on Akaike’s information criterion four-point scale: 1 (dry, well-drained), 2 (intermediate), (AIC; Burnham & Anderson 2003), implemented using 3 (saturated, moisture comes to the surface when the ‘step’ function in R. Beginning with the full model pressed by hand) and 4 (standing water within the plot) for each response (entrance width, entrance height or (Lawton et al. 2006). We measured average tussac burrow length), this function determines which model height to the nearest 0.25 m from the ground to the top term’s removal would result in the greatest reduction in of the grass, using a graduated pole.