Investigating aerodynamic factors in wind-driven grey-headed albatross crash-landings

Abstract

A significant portion (ca. 10%) of the global population of Grey-headed albatrosses (Thalassarche chrysostoma) breed on sub-Antarctic Marion Island. It has been observed that a large number of adult grey-headed albatrosses (GHAs) crash into the valley below an important inland breeding site, which proves fatal in most cases. The possible effect of the prevailing wind on the frequency and spatial distribution of crashes was unknown, prompting an investigation into the aerodynamic limitations of GHAs and their interaction with fine-scale wind patterns around this inland breeding site. The purpose of this study is to investigate the frequency of adult GHA crashes and identify any spatial patterns in these incidents. These data would then be compared with both measured and simulated wind vectors to determine if wind plays a significant role. Additionally, the study aimed to examine the aerodynamic limitations of the birds, specifically relating to lift generation, to ascertain which wind conditions are most likely to result in crashes. Field observations were carried out on Marion Island to study the way the GHAs interact with their environment. Flight observations were conducted to directly witness GHA crash-landings and link them to specific prevailing wind conditions, which were measured near the study area. The GHAs nest in dense colonies along an east-facing ridge called Grey-headed Albatross Ridge (maximum height of ca. 200 m a.g.l.). Carcass surveys were conducted in a 1 km2 area below the Ridge, spanning the length of this sub-colony (ca. 4000 breeding pairs) for four breeding seasons (2017-2020). Measured wind data were supplemented by computational fluid dynamics (CFD) simulations of wind vectors over Marion Island (Goddard et al., 2022) that were compared to the spatial distribution of carcasses. Additionally, an aerodynamic investigation of a GHA in flight was conducted using CFD. No geometry for a GHA wing (or body) was available at the initiation of the study except for a few studies noting the wingspan, aspect ratio, weight and planform of an adult GHA. Thus, a GHA geometry was created based on a dried GHA wing and photographs of GHAs in flight. The CFD simulations were conducted using Simcenter Star-CCM+ at a range of speeds, angles of attack and sideslip angles. Observations of albatrosses in flight indicate that most birds are killed when attempting to leave the colony, specifically when flying low above ground in strong wind conditions. During the prevailing westerly winds, the GHA chicks were sheltered, while adults flying low over the valley experienced highly variable wind vectors. In winds approaching from the north, east or south, GHAs were able to take off with a headwind to gain altitude after take-off. The spatial distribution of carcasses also ii showed a clear hotspot in an area of highly variable flow in the lee of the Ridge during the prevailing wind conditions. The CFD investigation showed that a GHA flying with fully extended wings can generate lift up to 9 times its weight at airspeeds above 35 m.s-1 and positive angle of attack (updrafts). A GHA in this gliding posture can generate sufficient lift at a minimum speed of 13 m.s-1 and angles of attack above 10°. For a given speed and angle of attack, the lift decreases with increasing sideslip angle. At high sideslip angles and negative angle of attack, negative lift is generated, resulting in downward acceleration. At high speeds, this downforce can be equal to the bird’s weight, most likely causing fatal injuries if forced to crash. A combination of high cross-winds and low-to-moderate downdrafts is most common in the valley during the dominant westerly winds. Most cases of crash-landings of low-flying GHAs occur during these conditions. Additionally, most GHA carcasses were located in areas where westerly winds create variable airflow in the Valley, leading to low or negative lift generation that increases crash risk, especially during departure flights towards the ocean. GHAs taking off in Westerly winds tend to lose altitude quickly, exposing them to cross-winds and downdrafts for much of their 2 km journey, raising the likelihood of crash-landings. By contrast, northerly, southerly, or strong headwinds allow GHAs to gain altitude faster, reducing the risk of crash-landing. It is proposed that the primary causes of crash-landings are a combination of variable wind vectors and low altitude: at low heights, GHAs lack sufficient time to recover from adverse gusts, even with flapping. Anthropogenic climate change affects wind patterns in the Southern Ocean, impacting GHA foraging and nest accessibility on Marion Island, where increased northerly winds create challenges for birds reaching inland nests. Northerly winds pose both risks and benefits, as moderate northerlies can improve take-off conditions. However, increased frequency and intensity of strong northerly winds could hinder feeding and chick-rearing, threatening the breeding success of this already endangered species. While continued surveys are required to monitor the effect of changes in wind conditions over the long term, the current level of wind-related mortality in GHAs is substantial for a long-lived species with a low natural mortality rate. This is the first study to document persistent wind-driven, land-based mortalities in albatrosses. It presents a unique insight into the way seabirds are affected by wind, not only in the open ocean, but also on land around breeding sites.