Ecology of tern flight in relation to wind, topography and aerodynamic theory

Abstract

One contribution of 17 to a theme issue 'Moving in a moving medium: new perspectives on flight'. Flight is an economical mode of locomotion, because it is both fast and relatively cheap per unit of distance, enabling birds to migrate long distances and obtain food over large areas. The power required to fly follows a U-shaped function in relation to airspeed, from which context dependent 'optimal' flight speeds can be derived. Crosswinds will displace birds away from their intended track unless they make compensatory adjustments of heading and airspeed. We report on flight track measurements in five geometrically similar tern species ranging one magnitude in body mass, from both migration and the breeding season at the island of Ö land in the Baltic Sea. When leaving the southern point of Ö land, migrating Arctic and common terns made a 608 shift in track direction, probably guided by a distant landmark. Terns adjusted both airspeed and heading in relation to tail and side wind, where coastlines facilitated compensation. Airspeed also depended on ecological context (searching versus not searching for food), and it increased with flock size. Species-specific maximum range speed agreed with predicted speeds from a new aerodynamic theory. Our study shows that the selection of airspeed is a behavioural trait that depended on a complex blend of internal and external factors.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Route simulations, compass mechanisms and long-distance migration flights in birds

“…In the simulations, birds are, furthermore, assumed to respond only to cues affecting their compass mechanisms (e.g., the geomagnetic field, stars, and the sun), and not to other local topography such as coastlines and mountain ridges that could affect their orientation. Coastlines could be used as visual cues for navigation and wind drift compensation at low flight altitudes (e.g., Åkesson 1993b; Bruderer and Liechti 1998; Hedenström and Åkesson 2016), while ecological barriers could require a change of strategy such, as, for example, crossing deserts in the fastest possible way (e.g., Åkesson et al 2016), or by performing detours to avoid large inhospitable regions such as oceans or large mountains (e.g., Gudmundsson et al 1995). …”

“…Strong sidewinds may further cause birds to drift off intended courses during natural migrations (Alerstam 1979 ; Klaassen et al 2011 ). To compensate for wind drift, migratory birds may fly along topographical features such as coastlines, mountain ridges, or above cities at night for part of the journey providing visual contact with the ground below (Åkesson 1993b ; Karlsson et al 2010 ; Hedenström and Åkesson 2016 ). Most of the migrations, however, occur at much higher altitudes (>1000 m asl; e.g., Able 1970 ; Bruderer and Liechti 1998 ; Zehnder et al 2001 ), where they may be less affected by topography (Zehnder et al 2001 ; Nilsson et al 2014 ).…”

The Ornithodolite as a tool to quantify animal space use and habitat selection: a case study with birds diving in tidal waters

“…To examine how maximum distance varied in relation to body size, multiple sequential locations of birds migrating past Ottenby observatory, southern Sweden were collected from 2012 to 2017. The methods are detailed in full by Hedenström and Åkesson (2016). Each series of locations from an individual bird is hereafter referred to as a "run."…”

“…This system is now based on a laser rangefinder incorporated in a pair of Vector 21 binoculars (Pennycuick et al 2013), which measures distances directly and provides improved accuracy and precision. This system was specifically developed to quantify airspeed in birds and it is only very recently that it has been used to examine animal distributions (Hedenström & Åkesson 2016;Shepard et al 2016). We suggest that this technique has potentially broad ecological applications, which have yet to be fully realized.…”

Moving in a moving medium: new perspectives on flight

“…In this volume, Reynolds et al [22] review the different cues that insects may use to detect wind direction, and present a new mechanism for sensory detection based on the 'jerks' that insects experience in flight. While visual cues may or may not be important for high-flying insects, they are certainly used by insects in other scenarios [23] as well as birds, which can use the apparent ground movement to correct for wind drift [4,18,24]. rstb.royalsocietypublishing.org Phil.…”