Variability in the area, energy and time costs of wintering waders responding to disturbance

Abstract

Birds’ responses to human disturbance are interesting due to their similarities to anti‐predator behaviour, and understanding this behaviour has practical applications for conservation management by informing measures such as buffer zones to protect priority species. To understand better the costs of disturbance and whether it will impact on population size, studies should quantify time‐related responses as well as the more commonly reported flight initiation distance (FID). Using waders wintering on an estuarine area, we experimentally disturbed foraging birds on the Wash Embayment, UK, by walking towards them and recording their responses (FID, alert time, time spent in flight, time taken to resume feeding, and total feeding time lost). We present data for 10 species of conservation concern: Curlew Numenius arquata, Oystercatcher Haematopus ostralegus, Bar‐tailed Godwit Limosa lapponica, Grey Plover Pluvialis squatarola, Redshank Tringa totanus, Knot Calidris canutus, Turnstone Arenaria interpres, Ringed Plover Charadrius hiaticula, Sanderling Calidris alba and Dunlin Calidris alpina. Larger species responded more strongly, response magnitude was greater under milder environmental conditions, and responses varied over both small and large spatial scales. The energetic costs of individual responses, however, were low relative to daily requirements and disturbance events were unlikely to be frequent enough to seriously limit foraging time. We suggest, therefore, that wintering wader populations on the Wash are not currently significantly negatively impacted by human disturbance during the intertidal foraging period. This is also likely to be the case at other estuarine sites with comparable access levels, visitor patterns, invertebrate food availability and environmental conditions.

Disturbance increases high tide travel distance of a roosting shorebird but only marginally affects daily energy expenditure

“…We believe that our calculations are robust to the assumptions made. For example, our estimate for flight time upon walker disturbance (15.5 s) is similar to the flight time measured in an earlier study (21.2 s;Collop et al 2016). Even if travel distances were underestimated two-or threefold in our study due to birds not moving in a perfectly straight line between two location measurements, the impact of disturbed movement on the daily energy budget would still be minimal (~ 0.2%).…”

“…We note that our energy calculations are based on average displacement responses of roosting oystercatchers on Vlieland, incorporating only a limited set of conditions. Bird species show great variability in responses to disturbance (Kirby et al 1993;Smit and Visser 1993;Laursen et al 2005;Collop et al 2016), with larger species generally being more easily disturbed than smaller ones (Laursen et al 2005;Collop et al 2016). We observed that, in our study area, oystercatchers are far less susceptible to disturbance than Eurasian Curlews (Numenius arquata, 1.6 times as many disturbances per bird per hour) and especially Bar-tailed Godwits (Limosa lapponica, 4.6 times as many disturbances per bird per hour) (van der Kolk et al unpublished data).…”

“…Anthropogenic disturbance can negatively impact a bird's energy budget by evoking energetically costly responses such as increased movement (e.g. Stillman and Goss-Custard 2002;Collop et al 2016), physiological changes (Ackerman et al 2004;Walker et al 2005) and deviations from optimal foraging patterns (Gill et al 1996). Immediate flight responses have proved the most accessible indicator of the impact of disturbance (Gill 2007) as they are relatively straightforward to measure in the field through observation.…”

“…the distance at which a bird takes flight when approached by a disturbance source) have been determined for a range of species and disturbance sources in both experimental and observational studies (e.g. Fitzpatrick and Bouchez 1998;Lord et al 2001;Stillman and Goss-Custard 2002;Glover et al 2011;Collop et al 2016). Flight distances (i.e.…”

“…the distance travelled after disturbance) and flight time are reported in fewer studies (e.g. Kirby et al 1993;Spaans et al 1996;Beale and Monaghan 2004;Collop et al 2016), likely because these responses are difficult to quantify when birds fly for longer distances after being disturbed.…”