Preliminary assessment of nocturnal bird migration at the Bay of Biscay as observed by moon-watching.

Abstract

The East-Atlantic flyway represents one of the main bird migration routes worldwide, in which the Bay of Biscay acts as a geographic barrier. So far, the significance of the Bay of Biscay for migrants and its impact on their routes has not received a great deal of attention. The main goal of this study is to characterize the composition of nocturnal bird migration along the coast of the Bay of Biscay in northern Iberia. For this purpose, moon-watching was carried out at two study sites (Punta Galea, Cape Higuer) during spring and autumn 2014. The vast majority of observed migrants consisted of singly flying passerines, with a number of interspersed groups of non-passerines such as waterfowl, waders and swifts.Key words: East-Atlantic flyway, Basque coast, passerines, spring and autumn migration. ResumenLa ruta del Atlántico Oriental es una de las rutas de vuelo migratorio mas importantes en el mundo e incluye el golfo de Vizcaya como barrera geográfica. La importancia y el impacto del golfo de Vizcaya para las aves migratorias y sus rutas no han recibido mucha atención en la investigación. El objetivo principal de este estudio es caracterizar el ensamblaje de las aves que migran de noche por la zona del golfo de Vizcaya en el norte de la Península Ibérica. 155Moon-watching at the Bay of Biscay Munibe, Cienc. nat. 64, 2016 • pp. 155-159 • Donostia/San Sebastián •

Radar wind profilers and avian migration: a qualitative and quantitative assessment verified by thermal imaging and moon watching

“…Only in the final wind data involving measurements from several beams flight directions generally point towards NE in spring. Thermal imaging indicated a mean direction of 45·9° for the major part of the migrants (>90%; Weisshaupt, Maruri & Arizaga ).…”

“…This could be theoretically explained by the RWP sampling method of recording 30 s vertically followed by a gap of 5 min – given the overall low proportion of bird flocks it could be concluded that they are not readily captured by the beam in this narrow time slot. Considering the TI and moon‐watching findings (Weisshaupt, Maruri & Arizaga , b) and previous literature (e.g. for an overview see Alerstam ), it can be assumed that the majority of the targets represent passerines.…”

“…() might not be accurate anymore because far‐flying and/or small passerines might not be detected equally well anymore. So even though Weisshaupt, Maruri & Arizaga () did record the entire spectrum of size classes, it is theoretically possible that migration actually evolved on a somewhat reduced height scale.…”

“…Flight altitude distribution was similarly estimated from the 30 s MTRs per gate. Importantly, while the altitudes required for MTR calculations of the thermal‐imaging camera were derived from target size classes (Weisshaupt, Maruri & Arizaga , b), altitudes in the RWP were predefined by the gates.…”

“…If birds are present in all beams flying in a narrow directional range, as is the case in high densities in spring (Weisshaupt, Maruri & Arizaga 2016a), their echoes are not removed because there is directional consensus. In case of low densities with potentially higher directional variability in autumn (Nilsson, B€ ackman & Alerstam 2014, Weisshaupt, Maruri & Arizaga 2016a, there is the possibility of having no consensus between the beams, whereby the aberrant strong signals (birds) are removed. So both higher variability in track directions and low densities could explain the fact that wind barbs indicating presence of birds are hardly observed in autumn data.…”