Do bird captures reflect migration intensity? – Trapping numbers on an Alpine pass compared with radar counts

Abstract

A limitation of standardized mist netting for monitoring migration is caused by the lack of knowledge about the relationship between trapped birds and birds flying aloft. Earlier studies related nocturnal radar counts with trapping data of the following day. In this study, we compared for the first time data gathered simultaneously by radar and mist netting, separately for diurnal and nocturnal migration. Trapping numbers were strongly correlated with migratory intensities measured by radar (r>0.6). A multiple regression analysis, including wind speed and wind direction explained 61% of variation in the number of captures. During the night, and particularly with favourable winds, birds flew at higher altitudes and hence escaped the nets to a higher proportion. The number of nocturnal migrants trapped during daytime was well correlated with migratory intensities observed by radar in the preceding night. The diurnal time patterns, however, revealed fundamental differences between trapping counts and radar observations. This was mainly due to increasing and decreasing flight altitudes in the course of the night, and by the limitations of the radar technique that underestimates migratory intensities during the day when birds aggregate in flocks. In relation to the migratory intensity recorded by radar, diurnal migrants are trapped in a much higher proportion than nocturnal migrants. Finally, our results confirm that trapping data from a site hardly used for stopover are well suited to represent the ongoing migration during the day and night.

Influence of weather on dragonfly migration and flight behaviour along the Baltic coast

“…We therefore cannot be sure whether the increase of passage intensity of A. mixta during tail-wind conditions (assuming a general southerly flight direction, based on the results of orientation tests) is limited to ground level or whether passage also intensifies at higher altitudes. In any case, at ringing stations where birds are captured during active migration, songbirds are usually captured in higher numbers when they are facing headwinds (Komenda-Zehnder et al 2010), flying lower in order to minimize the effect of the wind, which is also the case at Pape (M. Briedis, pers. comm.).…”

Effects of Regional Cold Fronts and Localized Weather Phenomena on Autumn Migration of Raptors and Landbirds in Southwest Idaho

“…The quantitative use of mist-net data in this way has been criticized on the basis of the data being too variable for statistical conclusions to be drawn (Remsen and Good 1996), although later studies have shown mist-netting of landbirds to be effective in studies of their migration (Wang and Finch 2002). For example, Komenda-Zehnder et al (2010) compared nocturnal measurements by radar with mist-net captures the next day and found a strong correlation in a similar mountainous environment. We feel confident that the overall number of new captures is a representative index of the total population on the move.…”

Comparison of visual bird migration counts with radar estimates

“…) and, especially in alpine areas, high concentrations of migrants can occur at these heights (Bruderer , Komenda‐Zehnder et al . ). According to radar measurements from five locations in Austria, on average 36% (±12.3 sd) of diurnal bird migration occurs within 200 m of ground level (Schmidt et al .…”

“…A current threat for migrating birds is the proliferation of wind energy production (Drewitt & Langston 2006) and wind farms should be situated at locations that minimize the risk of bird collisions (Drewitt & Langston 2008, Gove et al 2013. The majority of diurnal bird migration occurs at low altitudes, below 200 m above ground level (Bruderer & Liechti 1998, 2004, Gatter 2000, Bruderer et al 2012, Schmidt et al 2016) and, especially in alpine areas, high concentrations of migrants can occur at these heights (Bruderer 1996, Komenda-Zehnder et al 2010. According to radar measurements from five locations in Austria, on average 36% (AE12.3 sd) of diurnal bird migration occurs within 200 m of ground level (Schmidt et al 2016).…”