Wide ranging stopover movements and substantial fuelling in first year garden warblers at a northern stopover site

Abstract

Migratory birds use stopovers to replenish their fuel reserves and they generally spend more time at stopover sites than they do in actual fl ight. When arriving at a new stopover site birds may need to search extensively to fi nd a suitable feeding area and this search and settling period may aff ect the duration of stopover. Stopover behaviour can thus have profound eff ects on the migratory programme and studies on stopover behaviour are important to understand migratory strategies. We followed 51 fi rst-year garden warblers Sylvia borin with radio-transmitters at an autumn stopover site on the island of Gotland in southern Sweden. Our aim was to determine the distance birds relocated from the coastal capture site when searching for an area to settle in, and also to establish the duration of stopover and put it in relation to refuelling rate by recapturing a subset of the radio-tracked individuals. Sixteen birds made an extended stopover ( Ͼ 2 d), relocated inland from the capture site and settled on average 5.6 km from the capture site, with the longest recorded relocation being fourteen kilometres. Birds that relocated nocturnally settled in areas further away than birds that relocated diurnally. Th irteen birds that continued migration after a short stop carried larger fuel stores than birds that stopped over longer and they remained close to the capture site until departure. Th ree birds were re-trapped and showed high fuelling rates, between 0.3 and 1.1 g d Ϫ 1 . Th ey left the stopover site with fuel loads between 40 -56 percent of lean body mass, which possibly would have allowed them to reach the Mediterranean area without additional refuelling stops.

Are movements of daytime and nighttime passerine migrants as different as day and night?

“…We think, however, that the higher proportion of dunnocks observed to take detours in spring could hint to another phenomenon, so‐called landscape movements (see Schmaljohann & Eikenaar, 2017 for a recent review). As revealed from recent radio‐tracking studies, songbird migrants might sometimes leave a stopover site to search for another stopover site nearby (Mills, Thurber, Mackenzie, & Taylor, 2011; Stach, Fransson, Jakobsson, & Kullberg, 2015; Taylor et al., 2011) or might perform short exploratory flights, for example, to check wind conditions aloft (Schmaljohann et al., 2011).…”

Causes and characteristics of reverse bird migration: an analysis based on radar, radio tracking and ringing at Falsterbo, Sweden

“…To be able to continue on migration, especially if facing a sea crossing, it is very important for birds to have enough fuel. Consequently, fat score seems to be the factor that most reliably differs between forward and reverse migrants (Lindström and Alerstam , Åkesson et al , Sandberg and Moore , Deutschlander and Muheim , Schmaljohann and Naef‐Daenzer , Smolinsky et al , Stach et al ). In our study we find the expected effect of fat score on the likelihood of ringing recoveries in forward or reverse directions, with lean birds turning backwards to a higher degree (similar to the pattern observed in Smolinsky et al ).…”

“…Falsterbo peninsula is a migratory hotspot in southern Sweden, where millions of migrating birds pass each year. Coastlines are in many cases poor stopover habitats, especially at hotspots where many migrants pass, leading to high competition and high predation risk (Alerstam , Woodworth et al , see also Stach et al ). Poor stopover habitats may increase the amount of reverse migration, and birds have been shown to perform reverse movements to a higher degree at coastal stopover sites such as Falsterbo compared to inland stopover sites (Åkesson ).…”

Inability to regain normal body mass despite extensive refuelling in great reed warblers following the trans‐Sahara crossing during spring migration

“…It has, however, been shown in several studies that migrants are able to locate habitats that fulfil their species specific requirements, although it is unclear exactly how they manage to do it (cf. Moore and Aborn , Ktitorov et al , Chernetsov , Stach et al ). In spite of that this study took place in the peak of spring migration, relatively low numbers of migrants were ringed at Anapodaris river mouth and great reed warblers comprised 26.4 and 24.3% of all migrants ringed in 2015 and 2016, respectively.…”