When and where to fuel before crossing the Sahara desert – extended stopover and migratory fuelling in first-year garden warblers Sylvia borin

Abstract

Large numbers of passerine migrants cross the Sahara desert every year on their way to-and-from wintering areas in tropical Africa. In the desert, hardly any fuelling opportunities exist and most migrants have to prepare in advance. A central question is how inexperienced birds know where to fuel. Inexperienced garden warblers Sylvia borin were studied in Greece just before the desert crossing in autumn. Body mass data collected at two sites indicate that most birds do not fuel for the desert crossing further north. For the first time, detailed information about stopover duration close to the Sahara desert was studied by using light weight radio-transmitters. Results from Crete show that most first-year garden warblers arrive with relatively small fuel loads in relation to lean body mass (B30%), stay for 13Á20 d and depart with an average fuel load of about 100%. Radio-tagged birds performed small scale movements initially and took advantage of fig fruits. Birds trapped at fig trees were heavier than birds trapped with tape lures, showing that tape lures can bias the sample of migrants trapped. The precise fuelling pattern found indicates that first-year migrants must also include external spatial cues to make the preparation for crossing the desert in the right area.

Fuelling in front of the barrier - are there age based behavioral differences in Garden Warblers Sylvia borin?

“…Interestingly, the calculated departure body mass found in this study (30.5g) is very close to the one found using an average calculated fuel deposition rate from re-trapped birds in combination with stopover length on partly the same dataset of first-year Garden Warblers in Crete (30.3g;Fransson et al, 2008).…”

“…Mean body mass of birds trapped using tape lures on southern Crete was lower for first-year birds More than half of the adult birds (54%) disappeared from the study area during the first nine days while the same figure for first-year birds was much less, 24% (Mann-Whitney: Z=3.509, p<0.001; Fig 2) and only a few individuals disappeared in the interval 7-9 d. Due to the bi-modal distribution of number of days transmitter birds were present in the area (Fig 2) and flight range estimates indicating that garden warblers have to make a considerable stopover and fuelling to be able to cross the desert (Fransson, et al 2008;Barboutis, et al 2011), we have assumed that those that stayed longer than 9 d made a complete stopover in the study area. The median stopover duration for birds that stayed longer than 9 d was longer -1.993, p=0.046).…”

“…Ellegren, 1991;Rguibi-Idrissi et al, 2003). Both first-year and adult birds seem to make local movements during the first period of time after arrival, in adult birds this regularly involved leaving the study area while first-year birds, as shown by Fransson et al (2008), normally stayed within the study area. This means that some birds included in the study might have arrived from other sites on Crete and thus had a longer stopover duration than estimated.…”

“…All the birds equipped with radio transmitters were trapped using tape lures at the site close to the village of Kalivia (Fig 1 c). It has been shown that a great proportion of birds trapped with tape lures early in the morning are birds that have performed a migratory flight the night before (Schaub et al, 1999, Fransson et al, 2008 and combined with the fact that we only equipped lean birds with radio transmitters, we believe that the majority of birds studied were newly arrived at the study site. First-year birds were given transmitters between 29 th August and 14 th September while adults between 21 st August and 8 th September.…”

“…When estimating the departure body mass, we have used the median stopover period of birds with transmitters that we have classified to have stayed in the study area until they departed for sub-Saharan (Fransson et al, 2008) and that their body mass is increasing during their stopover. If the mean stopover duration at our site is χ, then the mean body mass of the (1/ χ *100) % of the heaviest birds represent the departure morning body mass (see Alerstam & Lindström, 1990 for the logic behind this).…”