Crossing the Sahara desert: migratory strategies of the Grasshopper Warbler Locustella naevia

Self Cite

To African-Palearctic migratory birds that spend the non-breeding season in sub-Saharan Africa, the Sahara desert is an ecological barrier that must be crossed using large energy reserves stored at stopover sites either side. Given the coincidence of spring migration with the height of the dry season in the west African Sahel, spring stopover conditions may be a limiting factor that regulates populations. By determining where, when and how reserves are accumulated prior to crossing the desert, we can identify possible population regulation mechanisms in Palearctic migrants and effectively target stopover sites for protection. We present data on the non-breeding season use of Djoudj National Park in the western Sahel by 38 Palearctic migrants based on constant effort mist-netting data. Twenty-four species showed evidence of using Djoudj either throughout the nonbreeding period or during spring migration. The remaining 14 species were largely absent from Djoudj and presumably accumulated energy reserves for spring migration south of the Sahel. Species preparing for spring migration at Djoudj began increasing their energy reserves in January or February. Energy reserves were generally accumulated at slow rates during long stopovers, and in all species examined, predicted take-off fuel loads were sufficient to cross the Sahara desert under certain conditions. Three species, Common Redstart Phoenicurus phoenicurus, Common Whitethroat Sylvia communis and Common Blackcap Sylvia atricapilla, achieved faster rates and had concurrently lower stopover durations. We predict that vulnerability to fluctuating environmental conditions in the Sahel region is greater in species that initiate fuelling later in the spring and in species with short stopover durations.

Section: Model Fuel Deposition Rate (Model Fdr)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

Fuelling for the Sahara crossing: variation in site use and the onset and rate of spring mass gain by 38 Palearctic migrants in the western Sahel

Bayly¹,

Atkinson

Rumsey³

2012

Self Cite

“…Nevertheless, a growing number of studies suggest that conditions encountered during migration may also be important for some species (Stokke et al 2005;Newton 2006;Bayly et al 2011), with a few studies providing evidence that mortality during migration may be higher than that experienced at the stationary stages (Ward et al 1997;Sillett and Holmes 2002;Klaassen et al 2014). These data coincide with occasional observations documenting substantial numbers of migratory bird deaths under adverse environmental conditions.…”

Section: Introductionmentioning

confidence: 81%

“…low food availability), it seems likely that a large proportion of the mortality which occurs in avian species is associated with this stage of the annual cycle. Several studies have provided evidence for this hypothesis (Newton 2006;Bayly et al 2011;Klaassen et al 2014, Hewson et al 2016. For example, Sillett and Holmes (2002) estimated that mortality rates of Blackthroated Blue Warblers Dendroica caerulescens were at least 15-fold higher during migration than during stationary periods.…”

Section: Discussionmentioning

confidence: 99%

Conditions at autumn stopover sites affect survival of a migratory passerine

Hałupka

Wierucka

Sztwiertnia³

et al. 2017

Weather is an important factor affecting many aspects of avian ecology, yet its importance for survival during various periods of the avian annual cycle has received relatively little attention and remains poorly understood. We have investigated the effect of weather conditions at the breeding and wintering grounds and during migration on the survival probability of Eurasian Reed Warblers Acrocephalus scirpaceus, a long-distance Palaearctic-African migrant species. We found that annual survival was significantly affected by precipitation at the autumn stopover sites in Spain and Morocco, where Reed Warblers accumulate energy reserves prior to crossing the Sahara desert: low rainfall was associated with decreased survival. We suggest that arid conditions at these crucial stopover sites may cause limitations in food availability, prevent proper refueling and hence result in lower survival.Survival estimates were not related to weather conditions in Africa during the bird wintering period or the preceding wet season. Likewise, meteorological conditions at the breeding area did not influence survival. Survival estimates for males and females did not differ, although recapture probability was significantly lower for females than for males. Our results indicate that weather significantly influences Reed Warbler survival, although its effect may vary for different stages of the annual cycle. Our findings suggest that marked climatic changes occurring along migratory routes, in particular at important stopover sites, may have far-reaching consequences on bird survival and population size.

“…We investigate whether temperate barrier-crossing migrants use fat reserves [because they need to optimise range (Jenni-Eiermann et al 2011)] much more than tropical migrants, which lack similar barriers and so may use protein [because other aspects such as water balance are optimised (Gerson and Guglielmo 2011a, b)]. Palearctic migrants that have to cross the Sahara and the Mediterranean Sea (Bayly et al 2011(Bayly et al , 2012 are likely to utilise more fat reserves compared to sub-Saharan intra-African migrants that are likely to experience more or less unbroken habitat (Hockey 2000) that is at least moderately suitable for most species over most of their assumed migration routes.…”

Section: Introductionmentioning

confidence: 99%

Body reserves in intra-African migrants

Nwaogu

Cresswell

2015

Avian migration has been shown to be a life history strategy for surviving environmental resource variability, but it requires increased body reserves for long distance flight. Fat reserves make excellent energy stores for barrier crossing, whereas proteins generate less energy for the same mass of fat but provide water during breakdown, which may become especially useful when birds become water stressed. Intra-African migrants are probably unlikely to have to cross barriers equivalent to the Sahara and the Mediterranean sea and so may have different patterns of mass reserves reflecting the utility of metabolizing fat versus protein in hot, tropical environments. We examined differences in proportions of body mass gain, pectoral muscle score, and fat score between intra-African migrants, Palearctic migrants, and resident African species. We tested whether intra-African migrants show a distinct seasonal peak in mass gain corresponding to expected peak migration period in a manner similar to Palearctic migrants, but maintain larger muscle tissues, because Palearctic migrants are more constrained by a need to heavily up-regulate fat in addition to fat-free reserves before migration due to the energy requirements of crossing the barrier of the Sahara. We found that intra-African migrants had a peak seasonal mass gain similar to Palearctics whereas African residents did not, and that Palearctics increased fat reserves with pectoral muscle reserves, so that they had much higher fat scores for any given level of pectoral muscle compared to intra-African migrants or resident species. Our results suggest that barrier crossing leads to a distinct increase in fat reserves rather than migration per se, and suggests that intra-African migrants are more similar in their reserve management to African residents. Mass gain devoid of visible fat accumulation in intra-African migrants may, therefore, suggest absence of barriers during migration.