Nicholas J. Bayly

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

AbstractTo 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 non-breeding 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.ZusammenfassungAuftanken für die Sahara-Überquerung—Variation bei der Gebietsnutzung und in Beginn und Ausmaß der Gewichtszunahme im Frühling bei achtunddreißig paläarktischen Zugvogelarten im westlichen Sahel. Für Zugvögel, die in der Paläarktis brüten und im subsaharischen Afrika überwintern, stellt die Sahara eine ökologische Barriere dar, für deren Überwindung sie große Energiereserven benötigen, die sie bei Zwischenstopps auf beiden Seiten einlagern. Da der Frühjahrszug mit dem Höhepunkt der Trockenzeit in der westafrikanischen Sahelzone zusammentrifft, könnten die dortigen Rastbedingungen im Frühjahr einen limitierenden, die Populationen regulierenden Faktor darstellen. Kann man ermitteln, wann, wo und wie die Energiereserven vor der Überquerung der Wüste angelegt werden, lassen sich mögliche Mechanismen der Populationsregulation bei paläarktischen Zugvögeln ausmachen und Rastplätze gezielt schützen. Hier präsentieren wir Daten über die Nutzung des Djoudj Nationalparks als Winterquartier durch 38 paläarktische Zugvogelarten, die auf der Grundlage standardisierter Netzfänge gewonnen wurden. Vierundzwanzig Arten nutzten Djoudj entweder während der ganzen Überwinterungszeit oder auf dem Frühjahrszug. Die restlichen 14 Arten suchten Djoudj kaum auf und legten ihre Energiereserven für den Frühjahrszug vermutlich südlich der Sahelzone an. Arten, die sich in Djoudj für den Frühjahrszug rüsteten, begannen im Januar oder Februar mit der Aufstockung ihrer Energiereserven. Diese wurden in der Regel allmählich während längerer Rastaufenthalte angelegt; bei allen untersuchten Arten waren die geschätzten Energiereserven beim Abflug ausreichend, um die Sahara unter bestimmten Bedingungen zu überqueren. Drei Arten—Gartenrotschwanz Phoenicurus phoenicurus, Dorngrasmücke Sylvia communis und Mönchsgrasmücke Sylvia atricapilla—erreichten schnellere Zunahmeraten und hatten folglich kürzere Verweildauern. Unsere Prognose wäre eine größere Anfälligkeit für veränderliche Umweltbedingungen in der Sahelzone bei Arten, die erst später im Frühling auftanken.

Fall Migration of the Veery (Catharus fuscescens) in Northern Colombia: Determining the Energetic Importance of a Stopover Site

ABSTRACT. Migration is a highly energy-demanding process, and migratory birds store energy at stopover sites along their migration routes to meet these demands. The Veery (Catharus fuscescens) performs one of the longest migrations of all Neotropical migratory landbirds, yet the stopover sites that it uses and their relative importance to migratory success are poorly known. We studied the Veery during two fall migrations (2009 and 2010) in northern Colombia, where we hypothesized that birds would replenish energy reserves after crossing the Caribbean Sea to fuel flights toward their wintering grounds. To determine the relative energetic importance of our study site, we combined estimates of fuel deposition rates, stopover durations, and flight ranges to estimate the percentage of the total migratory distance that could be covered following a stopover. Veeries arrived at our study site with low energy reserves, and recaptured birds increased their body mass at a mean rate of 3.6% of lean body mass day-1 over an average of 9 days. At departure, birds carried energy reserves equivalent to 34.5% of lean body mass, giving rise to a maximum potential flight range of 2,200 km, thereby allowing them to cover ≤30% of the total fall migration distance, depending on how reserves are used. By showing how a stopover site is used in energetic terms, we highlight the importance of a site in northern Colombia to long-distance Neartic—Neotropic migrants and also introduce a method that contributes to the prioritization of stopover sites across regions and species.

Fuel loads acquired at a stopover site influence the pace of intercontinental migration in a boreal songbird

Long-distance migratory organisms are under strong selection to migrate quickly. Stopovers demand more time than flying and are used by individuals to refuel during migration, but the effect of fuel loads (fat) acquired at stopover sites on the subsequent pace of migration has not been quantified. We studied stopover behaviour of Grey-cheeked Thrush (Catharus minimus) at a site in northern Colombia and then tracked their migration using an intercontinental radio-telemetry array. Tracking confirmed long-distance flights of more than 3000 km, highlighting the key importance of a single stopover site to the migration strategy of this species. Our results suggest that these songbirds behave as time-minimizers as predicted by optimal migration theory, and that fuel loads acquired at this South American stopover site, together with departure date, carry-over to influence the pace of migration, contributing to differences in travel time of up to 30 days in birds subsequently detected in the U. S. and Canada. Such variation in the pace of migration arising from a single stopover site, likely has important fitness consequences and suggests that identifying important fuelling sites will be essential to effectively conserve migratory species.

Seasonal variation in stopover site use: Catharus thrushes and vireos in northern Colombia

AbstractMigratory routes and stopover sites used by migrant landbirds may vary among seasons and years. Understanding the degree of such variation is an important aspect of migrant ecology, especially in South America, where routes and sites are not well known. We studied stopover site use by two Vireo species and three Catharus species in the Sierra Nevada de Santa Marta in northern Colombia during a two-year period. We used capture totals corrected for mist-net effort and density estimates derived from variable distance transect observations performed at sites located at three different altitudes and thus habitats to describe variation in stopover site use. Four of the five study species showed significant seasonal variation in terms of presence, such that ecologically similar species were present during opposite migration periods; for example, Vireo flavoviridis was an autumn migrant, while Vireo olivaceus was commonest in spring. A similar pattern was observed for Catharus fuscescens and Catharus minimus. We detected differences in density with habitat/altitude, with C. minimus and Catharus ustulatus most abundant in pre-montane forest, while C. fuscescens abundance peaked in lowland forest. At the same altitude, Catharus species were more abundant in pre-montane forest than in shade coffee plantations. The abundance of C. ustulatus varied markedly between years during autumn migration, potentially in association with changes in the prevailing wind conditions. The temporal and spatial variation in stopover site use described here provides important insights into migratory strategies for Neotropical migrants within northern South America, and also regarding the multiple factors that may have shaped these strategies.ZusammenfassungJahreszeitliche Variation in der Nutzung von Rastgebieten:Catharus-Drosseln und Vireos im nördlichen Kolumbien Vogelzugrouten und verfügbare Rastgebiete können saisonal und auch zwischen den Jahren variieren. Ein besseres Verständnis des Ausmaßes dieser Variation ist ein wichtiger Aspekt der Vogelzugökologie, besonders in Südamerika, wo Zugrouten und Rastgebiete oft nicht bekannt sind. Wir haben Rastgebiete von zwei Vireo- und drei Catharus-Arten in der Sierra Nevada de Santa Marta im nördlichen Kolumbien während zwei Jahren untersucht. Zur Beschreibung der Variation der Nutzung der Rastgebiete verwendeten wir die Gesamtzahl gefangener Vögel korrigiert um den Fangaufwand und Dichteschätzungen auf Grundlage von verschiedenen Distanz-Transekt Zählungen in drei verschiedenen Höhenlagen und verschieden Habitaten. Vier der fünf untersuchten Arten zeigten signifikante jahreszeitliche Variation in ihrer Anwesenheit. Ökologisch ähnliche Arten waren in den jeweils gegenläufigen Zugperioden anwesend. So war V. flavoviridis ein Herbstzieher, während V. olivaceus im Frühjahr häufiger war. Ein ähnliches Muster konnte zwischen C. fuscescens and C. minimus beobachtet werden. Wir beschreiben Unterschiede in der Dichtheit innerhalb der Habitate und Höhenlagen. C. minimus und C. ustulatus waren häufiger im submontanem Wald anzutreffen während C. fuscescens häufiger im Tieflandregenwald vorkam. Auf der gleichen Höhenlage war Catharus häufiger in submontanem Wald als in schattigen Kaffeeplantagen anzutreffen. Die Abundanz von C. ustulatus variierte während des Herbstzuges deutlich zwischen den Jahren, und ist möglicherweise assoziiert mit Veränderungen in den vorherrschenden Windkonditionen. Die zeitliche und räumliche Variation in der Nutzung von Rastgebieten liefert wichtige Erkenntnisse in der Zugstrategie neotropischer Zugvögel im nördlichen Südamerika und suggeriert, dass multiple Faktoren derartige Strategien gefördert haben können.

Grasshopper Warbler Locustella naevia autumn migration - findings from a study in southeast Britain

This study examines the migratory behaviour of Grasshopper Warblers Locustella naevia on the south coast of Britain with respect to phenology, fuel loads, stopover behaviour and potential flight ranges. Data were derived from 5,455 mist‐net captures gathered over 10 years between July and October at the Pannel Valley Reserve, East Sussex. For the majority of captures and recaptures, fat score, wing length and body mass were recorded. Data from recaptures were used to calculate fuel deposition rates and stopover durations, whilst fat scores and body masses were used to calculate fuel loads and potential flight ranges. The majority of birds were carrying moderate fuel reserves and very few birds refuelled at the reedbed site; indeed, many appeared to remain at the site for just one day. Estimated flight ranges suggest that most birds were capable of flying to the southern half of France. Therefore it is suggested that British birds typically accumulate fuel north of the south coast, possibly at or close to their breeding grounds, and do not refuel until reaching southern France. This is supported by the pattern of birds recovered abroad. That birds stop at the south coast at all is attributed to the English Channel acting as a barrier to onward flights.

Fall stopover strategies of three species of thrush (Catharus) in northern South America

ABSTRACT Northern South America is a geographic bottleneck that may be limiting the survival of Nearctic–Neotropic migrants. However, very little is known about the migration ecology of transcontinental migrants wintering in South America. We studied the fall migratory strategies of three species of thrush (Catharus ustulatus, C. fuscescens, and C. minimus) at 2 major migrant gateways into South America: the Darién and the Sierra Nevada de Santa Marta (SNSM) of northern Colombia. Assuming that migration route shapes the stopover strategy of birds and that our sites receive birds from 2 different routes, we predicted that (1) migrants traveling over land through Central America (Darién) would make short and frequent stopovers followed by short flights and (2) migrants crossing the Caribbean (SNSM) would make few and long stopovers to acquire fuel for longer flights. To test these predictions, we estimated condition on arrival, stopover duration, fuel deposition rates, and potential flight ranges after stopover, using 3 yr of capture–recapture data. Each species adopted a different stopover strategy, with Swainsons Thrush arriving in South America through the Darién, making the shortest stopovers and achieving the shortest flight ranges (800 km); Gray-cheeked Thrush arriving primarily through the Darién, making short stopovers but achieving longer flight ranges (1,200 km); and Veery arriving in both the Darién and the SNSM, making the longest stopovers and achieving the longest flight ranges (1,900 km). Our results suggest that stopover strategies are shaped by both migratory route (over land vs. over water) and distance to final destination in South America. Unraveling the breeding origins and wintering destinations of individuals passing through northern Colombia would greatly improve our understanding of hemispheric migration systems and will be critical if we are to protect the most important stopover sites and habitats.

Distribution of Nearctic-Neotropical migratory birds along a South American elevation gradient during spring migration

ABSTRACT The Sierra Nevada de Santa Marta in northern Colombia has been identified as a critical spring stopover site for at least one Neotropical migratory bird species prior to crossing the Caribbean sea on migration. The strategic location of the Sierra suggests that other South American wintering migrants may stopover there, but no information is available on the migrant community during spring or how they distribute themselves between habitats and across the broad elevational gradient. Here, we present species richness and densities of migratory landbirds obtained through standardized census and captures along an elevation gradient (100–2,100 m) covering two habitats, forest and shade coffee, during two consecutive spring migrations. The migrant community (~39 species) showed a peak in species richness and abundance at mid elevations (700–1,700 m), mirroring the pattern often observed in communities of resident Neotropical birds. However, individually the abundance of the commonest species peaked at different elevations and showed high annual variability. We also found within-species differences in density between shade-coffee and forest at the same elevation, possibly reflecting differences in habitat quality for some species. Factors such as food availability and predation risk are expected to be critical in shaping the distribution of migrants during stopover, and further research is required to identify the drivers of the observed elevational patterns. This study contributes to our knowledge of the life histories of migrants during stopover and highlights the habitats and elevations where conservation measures would protect the highest number of species and individuals at a South American stopover site.