Camila Gómez

Niche-tracking migrants and niche- switching residents: evolution of climatic niches in New World warblers (Parulidae)

Differences in life-history traits between tropical and temperate lineages are often attributed to differences in their climatic niche dynamics. For example, the more frequent appearance of migratory behaviour in temperate-breeding species than in species originally breeding in the tropics is believed to have resulted partly from tropical climatic stability and niche conservatism constraining tropical species from shifting their ranges. However, little is known about the patterns and processes underlying climatic niche evolution in migrant and resident animals. We evaluated the evolution of overlap in climatic niches between seasons and its relationship to migratory behaviour in the Parulidae, a family of New World passerine birds. We used ordination methods to measure seasonal niche overlap and niche breadth of 54 resident and 49 migrant species and used phylogenetic comparative methods to assess patterns of climatic niche evolution. We found that despite travelling thousands of kilometres, migrants tracked climatic conditions across the year to a greater extent than tropical residents. Migrant species had wider niches than resident species, although residents as a group occupied a wider climatic space and niches of migrants and residents overlapped extensively. Neither breeding latitude nor migratory distance explained variation among species in climatic niche overlap between seasons. Our findings support the notion that tropical species have narrower niches than temperate-breeders, but does not necessarily constrain their ability to shift or expand their geographical ranges and become migratory. Overall, the tropics may have been historically less likely to experience the suite of components that generate strong selection pressures for the evolution of migratory behaviour.

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.

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.

Major stopover regions and migratory bottlenecks for Nearctic-Neotropical landbirds within the Neotropics: a review

Nearly 300 species of landbirds, whose populations total billions, migrate between the Neotropics and North America. Many migratory populations are in steep decline, and migration is often identified as the greatest source of annual mortality. Identifying birds’ needs on migration is therefore central to designing conservation actions for Nearctic-Neotropical migratory birds; yet migration through the Neotropics is a significant knowledge gap in our understanding of the full annual cycle. Here, we synthesise current knowledge of Neotropical stopover regions and migratory bottlenecks, focusing on long-distance, migratory landbirds that spend the boreal winter in South America. We make the important distinction between “true” stopover—involving multi-day refuelling stops—and rest-roost stops lasting

Assessing seasonal changes in animal diets with stable-isotope analysis of amino acids: a migratory boreal songbird switches diet over its annual cycle

Tools to study seasonal changes in animal diets are needed to address a wide range of ecological questions. This is especially true of migratory animals that experience distinct environments where diets may be substantially different. However, tracking diets of individuals that move vast distances has proven difficult. Compound-specific isotope analysis has emerged as a valuable tool to study diets but has been little used to study dietary changes of migratory animals. Using this technique, we quantify seasonal variation in the annual diet of a migratory songbird (gray-cheeked thrush, Catharus minimus) and test the hypothesis that migrants change their diet in response to the energetic requirements of different periods of the annual cycle. By measuring δ13C and δ15N values of amino acids from feathers grown on the breeding grounds, blood formed during migration and claw grown on the wintering grounds, we found that migration is associated with greater consumption of fruit, compared to the breeding or wintering periods. This was confirmed by the lower trophic position of blood compared to feather and claw, by a decrease in the δ15N value of the source amino acid phenylalanine in blood as a function of days of stopover, and by the positive correlation between δ15N and δ13C values of phenylalanine in blood, and not in feather or claw. This study illustrates how isotopic analysis of amino acids can contribute to understand food webs, seasonal dietary changes and metabolic routing of nutrients in migratory animals.