Teresa Catry

Long-term declines of wader populations at the Tagus estuary, Portugal : a response to global or local factors?

Summary Migratory wader populations face global threats, mainly related to increasing rates of habitat loss and disturbance driven by human activities. To a large extent, the long-term survival of these populations requires the conservation of networks of sites along their migratory flyways. The Tagus estuary, Portugal, is among the most important wetlands for waders in the East Atlantic Flyway. Annual winter wader counts have been carried in this wetland since 1975 and a monthly roost-monitoring programme was implemented in 2007. Wintering populations of three out of the five most abundant species, Dunlin Calidris alpina, Grey Plover Pluvialis squatarola and Redshank Tringa totanus, showed significant population declines over the past three decades, which are most likely due to the loss and degradation of roost sites as a result of increasing human activity. The situation is unlikely to improve, as a high proportion of the wintering waders use roost sites that are situated in highly urbanised areas with no legal protection. The use of different roost sites by waders is highly variable both temporally and spatially, thus emphasizing the need for a network of good quality roost sites. Additionally, during migration, 60–80% of all the waders of the Tagus estuary concentrate at a single refuge, thus increasing the risk for wader populations during these periods.

Influence of environmental factors and energetic value of food on Little Tern Sterna albifrons chick growth and food delivery

Capsule Chick diet and early growth parameters differed between birds breeding on natural (sandy beaches) and alternative (salinas) habitats. Aims To describe diet and growth of Little Tern chicks, to examine the influence of tide and wind speed on food delivered to chicks in southern Portugal, and to determine whether diet and early chick growth parameters differed between birds breeding in natural (sandy beaches) and man-made alternative (salinas) habitats. Methods Nests were marked and visited regularly. Chicks were ringed and weighed at hatching. Between 12 and 14 nests were enclosed by a fence in order to measure and weigh chicks every day. Prey items delivered to fenced chicks were observed daily from hides, in different tidal periods and wind speed conditions. Prey items dropped in the colonies were collected. Foraging efficiency of adults was evaluated in relation to tide and wind speed. Fresh prey items were collected to determine their energetic content using a calorimeter. Results A chicks grew faster than B chicks, but both attained the same weight, tarsus-length and wing-length at 19 days old; C chicks attained the same values at day 21. There were significant differences in early growth parameters. A and B chicks grew faster on sandy beaches than on salinas. The main prey items were Atherina spp., Fundulus sp. and shrimps in salinas and Sardina pilchardus, Atherina spp. and Belone belone on sandy beaches. There was no correlation between ingestion rate and chick age, but mean length of prey ingested by chicks increased significantly with age. Conclusion Tide and wind speed influenced both size and type of prey delivered to chicks. Parents met the increasing energy demands of the growing chicks by adjusting prey size rather than prey delivery frequency. Differences in chick growth between salinas and sandy beaches could be related to both parental quality and diet. The presence of a high energy-content euryhaline fish species in the salinas, together with more sheltered conditions and no tidal influences, may be important in explaining the use of salinas by Little Terns.

Sex Promotes Spatial and Dietary Segregation in a Migratory Shorebird during the Non-Breeding Season

Several expressions of sexual segregation have been described in animals, especially in those exhibiting conspicuous dimorphism. Outside the breeding season, segregation has been mostly attributed to size or age-mediated dominance or to trophic niche divergence. Regardless of the recognized implications for population dynamics, the ecological causes and consequences of sexual segregation are still poorly understood. We investigate the foraging habits of a shorebird showing reversed sexual dimorphism, the black-tailed godwit Limosa limosa, during the winter season, and found extensive segregation between sexes in spatial distribution, microhabitat use and dietary composition. Males and females exhibited high site-fidelity but differed in their distributions at estuary-scale. Male godwits (shorter-billed) foraged more frequently in exposed mudflats than in patches with higher water levels, and consumed more bivalves and gastropods and fewer polychaetes than females. Females tended to be more frequently involved and to win more aggressive interactions than males. However, the number of aggressions recorded was low, suggesting that sexual dominance plays a lesser role in segregation, although its importance cannot be ruled out. Dimorphism in the feeding apparatus has been used to explain sex differences in foraging ecology and behaviour of many avian species, but few studies confirmed that morphologic characteristics drive individual differences within each sex. We found a relationship between resource use and bill size when pooling data from males and females. However, this relationship did not hold for either sex separately, suggesting that differences in foraging habits of godwits are primarily a function of sex, rather than bill size. Hence, the exact mechanisms through which this segregation operates are still unknown. The recorded differences in spatial distribution and resource use might expose male and female to distinct threats, thus affecting population dynamics through differential mortality. Therefore, population models and effective conservation strategies should increasingly take sex-specific requirements into consideration.

Mosaic-Level Inference of the Impact of Land Cover Changes in Agricultural Landscapes on Biodiversity: A Case-Study with a Threatened Grassland Bird

Changes in land use/land cover are a major driver of biodiversity change in the Mediterranean region. Understanding how animal populations respond to these landscape changes often requires using landscape mosaics as the unit of investigation, but few previous studies have measured both response and explanatory variables at the land mosaic level. Here, we used a “whole-landscape” approach to assess the influence of regional variation in the land cover composition of 81 farmland mosaics (mean area of 2900 ha) on the population density of a threatened bird, the little bustard (Tetrax tetrax), in southern Portugal. Results showed that ca. 50% of the regional variability in the density of little bustards could be explained by three variables summarising the land cover composition and diversity in the studied mosaics. Little bustard breeding males attained higher population density in land mosaics with a low land cover diversity, with less forests, and dominated by grasslands. Land mosaic composition gradients showed that agricultural intensification was not reflected in a loss of land cover diversity, as in many other regions of Europe. On the contrary, it led to the introduction of new land cover types in homogenous farmland, which increased land cover diversity but reduced overall landscape suitability for the species. Based on these results, the impact of recent land cover changes in Europe on the little bustard populations is evaluated.

Seasonal Variations in the Diet and Foraging Behaviour of Dunlins Calidris alpina in a South European Estuary: Improved Feeding Conditions for Northward Migrants

During the annual cycle, migratory waders may face strikingly different feeding conditions as they move between breeding areas and wintering grounds. Thus, it is of crucial importance that they rapidly adjust their behaviour and diet to benefit from peaks of prey abundance, in particular during migration, when they need to accumulate energy at a fast pace. In this study, we compared foraging behaviour and diet of wintering and northward migrating dunlins in the Tagus estuary, Portugal, by video-recording foraging birds and analysing their droppings. We also estimated energy intake rates and analysed variations in prey availability, including those that were active at the sediment surface. Wintering and northward migrating dunlins showed clearly different foraging behaviour and diet. In winter, birds predominantly adopted a tactile foraging technique (probing), mainly used to search for small buried bivalves, with some visual surface pecking to collect gastropods and crop bivalve siphons. Contrastingly, in spring dunlins generally used a visual foraging strategy, mostly to consume worms, but also bivalve siphons and shrimps. From winter to spring, we found a marked increase both in the biomass of invertebrate prey in the sediment and in the surface activity of worms and siphons. The combination of these two factors, together with the availability of shrimps in spring, most likely explains the changes in the diet and foraging behaviour of dunlins. Northward migrating birds took advantage from the improved feeding conditions in spring, achieving 65% higher energy intake rates as compared with wintering birds. Building on these results and on known daily activity budgets for this species, our results suggest that Tagus estuary provides high-quality feeding conditions for birds during their stopovers, enabling high fattening rates. These findings show that this large wetland plays a key role as a stopover site for migratory waders within the East Atlantic Flyway.

INTERCOLONY AND ANNUAL DIFFERENCES IN THE DIET AND FEEDING ECOLOGY OF LITTLE TERN ADULTS AND CHICKS IN PORTUGAL

Abstract We used pellets and prey dropped near nests to study the diet and feeding ecology of Little Tern (Sterna albifrons) adults and chicks in northern and southern Portugal during 2000–2004. We compared diets among study areas, years, and birds breeding in two main habitat types: sandy beaches and salinas (man-made salt-pans). We also sampled prey fish in one study area and used published data from another area to examine whether the diet of Little Tern adults and chicks corresponded with the abundance of prey found in primary foraging habitats. Adult Little Terns fed mainly on sand-smelts (Atherina spp.) and gobies (Pomatoschistus spp.), which were the most abundant fish species in our estuarine and lagoon study areas. Despite some differences among colonies we did not find clear differences in adult diets among study areas and habitat types. Little Tern chicks also ate primarily Atherina spp., but their diet was more diverse than that of the adults, and characterized by a significantly higher proportion of prey with a high energetic value such as sardines (Sardina pilchardus), garfish (Belone belone), and killifish (Fundulus spp.). Chick diets differed significantly between habitat types, which may be a consequence of the limited foraging range of adult breeding birds. Both adult and chick diets matched the abundance of the main prey in our study areas, confirming the opportunistic foraging character of this species. We suggest that annual variation in prey availability may be the most important factor explaining annual variation in adult diets.

Unravelling migration routes and wintering grounds of European rollers using light-level geolocators

We used light-level geolocators to track the migratory journey of a globally near-threatened trans-Saharan migrant, the European roller Coracias garrulus, from its breeding grounds in Iberia to its wintering grounds in southern Africa. During autumn migration, birds followed the western African coast with lengthy stopovers within sub-Saharan countries before crossing the equatorial rainforests towards the wintering areas, mainly in Angola. Although based in only two tracked birds, comparison of our results with other studies suggests that western European rollers use distinct migration routes and stopover sites towards shared wintering grounds. Time spent in widely separated and ecologically disparate countries highlights the vulnerability of the species facing the cumulative risks of each area used along their journey.ZusammenfassungAufklärung der Zugwege und Überwinterungsgebiete von Blauracken mittels Hell-Dunkel-Geolokatoren Mithilfe von Hell-Dunkel-Geolokatoren verfolgten wir die Zugroute eines weltweit potentiell gefährdeten Transsaharaziehers, der Blauracke Coracias garrulus, von ihren Brutgebieten auf der Iberischen Halbinsel zu ihren Winterquartieren im südlichen Afrika. Auf dem Herbstzug folgten die Vögel der westafrikanischen Küste, wobei sie längere Rastpausen in subsaharischen Ländern einlegten, um dann die äquatorialen Regenwälder zu überqueren und in die vor allem in Angola gelegenen Überwinterungsgebiete zu fliegen. Obwohl nur zwei Vögel verfolgt wurden, legen Vergleiche mit anderen Studien nahe, dass westeuropäische Blauracken auf dem Weg in die gemeinsamen Winterquartiere klar abgegrenzte Zugrouten und Rastgebiete nutzen. Es unterstreicht die Anfälligkeit dieser Vogelart, dass sie dabei Zeit in weit auseinander liegenden und ökologisch ganz verschiedenartigen Ländern verbringt und somit den geballten Risiken aller Gegenden ausgesetzt ist, die sie auf ihrer Reise aufsucht.

Influence of age and sex on winter site fidelity of sanderlings Calidris alba

Many migratory bird species show high levels of site fidelity to their wintering sites, which confers advantages due to prior knowledge, but may also limit the ability of the individual to move away from degrading sites or to detect alternative foraging opportunities. Winter site fidelity often varies among age groups, but sexual differences have seldom been recorded in birds. We studied a population of individually colour-marked sanderlings wintering in and around the Tejo estuary, a large estuarine wetland on the western coast of Portugal. For 160 individuals, sighted a total of 1,249 times between November 2009 and March 2013, we calculated the probability that they moved among five distinct wintering sites and how this probability is affected by distance between them. To compare site fidelity among age classes and sexes, as well as within the same winter and over multiple winters, we used a Site Fidelity Index (SFI). Birds were sexed using a discriminant function based on biometrics of a large set of molecularly sexed sanderlings (n = 990). The vast majority of birds were observed at one site only, and the probability of the few detected movements between sites was negatively correlated with the distance among each pair of sites. Hardly any movements were recorded over more than 15 km, suggesting small home ranges. SFI values indicated that juveniles were less site-faithful than adults which may reflect the accumulated knowledge and/or dominance of older animals. Among adults, females were significantly less site faithful than males. A sexual difference in winter site fidelity is unusual in shorebirds. SFI values show site-faithfulness is lower when multiple winters were considered, and most birds seem to chose a wintering site early in the season and use that site throughout the winter. Sanderlings show a very limited tendency to explore alternative wintering options, which might have implications for their survival when facing habitat change or loss (e.g., like severe beach erosion as can be the case at one of the study sites).

Comparative Feeding Ecology of Shorebirds Wintering at Banc d’Arguin, Mauritania

Limited knowledge of shorebird feeding ecology in the tropics hinders interpretation of considerable changes in numbers recently observed at the most important shorebird nonbreeding area along the East Atlantic Flyway, the Banc d’Arguin in Mauritania. We used droppings and video recordings to compare the diet and foraging behaviour of six shorebird species at Banc d’Arguin: dunlin, sanderling, red knot, ringed plover, grey plover and bar-tailed godwit. In four of those species, the detail achieved in prey identification allowed us to calculate niche width and foraging niche overlap. Sanderling and ringed plover consumed a wide range of macroinvertebrate taxa, while red knot consumed mainly bivalves with some gastropods, and both grey plover and bar-tailed godwit fed mainly on polychaete worms. A large proportion of dunlin droppings had no recognizable food items, suggesting a soft-bodied prey that we could not identify. Dunlin was the only strictly tactile forager, while sanderling and red knot used both visual and tactile methods. The remaining species resort to visual cues for catching prey, even though tactile cues may be important for bar-tailed godwits. Dunlin, sanderling, red knot and ringed plover all showed relatively narrow trophic niches and non-significant levels of foraging niche overlap. Overall, the diet of each shorebird was broadly similar to that reported from other parts of their wintering ranges, with more gastropods in the red knot diet than previously described. Our data suggest that the high shorebird densities at Banc d’Arguin despite low benthic food densities (Hydrobiologia 258:211-222, 1993a; Journal of Sea Research 60:235-243, 2008) may result partly from partitioning of available resources via narrow trophic niches with little overlap. Given these distinct trophic niches, the widespread declines in shorebird numbers at Banc d’Arguin may reflect changes in a wide range of prey types and suggest a shift in the overall properties of this unique ecosystem.

Mechanisms and fitness consequences of laying decisions in a migratory raptor

Seasonal decline in breeding performance is a commonly observed pattern in birds, but disentangling the contributions of environmental conditions (“timing” hypothesis) and individual quality (“quality” hypothesis) to such a pattern is challenging. Moreover, despite the strong selection for early breeding, the individual optimization model predicts that each individual has an optimal breeding window. We investigated the causes and consequences of laying decisions in the lesser kestrel (Falco naumanni) by combining a long-term dataset on reproductive traits with information on food availability. A marked seasonal decline was found in breeding success, mostly mediated by a decline in clutch size. The “timing” hypothesis, supported by the decline in consumption of mole crickets, key prey for prelaying females, seems to explain the seasonal trend in clutch size, as this pattern was recorded in both higher (adults) and lower (yearlings) quality individuals. Contrarily, the higher proportion of yearlings breeding late in the season, rather than a decay in food availability during chick rearing, seems to drive the decline in fledging success, giving support to the “quality” hypothesis. Advanced breeding and increased clutch size, as proxies of reproductive effort, were not offset by lower survival. Low repeatability in both these traits suggests that individual quality is a dynamic attribute and reproduction costs are minimized by individual optimization. Understanding the mechanisms driving individual breeding decisions is critical to anticipate species’ ability to cope with environmental changes. Here, we show that lesser kestrels failing the prelaying food window opportunity compromise reproductive performance, mostly regardless of their individual quality.