Alejandro Pérez-Hurtado

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free‐living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual‐based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro‐invertebrates. Intake rate is measured as the ash‐free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.

IMPORTANCE OF THE SUPRATIDAL HABITATS FOR MAINTAINING OVERWINTERING SHOREBIRD POPULATIONS: HOW REDSHANKS USE TIDAL MUDFLATS AND ADJACENT SALTWORKS IN SOUTHERN EUROPE

Abstract The prey-size selection, foraging behavior, and intake rate of overwintering Redshanks (Tringa totanus) were studied in a supratidal-intertidal system with high intertidal densities of shorebirds (100 birds ha−1). For assessing the importance of the energy obtained in the supratidal habitat (saltworks), daily consumption in this habitat was compared with the total daily energy requirement. Redshanks passively select prey within a certain size range from those accessible on the intertidal area. Despite the high prey biomass on the mudflats, Redshanks exhibited a low intake rate during winter (0.321 kJ min−1). This low intake seems to be related to the influence of the high densities of foraging shorebirds on the behavior of prey. Although intake rate was higher in the saltworks, the majority of Redshanks did not choose to feed there in winter. This foraging pattern seems to be related to density-dependent effects in habitat occupancy, as social interactions could have kept Redshanks out of the saltworks in winter according to the predictions of the ideal-despotic model. Energy intake in the supratidal habitat contributed 23% and 82% of the total daily energy requirement in winter and the pre-migration period, respectively. Redshanks were able to meet the total daily energy requirement during the pre-migration period by increasing foraging time in the saltworks. The availability of supratidal foraging habitats seems to contribute significantly to the maintenance of the population of overwintering Redshanks.

SMALL-PREY PROFITABILITY: FIELD ANALYSIS OF SHOREBIRDS' USE OF SURFACE TENSION OF WATER TO TRANSPORT PREY

Abstract Previous laboratory studies have shown that Red-necked Phalarope (Phalaropus lobatus), Wilson’s Phalarope (P. tricolor), Western Sandpiper (Calidris mauri), and Least Sandpiper (C. minutilla) use the surface tension of water surrounding a prey item to transport it from bill tip to mouth. Although such experimental work suggests that many species of shorebird may be capable of surface-tension feeding, no field studies have been done that examine this possibility. We studied the occurrence and interspecific variation in the performance of surface-tension transport (STT) in wild shorebirds feeding on identical prey items in shallow water. All shorebirds videotaped—Little Stint (C. minuta), Dunlin (C. alpina), Sanderling (C. alba), Curlew Sandpiper (C. ferruginea), Common Redshank (Tringa totanus), and Black-winged Stilt (Himantopus himantopus)—used STT to feed on small prey items. Individuals employing STT used one or several cycles of jaw spreading to transport the prey contained in a drop of water upward along the bill cavity, an action indicative of STT. Two distinct types of prey transport were observed: (1) use of STT in isolation by calidridine species following the description given in previous studies (i.e., an absence of other feeding mechanisms such as tongue movements, suction, or inertial transport), and (2) STT aided by inertial transport (head jerks) as seen in Common Redshank and Black-winged Stilt. Measured prey-transport variables (number of cycles, total time, and speed of transport) varied among species. The absence of significant relationships between these variables and measures of external morphology (bill length, bill length-to-width ratio, and bill length-to-depth ratio) suggests that some interspecific variations in STT performance may be attributable to differences in internal bill morphology. We show that STT is a common feeding mechanism in small or medium- sized shorebird species that feed on small prey items in shallow water. Birds using STT transported ≤3.6× faster than the theoretical value predicted by a previous model and can achieve high intake rates when foraging on high densities of available small prey items. Ventajas de las Presas Pequeñas: Análisis de Campo del Uso de la Tensión Superficial del Agua por las Aves Playeras para Transportar las Presas

Human Pressure on Sandy Beaches: Implications for Trophic Functioning

The effect of coastal development and tourism occupancy on the structure and trophic networks of sandy beaches was analysed for the first time, using mass-balanced trophic models. Ecopath models were applied to two beaches, representative of different anthropogenic pressures, a beach located inside a protected area and an urbanised beach with tourism infrastructure and high levels of visitors. Models comprised 28 compartment at the protected beach and 27 compartments at the urbanised beaches, including detritus, phytoplankton, zooplankton, invertebrates, fishes and birds. Results revealed that the protected area had higher values of total system throughput, biomass, ascendency and capacity, reflecting a more complex, organised, mature and active system, compared to the urbanised beach. Finally, different indicators of stress were analysed and we suggest the Finn cycling index as an indicator of anthropogenic impact on sandy beaches.

Energy and Macronutrient Assimilation Efficiencies of Snowy Plover (Charadrius Alexandrinus) Feeding on the Polychaete Ragworm Nereis Diversicolor

Abstract Macronutrient (protein, lipids, and carbohydrates) assimilation efficiencies of wild birds have rarely been studied, but they may be particularly important for estimating the optimal diet. We assessed experimentally the energy and macronutrient assimilation efficiencies of captive Snowy Plover (Charadrius alexandrinus) feeding on the polychaete ragworm Nereis diversicolor, a major natural prey for migratory shorebirds (Charadrii) along intertidal habitats worldwide. Overall, Snowy Plover consumed 8.64 ± 1.54 (SD) g of dry ragworms per day, and the apparent metabolizable energy was 136.59 ± 27.69 kJ·day−1 (3.5–4.0 times the basal metabolic rate). Assimilation efficiencies were 80.04 ± 2.04%, 93.48 ± 1.08%, 84.75 ± 2.01%, and 89.23 ± 2.61% for energy, proteins, lipids, and carbohydrates, respectively. Macronutrient assimilation efficiencies were similar during early winter and premigration and between male and female Snowy Plover. Relatively high protein assimilation by Snowy Plover may be associated with their specialized high-protein diet.

Land use and land cover change analysis in predominantly man-made coastal wetlands: towards a methodological framework

In areas with a long history of human occupation, coastal wetlands have undergone extensive modification to accommodate extractive activities as salt-extraction and aquaculture. These man-made wetlands maintain some of the ecological functions of natural wetlands in spite of their artificial character: their suitability as complimentary waterbird habitat is well documented. In cases of wetlands composed of mixed natural and man-made areas, similarities in substrate-vegetation-water compositions may pose challenges in the applicability of remote sensing and GIS techniques for the study of landscape changes, requiring tailor-made, case-specific methods. We explored this supposition by testing these techniques for the study of the Bahia de Cadiz Nature Park (Spain). Using Landsat imagery spanning the 1985–2011 period, natural and man-made marsh areas were classified separately and results merged to produce land cover classification maps. Different change dynamics were observed for the natural and man-made areas, the latter exhibiting prominent changes, including widespread vegetative succession. Further, through the overlay of ancillary land use data for 2011, an integrated land use and cover map was produced for this year. Different scenarios arising from the abandonment of extractive activity and structural negligence were highlighted. Furthermore, a methodological framework for the classification of predominantly man-made wetlands was designed. The method is cost-effective and open for integration of additional datasets, and is considered a beneficial input to conservation and land use management. Its applicability for monitoring of landscape change not only pertains to the study area, but also extends to other coastal wetland areas of a similar nature.

Individual egg camouflage is influenced by microhabitat selection and use of nest materials in ground-nesting birds

Camouflage is a widespread strategy to avoid predation and is of particular importance for animals with reduced mobility or those in exposed habitats. Camouflage often relies on matching the visual appearance of the background, and selecting fine-scale backgrounds that complement an individual’s appearance is an effective means of optimising camouflage. We investigated whether there was an active selection of microhabitats and nest materials in three ground-nesting birds (pied avocet, Kentish plover, and little tern) to camouflage their eggs using avian visual modelling. Plovers and avocets selected substrates in which their eggs were better camouflaged, and that choice was done at an individual level. Terns have lighter, less spotted eggs, and while they did select lighter background than the other species, their eggs were a poor match to their backgrounds. The worse matching of the tern eggs was likely due to a compromise between thermal protection and camouflage because they breed later, when temperatures are higher. Finally, the addition of nest materials improved egg camouflage in terms of luminance, although the materials reduced pattern matching, which may be associated with the different roles that the nest materials play. Active selection of substrates at an individual level may be crucial to improve nest success in species that nest in exposed sites.Significance statementMany bird species nest on the ground at sites with no vegetation cover where their nests are exposed to visual predators. We studied whether individual females chose nest substrates that improved the camouflage of their eggs. Using images of nests and avian vision modelling, we found that the choice of nesting substrates in two species was done at the individual level, so that egg camouflage was optimised. In addition, such species were even able to improve egg camouflage by the addition of materials into the nests. However, this pattern was not observed in another species with paler and less spotted eggs, which may reflect a trade-off between camouflage and overheating of eggs because it breeds later in the season. Although individuals may try to choose substrates in which their eggs are better camouflaged, other factors that compromise offspring survival may hamper this.