Feeding ecology of wintering terns in Guinea-Bissau

We studied the feeding ecology of Little Terns Sterna albifrons , Sandwich Terns S. sandvicensis and Royal Terns S. maxima in the Archipélago dos Bijagós (11°40'N, 15°45'W) in Guinea-Bissau (West Africa) during the winter of 1992/1993. More than 95% of all prey taken by these terns were roundfish, ranging in weight from 0.3 to 40 g. Birds usually fed alone, but sometimes they were observed feeding in mixed-species flocks consisting of 15–200 individuals. Capture rate ( n fish per hour foraging) in these flocks was higher than that of solitary birds. However, smaller fish were caught by birds foraging in flocks, so food intake rate (g/h) did not differ between solitary and flock-feeding birds. The relationships between foraging behaviour of the three tern species and abiotic factors, such as time, tide and water clarity, have been investigated. Capture rate of Royal Terns increased with water clarity. For Little Terns and Sandwich Terns, food intake rate was lower in the most turbid waters compared to clearer waters. There was very little foraging activity during high tide. For Little Terns and Royal Terns, food intake rate was about twice as high during receding and low tides as during an incoming tide. Food intake rate averaged 8 g/h in Little Terns, 60 g/h in Sandwich Terns and 45 g/h in Royal Terns. With a rough model, we estimate the maximum rate of daily energy expenditure of terns wintering in the tropics at 3 × BMR (defined as energy expenditure of inactive bird at thermoneutrality in a post-absorptive state during the resting phase of the daily cycle). From an energetic viewpoint, wintering Sandwich Terns in Guinea-Bissau seem to have an easy living.     

Unterschiedliche Ern?hrungsstrategien von Flu?- und Küstenseeschwalbe (Sterna hirundo undS. paradisaea) im Wattenmeer

In 1991 and 1992 Common and Arctic Terns differed in chick-feeding and chick-rearing strategies. Arctic Terns favoured smaller prey, primarily crustaceans, for feeding their chicks than did Common Terns. Consequently, Arctic Tern feeding rates were higher. The influence of the tide on food parameters was greater in Arctic Terns. In both years breeding success was better in Common Terns.     

Assessing alternative anthropogenic habitats for conserving waterbirds: salinas as buffer areas against the impact of natural habitat loss for shorebirds

Because many natural waterbird habitats are threatened by human disturbance and sea level rise, it is vitally important to identify alternative wetlands that may supplement declining natural habitats. Coastal salinas are anthropogenic habitats used for obtaining salt by evaporation of sea water. These habitats support important numbers of waterbirds around the world, but their importance as feeding habitats is poorly understood. I evaluated salinas as feeding habitats relative to natural intertidal habitats by comparing time spent foraging, prey-size selection, and net energy intake rate of four overwintering small-sized shorebird species on intertidal mudflats and on adjacent salinas. In winter, Dunlin Calidris alpina, Curlew Sandpiper C. ferruginea and Sanderling C. alba predominantly used the mudflats, whereas Little Stint C. minuta fed mainly on the salina. In the pre-migration fattening period, all species preferred to feed on the salina, significantly increasing the time they spent feeding in the supratidal pans. Net energy intake rates (kJ min^–1) were significantly higher on the salina than on the intertidal mudflats in 60% of all comparisons. On average, salina contributed 25.2 ± 24.2% (range: 4–54%) of the daily consumption in winter and 78.7 ± 16.4% (range: 63–100%) of the daily consumption in the pre-migration period. I recommend that modern active salinas maintain flooding conditions in the evaporation pans throughout winter, thus increasing the available surface for foraging waterbirds. I conclude that the conservation of salinas at coastal wetlands is a viable approach for shorebird conservation.     

Are salinas a suitable alternative breeding habitat for Little Terns Sterna albifrons?

This paper describes the breeding population, breeding habitats and reproductive variables of Little Terns Sterna albifrons in natural (sandy beaches) and alternative (salinas) habitats. Studies of nesting success conducted between 1998 and 2002 in these two types of habitat were combined with a literature review of census work from the past 30 years in order to assess whether salinas are suitable alternative breeding habitat for Little Terns. Most of the Portuguese Little Tern population now breeds in salinas. Census data from the last 30 years show that this is a recent breeding behaviour, because until the 1990s most colonies were located on sandy beaches. Destruction and disturbance of the natural habitat has caused this habitat shift. Despite this shift, the Portuguese Little Tern breeding population did not decline during this period and no significant differences were found in nesting success between natural and alternative habitats. This might indicate that salinas are a suitable alternative breeding habitat for Little Terns, but differences in laying period, clutch size and egg size were recorded between birds nesting on sandy beaches and in salinas in the same area. Birds nested earlier on sandy beaches and laid larger clutches and eggs than in salinas. These data suggest that, when both habitats are available, older and/or higher quality birds prefer sandy beaches for breeding, presumably trying to re-nest in salinas when first breeding attempts failed. We discuss conservation priorities and management actions for both habitats.     

Specialized foraging habits of adult female California sea lions Zalophus californianus

Individual foraging specialization occurs when organisms use a small subset of the resources available to a population. This plays an important role in population dynamics since individuals may have different ecological functions within an ecosystem related to habitat use and prey preferences. The foraging habitat fidelity and degree of specialization of California sea lions (Zalophus californianus) were evaluated by analyzing the stable isotopes values of carbon and nitrogen in vibrissae collected from 16 adult females from the reproductive colony on Santa Margarita Island, Magdalena Bay, Mexico, in 2012 and 2013. Based on the degree of individual specialization in δ¹⁵N, 62.5% of the females assessed can be considered specialist consumers focusing on the same prey or different prey from the same trophic level. The degree of individual specialization in δ¹³C indicated that 100% of the individuals showed fidelity to their foraging habitat as some fed in the lagoon, others foraged along the coast, and a third group preferred prey from the pelagic environment during both the breeding and nonbreeding seasons, suggesting diversification of foraging areas. Foraging area fidelity persisted despite the 2°C increase in the sea surface temperature over the course of the study period.     

Luxury in freshwater and stress at sea? The foraging of the Common Tern Sterna hirundo

We compared the foraging strategies of Common Terns Sterna hirundo in freshwater (Lake Jeziorsko, Brzeg, Poland) and marine environments (Minsener Oldeoog, German Wadden Sea). Body mass changes, nest relief and duration and number of feeding trips per day were studied by automatically weighing the adults, using electronic balances under the nests. At the freshwater site, adults were lighter both before and after feeding and gained less mass during a trip. in the Wadden Sea, single feeding trips lasted longer than at the freshwater site and the terns made fewer trips per day. To achieve the same mass gain per day as in birds in freshwater, trips at sea had to be longer and food intake per trip was higher. The daily duration of absence for feeding and the daily mass gain were about the same in both areas. The limnetic feeders finished foraging earlier in the evening than the terns foraging at sea. These differences are consistent with the hypothesis that limnetic prey availability was consistent, whereas the tides limited the availability of marine prey. In consequence, foraging over freshwater presents several advantages, such as higher colony attendance, better mate coordination and better parental care.     

Relation between climatic factors,diet and reproductive parameters of Little Terns over a decade

We used 10 years of data on clutch size, egg size and diet, and 8 years of data on timing of laying on Little Terns (Sternula albifrons) breeding in Ria Formosa lagoon system, Algarve, Portugal to assess whether diet acts as an important intermediary between climatic conditions and breeding parameters. We used Generalized Linear Models to relate (1) the relative occurrence and size of the main prey species, sand smelts (Atherina spp.), with environmental variables, a large-scale climate variable, the North Atlantic Oscillation (NAO) index, and a local scale variable, the sea-surface temperature (SST), and (2) the respective effects of sand smelts relative occurrence, NAO index and SST on Little Tern breeding parameters. The diet of Little Terns was dominated by sand smelts, with a frequency occurrence of over 60% in all years. The winter SST (February) was negatively associated with the relative occurrence of sand smelts in the diet of Little Terns during the breeding season which, in turn, was positively associated with Little Tern clutch size. Our results suggest that negative NAO conditions in the Atlantic Ocean, often associated with rougher sea conditions (greater vertical mixing, stronger winds and lower SST) were related with earlier breeding, and lower SST in the surroundings of the colony during winter–spring favour the abundance of prey fish for Little Terns as well as their reproductive parameters. Climate patterns at both large and local scales are likely to change in the future, which may have important implications for estuarine seabirds in Southern Europe.     

Foraging by little auks in the distant marginal sea ice zone during the chick-rearing period

During the chick-rearing period, little auks Alle alle adopt a bimodal foraging strategy, alternating long trips with several short ones. It has been postulated that they reach more remote areas during long feeding trips than during short ones. However, the range of their foraging flights has never actually been measured. The aims of this study were to find the exact location of the little auk feeding grounds and to investigate whether they reach remote areas during long foraging trips using miniature GPS and temperature loggers. The study was conducted in 2009 in Magdalenefjorden (79°34′N, 11°04′E), one of the main breeding grounds of little auks on Spitsbergen. The temperature logger records indicated that during short trips, little auks visit warmer waters (situated close to the colony) than during long ones. The tracks of two GPS-equipped birds indicated that during long trips little auks foraged in the distant, food-abundant marginal sea ice zone, at least 100 km away from the colony. During long trips, birds make several stops at sea, perhaps sampling the foraging area with respect to prey distribution. Since food conditions near the studied colony are usually suboptimal, little auks may be exploiting distant feeding areas to compensate for the poorer-quality food available at nearby foraging grounds. The extended duration of long foraging trips may enable birds to collect food for chicks on food-abundant, remote foraging grounds as well as acquire, process and excrete food needed for self-maintenance, reducing the costs of flight to the colony.     

Seabird and fur seal responses to vertically migrating winter krill swarms in Antarctica

Summary The foraging behaviour of fur seals and two species of surface feeding seabirds was observed over swarms of vertically migrating krill along the Antarctic Peninsula in July 1987. Fur Seal haul out patterns were correlated with krill in the upper 30 m of the water column. Krill moved to the surface at night; seals subsequently foraged from 1400-0700 hours before returning to floes. Foraging was continuous through the night. Dive duration decreased as krill moved up to the surface; shorter dives may have been more successful than longer ones. It is possible that very deep dives, which occur early in a foraging bout, represent more of an attempt to assess krill depth and distribution rather than being a genuine foraging effort. Seabirds responded to the presence of a surface krill swarm by circling over it and foraging; krill at depths greater than 30 m elicited directional flight and low frequencies of prey capture attempts. Both Snow Petrels and Antarctic Terns preyed on krill, but each species approached the swarms from different habitats. Snow Petrels primarily overflew areas covered by ice; terns preferred open water. This suggested that prey encounters are essentially opportunistic, although the search for prey is limited to rather specific marine habitats. This feature may be important to our understanding of the factors that determine the pelagic distribution of seabirds.     

Niche segregation of Himalayan river birds

ABSTRACT.   The Himalayan Mountains have more specialist river birds than anywhere on earth, but factors permitting their coexistence have not been evaluated. We examined potential ecological segregation of five sympatric insectivores based on diet, morphology, and foraging behavior during the breeding and nonbreeding seasons in central Nepal. Little Forktails ( Enicurus scouleri ) and Brown Dippers ( Cinclus pallasii ) were highly aquatic in habitat use and diet (89–96% of prey of aquatic origin), but took prey that differed in size and composition. Spotted Forktails ( E. maculatus ) foraged primarily along channel margins, and took more aquatic than terrestrial prey (59% vs. 40%). White-capped ( Chaimorrornis leucocephalus ) and Plumbeous ( Rhyacornis fuliginosus ) water redstarts took primarily terrestrial or aerial prey by gleaning marginal surfaces and flycatching over channels. Two species were similar morphologically (Little Forktail and Plumbeous Water Redstart), but had different diets. Our results illustrate clear ecological separation among this guild of co-occurring river birds on one or more niche dimensions that we suggest is mediated by the physical complexity of mountain, riverine habitats.     

The foraging ecology of two sympatric gobiid fishes: importance of behavior in prey type selection

Synopsis The foraging ecology of two temperate marine gobies (Pisces: Gobiidae) was studied in rocky subtidal habitats off Santa Catalina Island, California. The bluebanded goby, Lythrypnus dalli, foraged from exposed ledges and fed on planktonic and benthic prey, although planktonic prey were more important in diets by number and weight. The more cryptic zebra goby, Lythrypnus zebra, remained hidden under rocks and in crevices feeding on benthic prey almost exclusively. The active selection of particular prey taxa from the two prey sources (water column and substratum), mediated by species-specific differences in foraging behavior, resulted in interspecific differences in type, number, size and weight of prey consumed. Interspecific differences in foraging ecology reflect the selection of prey most readily available to these fishes that occupy specific and fixed microhabitats within rocky reefs.     

Inter-colony and sex differences in the effects of parental body condition and foraging effort on the brood growth of Adélie penguins

Among colonies with different foraging distances, central-place-foraging seabirds may change their foraging and reproductive efforts. We compared the body condition, meal frequency, and diving behavior of male and female Adélie penguins at two locations: Dumont d'Urville, where there was little sea ice and they foraged in open waters far from the colony; and Syowa, where there was heavy, fast sea ice and they foraged in ice cracks close to the colony. The parental mass decrease rate during the chick-rearing period was similar between colonies and between sexes. A large individual variation in meal frequency positively affected the brood growth rate, but daily underwater time did not. A weak but significant positive effect of body condition on brood growth rate was found only in males at Syowa. It was suggested that males work with better body condition than females. We propose the hypothesis that the regional difference in the distance to the feeding sites and the sex difference in body energy reserve might constrain the capacity to regulate reproductive effort.     

Foraging behaviour of non-breeding Western Sandpipers Calidris mauri as a function of sex, habitat and flocking

Individuals within a population may vary considerably in the way they exploit available food resources. If the sexes differ in the size of their feeding apparatus, there can be differences in foraging behaviour and habitat use, hence one sex may be more susceptible to competition. We examined relationships between sexual dimorphism in bill size and foraging behaviour, and habitat and microhabitat use of non-breeding Western Sandpipers Calidris mauri at Bahía Santa María, northwestern Mexico. Western Sandpipers are sexually dimorphic, with females about 15% longer-billed than males. Males used a higher proportion of pecks, had a lower probing–pecking rate, walked at higher rates, foraged at sites with lower water content, and had greater variation in foraging technique than females. Moreover, males decreased their proportion of pecks and foraged at a higher rate than females when they changed from feeding alone to feeding in flocks, suggesting a greater safety advantage or susceptibility to conspecific interference when flock feeding. We compared behaviour and microhabitat usage in three habitats: brackish flats, mangroves, and cattail marshes. Sex-related differences in foraging behaviour and microhabitat use were consistent among habitats. Birds in brackish flats and mangroves used a higher proportion of pecks, foraged at lower rates and walked at higher rates, and foraged at deeper sites, with a lower proportion of water cover, than those in cattail marshes. Sex-related differences in foraging behaviour and microhabitat should reduce the level of competition between sexes, and may account for sex differences in Western Sandpiper distribution observed between habitats in Bahía Santa María.     

The choice of foraging methods of the Brown Dipper,Cinclus pallasii (Aves: Cinclidae)

The Brown Dipper,Cinclus pallasii, forages for large prey by diving deeply among submerged rocks, and for small prey in shallow water by wading and pecking on the bottom. Foraging by diving entailed a higher energy cost but resulted in taking larger prey than foraging by wading-and-pecking. Foraging by diving was seldom observed from May to October, but increased from December to April, when the dippers came into breeding condition and the proportion of large prey in the benthic fauna increased. During the breeding season, adult dippers fed themselves mainly by foraging by wading-and-pecking, whereas those feeding nestlings and fledglings foraged by diving. The foraging methods employed was determined by the individuals being fed and the abundance of the large prey. Foraging by diving may be advantageous for individuals provisioning their young because of the high gross intake rate, but may be less advantageous for the foragers themselves because of the high energy cost.     

Agroecosystems and conservation of migratory waterbirds: importance of coastal pastures and factors influencing their use by wintering shorebirds

Coastal pastures are common agroecosystems adjacent to estuarine areas that can provide valuable habitat for wildlife, particularly for migratory shorebirds. Disentangling the factors that influence coastal pasture use by wintering shorebirds will provide new insights into its role for buffering human disturbances and habitat loss in intertidal areas. We examined whether numbers of two shorebirds (Eurasian curlew and Black-tailed godwit) foraging actively on coastal pastures was affected by weather conditions, tidal stage (low/high tide) and number of harvesters at intertidal areas throughout winter. Both species frequently used coastal pastures and most individuals foraged actively there. The average percentage of the total wintering population of curlews and godwits foraging on coastal pastures was 27.4 and 7.8 %, respectively, and was significantly higher during high tide compared to low tide. The number of harvesters on mudflats also had a positive significant effect in explaining the presence of curlews, and to a lesser extent for godwits, on coastal pastures, and accumulated rainfall had a positive effect for both species too. These supratidal areas were consistently used as alternative foraging grounds during low tide by curlews, as well as supplementary foraging areas during high tide by wintering populations of both large shorebirds. By supplementary foraging, wintering curlews, and probably godwits, seemed to compensate for a negative effect of the presence of harvesters on their foraging activity. We recommend managing of those coastal agricultural fields adjacent to intertidal foraging grounds in order to increase the availability of supratidal foraging habitats for declining shorebird populations. These habitats may thus have a beneficial role in sustaining populations of wintering shorebirds, but further studies are needed to estimate if birds can compensate for any shortfall in daily energy budget by supplementary foraging on coastal pastures, thus providing insights into whether they are involved in large-scale population regulation of migratory birds.     

Use of dredge islands by a declining European shorebird, the Kentish Plover Charadrius alexandrinus

Colonization of newly created habitats is a challenge for waterbird populations in a changing world. Knowing which habitat characteristics are required by waterbird populations is a research challenge for rational management of the new ecosystems and their aquatic bird populations. Since 1989 intertidal dredge islands have been built in the lagoon of Venice using sediments coming from regular dredging of lagoon channels and inlets. Kentish Plover, a species declining in Europe, readily uses these new sites as soon as they become available. Between 2005 and 2007, 75 dredge islands were surveyed each year and the number of breeding pairs of Kentish Plover estimated. Each year about one-third of available dredge islands was used by Kentish Plover. Between 34 (in 2005) and 131 (in 2007) breeding pairs were found, and possible differences in vegetation and morphological characteristics between occupied and unoccupied sites were investigated. Only age, mean elevation above sea level and extension of bare ground were statistically different; Kentish Plover preferred younger sites, with higher elevation and with larger areas of bare ground. The largest groups of breeding pairs, up to thirty pairs, were found on islands which also supported colonies of Little Terns. In the study period dredged islands supported about 60 % of the total breeding population of the lagoon of Venice and 4–6 % of the estimated Italian population. Along coastal sites where human pressure on beaches is particularly heavy, man made habitats such as dredge islands may become a valuable alternative breeding site for this and other species of conservation concern. Management works aimed at promoting the occurrence of this species at selected dredge islands have been made in the lagoon of Venice.     

Visual tracking from a rigid‐hulled inflatable boat to determine foraging movements of breeding terns

ABSTRACT Defining the at‐sea foraging movements of seabirds is fundamental to understanding their ecology and can also be important in assessing the potential impact of marine developments such as offshore wind farms (OWFs). Surveys of predefined areas using aerial or boat‐based transect surveys may not allow adequate assessment of the relative importance of different areas to birds. Individual‐based satellite or radio‐telemetry can be effective in identifying foraging ranges and preferred areas, but may not be suitable for some species. We developed a method to determine the foraging movements of breeding terns (Sterna spp.) by visually tracking individuals using a rigid‐hulled inflatable boat (RHIB). Sandwich Terns (S. sandvicensis), Common Terns (S. hirundo), and Arctic Terns (S. paradisaea) were tracked from colonies in Norfolk and Anglesey, United Kingdom, from 2006 to 2008. The proportion of complete (from and to colony) trips varied from 29–60% among species, years, and colonies. Individual Sandwich Terns were tracked for periods up to 126 min over distances up to 72 km and as far as 54 km from the breeding colony, further than Arctic (up to 57 km and 29 km from the colony) and Common (to 29 km and <9 km from the colony) terns. Mean values were much smaller. Multivariate modeling of Sandwich Tern foraging trips indicated that flight speeds >50 km/hr coupled with greater distances from shore (>25 km) significantly reduced the likelihood of tracking a bird for an entire foraging trip. Use of different boats that differ in speed and performance may alleviate such issues. Visual tracking allowed us to collect data on foraging behavior, flight height, and prey capture rates, and also permitted comparisons between species. Our results indicate that visual tracking may be an effective means of determining the foraging movements and at‐sea behavior of a variety of short‐ranging, day‐active seabirds.     

Intraspecific differences in benefits from feeding in mixed-species flocks

The Madagascar Paradise Flycatcher Terpsiphone mutata and Common Newtonia Newtonia brunneicauda frequently form two-species flocks in the deciduous dry forest of western Madagascar. In T. mutata , some males have long tails, while other males and females have short tails. When foraging in mixed flocks, each type of bird captured prey more rapidly than otherwise, but the degree of increase in feeding rate was smaller in long-tailed males. When in mixed flocks, all T. mutata caught prey on leaves in the canopy where N. brunneicauda foraged. Long-tailed males changed feeding habits from sallying when not in mixed flocks, whereas short-tailed birds showed no change of feeding habit. The elongated tails of long-tailed males may have made their foraging less efficient owing to decreased agility in the canopy. N. brunneicauda is monomorphic and often formed groups of three to five individuals. In monospecific flocks, subordinates fed at low rates on branches owing to frequent hostile encounters. When foraging in mixed flocks, however, subordinates foraged among leaves, and their feeding rates increased because the frequency of intraspecific interference decreased greatly. Dominants did not show any difference in feeding pattern with social situation. Thus, heterospecific flocking was more advantageous for subordinates.     

Temporal variations in the feeding habits of the Purple Heron Ardea purpurea during the breeding season

The foraging strategies of adult Purple Herons Ardea purpurea from an increasing colony in northern Spain were studied in the feeding areas (nonagricultural habitats) during the breeding season (May-July). In this area, herons were mainly piscivorous, and there was no seasonal variation in prey type. Small fish (<12.5 cm) were more abundant in the diet in June and medium-sized fish (12.5–25 cm) in May and July. Foraging strategies varied seasonally: (1) More time was spent by the herons in the feeding areas in June than in May or July, whereas foraging time was only significantly greater in June than in July. (2) The biomass captured by the adults in each foraging session was similar in May and June but greater in July. (3) Feeding attempts were more numerous (but not more successful) in shallow water in June and July. (4) Aggression rates between conspecifics were higher in June than in both May and July. Probably the hatching of chicks (June) was the cause of these changes in diet and foraging strategies as well as increased intraspecies competition for foraging habitat.     

Foraging movements of emperor penguins at Pointe Géologie, Antarctica

The foraging distributions of 20 breeding emperor penguins were investigated at Pointe Géologie, Terre Adélie, Antarctica by using satellite telemetry in 2005 and 2006 during early and late winter, as well as during late spring and summer, corresponding to incubation, early chick-brooding, late chick-rearing and the adult pre-moult period, respectively. Dive depth records of three post-egg-laying females, two post-incubating males and four late chick-rearing adults were examined, as well as the horizontal space use by these birds. Foraging ranges of chick-provisioning penguins extended over the Antarctic shelf and were constricted by winter pack-ice. During spring ice break-up, the foraging ranges rarely exceeded the shelf slope, although seawater access was apparently almost unlimited. Winter females appeared constrained in their access to open water but used fissures in the sea ice and expanded their prey search effort by expanding the horizontal search component underwater. Birds in spring however, showed higher area-restricted-search than did birds in winter. Despite different seasonal foraging strategies, chick-rearing penguins exploited similar areas as indicated by both a high ‘Area-Restricted-Search Index’ and high ‘Catch Per Unit Effort’. During pre-moult trips, emperor penguins ranged much farther offshore than breeding birds, which argues for particularly profitable oceanic feeding areas which can be exploited when the time constraints imposed by having to return to a central place to provision the chick no longer apply.