Impact of wind turbines on birds in Zeebrugge (Belgium)

We studied the impact of a wind farm (line of 25 small to medium sized turbines) on birds at the eastern port breakwater in Zeebrugge, Belgium, with special attention to the nearby breeding colony of Common Tern Sterna hirundo, Sandwich Tern Sterna sandvicensis and Little Tern Sterna albifrons. With the data of found collision fatalities under the wind turbines, and the correction factors for available search area, search efficiency and scavenging, we calculated that during the breeding seasons in 2004 and 2005, about 168 resp. 161 terns collided with the wind turbines located on the eastern port breakwater close to the breeding colony, mainly Common Terns and Sandwich Terns. The mean number of terns killed in 2004 and 2005 was 6.7 per turbine per year for the whole wind farm, and 11.2 resp. 10.8 per turbine per year for the line of 14 turbines on the sea-directed breakwater close to the breeding colony. The mean number of collision fatalities when including other species (mainly gulls) in 2004 and 2005 was 20.9 resp. 19.1 per turbine per year for the whole wind farm and 34.3 resp. 27.6 per turbine per year for 14 turbines on the sea-directed breakwater. The collision probability for Common Terns crossing the line of wind turbines amounted 0.110–0.118% for flights at rotor height and 0.007–0.030% for all flights. For Sandwich Tern this probability was 0.046–0.088% for flights at rotor height and 0.005–0.006% for all flights. The breeding terns were almost not disturbed by the wind turbines, but the relative large number of tern fatalities was determined as a significant negative impact on the breeding colony at the eastern port breakwater (additional mortality of 3.0–4.4% for Common Tern, 1.8–6.7% for Little Tern and 0.6–0.7% for Sandwich Tern). We recommend that there should be precautionary avoidance of constructing wind turbines close to any important breeding colony of terns or gulls, nor should artificial breeding sites be constructed near wind turbines, especially not within the frequent foraging flight paths.     

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.     

Does interference competition explain why White Terns of Aride Island,Seychelles, breed predominantly when marine productivity is lower?

White Terns Gygis alba breed throughout the year on Aride Island but show a marked preference to nest during the northwest monsoon (November–March), when interspecific competition and also marine productivity are considered to be lower. In this study we investigated and compared breeding and foraging parameters of White Terns between the northwest and southeast (May–September) monsoons to assess whether interspecific competition affected foraging activity and explained the timing of breeding of White Terns. Goatfish (Mullidae) dominated the diet of White Terns and no significant differences were found in diet composition between the northwest and southeast monsoons. Similar diets and patterns of food provisioning of White Terns between the two monsoons do not support the hypothesis of strong exploitation competition during the southeast monsoon. On the other hand, foraging behaviour and frequency of capture attempts of White Terns differed between the two monsoons. The percentages of multi‐species flocks with White Terns and of large flocks with more than 25 noddies Anous and terns Onychoprion were significantly higher during the southeast monsoon. Moreover, capture attempt frequencies of White Terns foraging in multi‐species flocks were lower than those of solitary individuals, and both solitary and flocking White Terns had significantly lower capture attempt frequencies during the southeast monsoon in comparison with the northwest monsoon. These results suggest that interference competition depresses foraging activity of flock‐foraging White Terns, particularly during the southeast monsoon, which could potentially explain why White Terns breed preferentially during the northwest monsoon. However, and despite avoiding interference competition, White Terns nesting during the northwest monsoon did not show higher breeding success than those nesting during the southeast monsoon. Other hypotheses that might explain the preference of White Terns for breeding during the northwest monsoon, such as avoidance of agonistic interactions with Lesser Noddies Anous tenuirostris at nest sites, should also be investigated.     

Prey capture success of Sandwich Terns Sterna sandvicensis varies non‐linearly with water transparency

Human impacts on water transparency may affect plunge‐diving seabirds. We studied prey capture success of Sandwich Terns Sterna sandvicensis as a function of six environmental variables during the breeding season. We observed diving terns in the south eastern North Sea and found a non‐linear optimum curve for the relationship between Secchi transparency and prey capture probability. High capture probability was found at 1.5–2.0 m, with an optimum of 63% at 1.74 m. At a minimum transparency of 0.4 m and at a maximum transparency of 3.2 m, capture probabilities were about halved. Conversion of transparency to total suspended matter (TSM) concentration showed that the optimum concentration for foraging terns would be 5–10 mg/L under mean summer conditions for chlorophyll‐a. However, the summer‐averaged TSM concentrations in the nearshore Dutch coastal waters range between 10 and 30 mg/L, which implies that foraging Terns in the breeding season do not encounter optimal foraging conditions. The Full Plunge Dive, with which the largest diving depth can be achieved, was applied dominantly in clear water, while the Partial Plunge Dive and Contact Dip were applied more frequently in turbid water, thus showing that Sandwich Terns adjust their diving technique in response to water transparency.     

Foraging habitat selection by Little Terns Sternula albifrons in an estuarine lagoon system of southern Portugal

We assessed the effects of environmental variables on the distribution and feeding behaviour of adult Little Terns Sternula albifrons in Ria Formosa Natural Park, Algarve, southern Portugal, in different foraging habitats (main lagoon, salinas and sea) during the breeding seasons, April–July, of 2003–05. Foraging density was higher in the lagoon than in the sea, and at low tide. The number of foraging individuals at sea was independent of tide. Individual Little Terns foraged further from the nearest breeding colony in April and May (courtship feeding and incubation) than in June and July (chick-rearing). During intermediate tidal phases, individuals foraged further from the nearest colony, and followed main lagoon channels, perhaps because stronger currents increased prey availability. Diving activity and foraging success were higher in 2003 than 2004 or 2005, perhaps because of greater availability of marine prey in 2003. Diving rate was higher in July (when independent juveniles began learning how to forage) but diving success was higher in June (chick-rearing) than in other months. The variables selected by the final logistic models reflected four basic needs for the selection of feeding habitats by Little Terns: (1) association between foraging individuals, (2) areas with abundant feeding resources, (3) entrance channels and main lagoon channels with strong currents, and (4) the proximity to areas with alternative feeding resources, the salinas. Areas subjected to strong human pressure were avoided by foraging Little Terns.     

The role of the Ghana coast in the annual cycle of migratory terns

Ntiamoa-Baidu, Y. & Nuoh, A. A. 2000. The role of the Ghana coast in the annual cycle of migratory terns. Ostrich 71 (1 & 2): 183.

Data are presented on counts of migratory terns on the Ghana coast over a ten-year period, 1986–1996. Fourteen species of terns were recorded during the period. Influx of terns begins in late August, peaking in September/October, with the highest likelihood of tern occurrence on the Ghana coast recorded in August-November and April-May. Terns fed mostly offshore and used the coastal lagoons as roosting sites, although a few species e.g. Black Tern Chlidonias niger and Little Tern Sterna albifrons, foraged also on the lagoons. Two sites, Songor Lagoon and Densu Delta, regularly held over 50 % of the total count recorded for all sites. A comparison of the day-time counts with dusk roost counts indicate that the day time counts under-represent the total numbers of terns using the Ghana coast. Trends in the populations of the four most abundant tern species: Black Tern, Common Tern Sterna hirundo, Sandwich Tern S. sandvicensis and Royal Tern S. maxima, are analysed; and the implications for conservation of the species are discussed.     

SEA-BIRDS AT LAGOS AND IN THE GULF OF GUINEA

Regular observations of sea-birds were made in the vicinity of Lagos, southwest Nigeria, between October 1967 and June 1971. Records in the Gulf of Guinea were obtained in April 1969 and, together with others from Ghana and Sierra Leone, were used to complement this study. While few large populations of sea-birds are supported, the diversity of species occurring at Lagos and over offshore deep water was great. The occurrence of nine species of sea-bird new to Nigeria was established or confirmed; most of these were also new to West Africa southeast of Gambia. Details are given of fluctuations in numbers these and a further 16 species.
All coastal habitats at Lagos supported a large population of immature Black Terns, and groups of immatures made up a high proportion of observations of other Palaearctic terns and of Royal Terns, which breed only in northwest Africa. In contrast, the Lesser Black-backed Gulls and Damara Terns reaching Lagos (from breeding grounds almost two continents apart) were mainly adults; both these species are amongst those showing a current increase in numbers. A pronounced spring passage was observed among species including Royal and Sandwich Terns, but adult movements in autumn were not elucidated. The occurrence of the European race of the Little Tern in southwest Nigeria was confirmed, it is at least a regular spring migrant. The frequency of vagrants in January and February was linked to local climate as much to general distribution. Brief comment is also given on immature plumages, and the growing exploitation and pollution of coastal waters.     

Survival estimates for Royal Terns in southern California

ABSTRACT.   Once the most abundant large tern in California, populations of Royal Terns ( Thalasseus maximus ) are now greatly reduced statewide. However, these terns are still common in southern California where the population appears to be increasing. We estimated survival of this species, for which no robust estimate of survival is available, using mark-resighting data. Apparent survival of birds >3 yr of age was estimated to be ∼0.95; first-year survival was lower at 0.86. The lower survival estimate for first-year birds probably reflects increased mortality, but could also be a function of permanent emigration by individuals in this age class. The adult survival probability is the highest reported among terns and suggests that survival is playing an important role in the recovery of this tern population and that the current population growth is not solely due to immigration. Improvements in resighting protocols, marking technology, and mark-recapture methods are recommended to better use live resighting, dead recovery, and multistate data.     

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.     

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.     

Choosing the best foraging microhabitats: individual skills constrain the choices of dunlins Calidris alpina

Estuarine mudflats, among the most important foraging grounds for waders during the non-breeding season, consist of complex mosaics of shallow pools and dry areas during low tide. In this study, we carried out close-range focal observations to determine foraging parameters of dunlins Calidris alpina, foraging in the mudflat microhabitats of the Tagus estuary, Portugal. Birds foraging in wet patches mostly targeted the siphons of the bivalve Scrobicularia plana , while in dry patches they mostly fed on mudsnails Hydrobia ulvae . Surface visibility of prey, rather than their abundance in the sediment, explained the microhabitat-related differences in prey selection. Birds using dry patches obtained 40% less energy intake than those using wet patches, still many extensively used this poor microhabitat. Because siphons retract quickly when the sediment is disturbed, birds often failed to catch them. We found that birds that were less efficient in capturing siphons in wet patches tended to spend more time foraging on mudsnails in dry patches. This suggests that lack of skills in siphon cropping represents a major foraging constraint for dunlins wintering in the Tagus estuary. It may even cause them to forage during high tide in order to achieve their daily energetic requirements.     

Trends in the use of a small coastal lagoon by waterbirds: Muni Lagoon (Ghana)

This paper presents data from a twelve-year (1986–98) monitoring of waterbird populations at Muni Lagoon, and discusses the ornithological importance of the lagoon and trends in the use of the site by waterbirds. The small brackish lagoon forms part of the Muni-Pomadze Ramsar site situated on the Ghana coast. A total of 48 species of waterbirds were recorded at the site comprising of 29 species of waders, eight species of terns, two species of gulls, seven species of herons and egrets, one species each of duck and cormorant. The most abundant waterbirds species recorded on the site were the Curlew Sandpiper, Ringed Plover, Greenshank, Black-winged Stilt, Common Tern, Black Tern, Royal Tern and the Sandwich Tern. Three species of waders, Black-winged Stilt, Pratincole and Little Tern were observed to breed at the site. The highest numbers of waterbirds occurred on the site in the period September to March. The site remains an internationally important wetland site by virtue of supporting internationally important numbers of four species of terns: Royal Tern, Sandwich Tern, Common Tern and Black Tern. The value of Muni lies more in its use as a feeding/staging site for migrating waders that spend the non-breeding season in Ghana or pass through to winter further south, and as a roosting site for terns which feed mainly at sea. The use of the site by waterbirds has increased by some 400% between 1986 and 1998. This is attributed to improved prey availability resulting from the opening of the lagoon into the sea during the 1994 floods, and reduced disturbance of feeding and roosting birds. This would imply that the value of the site as waterbird habitat could improve considerably with appropriate habitat management interventions and continued protection.     

EFFECT OF AGE ON THE FISHING ABILITY OF SANDWICH TERNS STERNA SANDVICENSIS

The fishing ability of first-winter and older Sandwich Terns was compared at foraging sites used by both age-groups on the coast of Sierra Leone between January and March 1970 when first-winter birds were about 7–9 months old. The methods used for measuring various parameters of fishing ability are described. It was found that a significantly larger proportion of plunge dives by the older age-group yielded prey although the difference between the two age-groups was only slight.
There were no major differences between the age-groups in diving rates or in size of fish captured. Older birds tended to dive from higher than first-winter birds.
By driving shoals of small fish to the surface, predatory fish enhanced the fishing success of both age-groups on certain days. Where man's beach seining operations yielded a superabundance of small fish near the tideline, these were captured equally well by both age-groups.
First-winter birds were not seen being fed by older birds, and although food-begging behaviour was observed once in late January it is clear that the first-winter birds were self-sufficient for food by the time this study was made.
Some implications of the observed differences in fishing success are discussed. It is suggested that, after fledging, Sandwich Terns gradually gain the judgment and skill necessary to dive profitably from increasing heights and so learn to exploit prey in a greater range of water depth as they get older.     

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.     

Composition of mixed‐species flocks and shifts in foraging location of flocking species on Hainan Island,China

A total of 134 bird species were recorded at Jianfengling, Hainan Island, in China from May 2000 to September 2004, of which 44 participated in one or more of 134 mixed‐species flocks. These flocks averaged 3.8 ± 0.2 species and 20.3 ± 1.2 individuals. Flocking propensity in a given species ranged from 1.5 to 100%. For flocking species, frequency of flocking and number of individuals in flocks was positively correlated with frequency and number in point counts. Among all species pairs with flocking frequency above 5%, cluster and correlation analysis indicated there were two principal groups of flocking birds – canopy species and understorey species: associations were positive within a group, but negative between groups. Canopy birds had a higher flocking propensity than understorey birds. They also made significantly less use of inner branches and trunks and greater use of middle branches, and foraged at a significantly greater height when in mixed‐species flocks than when solitary. For understorey bird species, there were no significant differences in foraging locations between solitary and mixed‐species flocks. Higher flocking frequency occurred in the wet season for canopy birds, but in the dry season for understorey birds. Overall patterns were consistent with the explanation that flocking enables an expansion of foraging niche by reducing the risk of predation.     

Genetic relatedness in wintering groups of house sparrows (Passer domesticus)

Social behaviour of group-living animals is often influenced by the relatedness of individuals, thus understanding the genetic structure of groups is important for the interpretation of costs and benefits of social interactions. In this study, we investigated genetic relatedness in feeding aggregations of free-living house sparrows ( Passer domesticus ) during the nonbreeding season. This species is a frequent model system for studies of social behaviour (e.g. aggression, social foraging), but we lack adequate information on the kin structure of sparrow flocks. During two winters, we ringed and observed sparrows at feeding stations, and used resightings to identify stable flock-members and to calculate association indices between birds. We genotyped the birds using seven highly polymorphic microsatellite loci, and estimated pairwise relatedness coefficients and relatedness categories (close kin vs. unrelated) by maximum likelihood method. We found that most birds were unrelated to each other in the flocks (mean ± SE relatedness coefficient: 0.06 ± 0.002), although most individuals had at least a few close relatives in their home flock (14.3 ± 0.6% of flock-mates). Pairwise association between individuals was not significantly related to their genetic relatedness. Furthermore, there was no difference between within-flock vs. between-flock relatedness, and birds had similar proportions of close kin within and outside their home flock. Finally, relatedness among members of different flocks was unrelated to the distance between their flocks. Thus, sparrow flocks were not characterized by association of relatives, nevertheless the presence of some close kin may provide opportunity for kin-biased behaviours to evolve.     

Long-term changes in breeding phenology at two seabird colonies in the western North Sea

There is compelling evidence that the breeding phenology of many species has changed substantially in recent decades. However, taxonomic and spatial variation in the direction and rate of change is still not well understood. We explored these issues by analysing a dataset containing information on first egg dates of 10 species of seabird at two major breeding colonies (86 km apart) in the western North Sea over a period of 35 years. Within a species, timing of breeding was positively correlated between the two colonies, suggesting that factors affecting the phenology of these species operated at a regional rather than a colony scale. Comparison of time trends among the species revealed contrasting patterns, with some showing no systematic change, others becoming earlier and others later. The clearest species groupings appeared to be among the terns with arrival and/or first egg dates becoming earlier in Arctic Terns Sterna paradisaea , Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis , and among the auks (Common Guillemot Uria aalge , Razorbill Alca torda and Atlantic Puffin Fratercula arctica ) and Black-legged Kittiwake Rissa tridactyla where the trend was in the opposite direction towards later breeding. This general trend towards later breeding in the latter group of species contrasts with correlational evidence from many other organisms indicating that breeding phenology is advancing in response to climate change.     

Composition and foraging behaviour of mixed-species flocks led by the Grey-cheeked Fulvetta in Fushan Experimental Forest, Taiwan

Thirty-two species were recorded in mixed-species bird flocks led by the Grey-cheeked Fulvetta Alcippe morrisonia in Fushan Experimental Forest, Taiwan. Flocks averaged (± se) 5.8 ± 0.2 species and 51.4 ± 2.7 birds. Most participants were resident species (86.3%), some were elevational migrants (12.6%) and a few were latitudinal migrants (1.1%). Flock size was determined primarily by the abundance of Grey-cheeked Fulvettas, the most abundant species (68.1%). Flocks moved at an average rate of 10.8 ± 0.7 m/min, with larger flocks moving faster than smaller flocks. In moving flocks, canopy species were usually near the front, while understorey species usually followed. Fulvettas gave higher-intensity alarm calls and dived down more frequently in response to avian threats, especially raptors, than to non-avian threats. The overall foraging niche-breadth of the fulvetta was greater than that of any attendant species. Each species in a flock had a unique foraging niche. Most attendant species exhibited low foraging niche-overlap with the Grey-cheeked Fulvetta. Both the predator avoidance and the foraging efficiency hypotheses for mixed-species flocking were supported. The Grey-cheeked Fulvetta plays a critical role in the function of mixed-species flocks. A large flock formed around the Grey-cheeked Fulvetta provides attendant species with numerous opportunities for obtaining food and protection from predators.     

Foraging behavior of redheads (Aythya americana) wintering in Texas and Louisiana

Redheads, Aythya americana, concentrate in large numbers annually in traditional wintering areas along the western and northern rim of the Gulf of Mexico. Two of these areas are the Laguna Madre of Texas and Chandeleur Sound of Louisiana. We collected data on 54,340 activities from 103 redhead flocks in Texas and 51,650 activities from 57 redhead flocks in Louisiana. Males and females fed similarly, differing neither in levels of feeding (percent of all birds in flock that were feeding) (p>0.90) nor in percentages of birds feeding by diving, tipping, dipping, or gleaning from the surface (p>0.10). The foraging level of redheads in the upper Laguna Madre region was relatively constant throughout two winters. Foraging of redheads in early winter in Louisiana was significantly greater than redhead foraging in the upper Laguna Madre, but by late winter, foraging by redheads in Louisiana had declined to the same level as that shown by redheads foraging in the upper Laguna Madre. The overall foraging level of redheads from Chandeleur Sound was greater (41%) than that of redheads in the upper Laguna Madre (26%), yet it was quite similar to the 46% foraging level reported for redheads from the lower Laguna Madre. Redheads in the upper Laguna Madre region of Texas fed more by diving than did those in the Chandeleur Sound and the lower Laguna Madre. Diving increased in frequency in late winter. Greater reliance by redheads on diving in January and February indicates that the birds altered their foraging to feed in deeper water, suggesting that the large concentrations of redheads staging at this time for spring migration may have displaced some birds to alternative foraging sites. Our results imply that the most likely period for food resources to become limiting for wintering redheads is when they are staging in late winter.     

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.