Despite increasing interest in long‐distance migration, the wintering areas, migration corridors, and population mix in winter quarters of most pelagic marine predators are unknown. Here, we present the first study tracking migration movements of shearwaters through the non‐breeding period. We used geolocators (global location sensing [GLS] units based on ambient light levels) to track 22 Cory's shearwaters (Calonectris diomedea) breeding in three different areas. Most birds wintered in one or more of three relatively small areas, all clearly associated with major coastal upwelling systems of the tropical and south Atlantic. Trans‐equatorial movements were dominated by prevailing trade winds and westerlies, while calm, oligotrophic areas were avoided. Breeding populations clearly differed in their preference amongst the three major wintering areas, but showed substantial mixing. This illustrates the exceptional value of GLS, not only for determining and describing the influence of oceanographic features on migration patterns, but also for assessing population mix in winter quarters. This knowledge is essential to understanding the impacts of population‐level threats, such as longlining, offshore windfarms, and oil spills on multiple breeding sites, and will be critical in devising conservation policies that guarantee the sustainable exploitation of the oceans.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Waterbird Society is collaborating with JSTOR to digitize, preserve and extend access to Colonial Waterbirds.Abstract.-The natural wetlands of the Mediterranean have been reduced to 10-20% of their original area over the past few centuries. However, the rice fields that have supplanted them in some regions may be valuable habitats for waterbirds. We describe the use of rice fields by waders, gulls, terns, ducks and herons in Italy, Spain, France, Greece and Portugal where this cultivation is most prevalent in certain restricted areas. Rice field systems, including the irrigation canals, are used by a variety of waterbirds, primarily as feeding habitats, and to a lesser extent as breeding sites. The long duration of flooding allows waterbird use through winter in Spain and France; it is limited to spring in other regions. However, the heavy dependence of waterbirds on rice fields is hazardous, because rice cultivation is subject to suddenly changing agricultural practices. Threats include risk from pesticide contamination, changes in the management of rice cultures, the rapidly spreading practice of cultivating rice on dry fields, and from large-scale hydrological changes. We urge authorities to recognize the importance of rice fields for the conservation of Mediterranean waterbirds and propose that cultivation and bird conservation issues be addressed in a holistic fashion.

The ~nfluence of a trawling moratorium on the breedlng ecology of Audouin's gulls (Larus audouinii Payraudeau) was investigated during 1992 and 1993 in the largest colony of the species in the world (70% of total population), at the Ebro Delta, NE Spain. western Mediterranean. In this colony, Audouin's gull behaves partly as a scavenger following the trawling fleet and exploiting fishing discards. Each year, the trawling moratorium affected different breeding stages of the species: in 1992 it overlapped with pre-laying and laying stages, while in 1993 it affected most of the chick-rearing stage. We examined differences in laying dates, laying interval, clutch and egg slzes, nest desertion during laying, hatching success, hatchling mass and condition, growth rates, chick survival and breeding success. The moratorium greatly affected egg production in 1992, delaying the start of l a y~n g and dramatically decreasing clutch and egg sizes. Intraclutch egg size variation between the 2 years suggests that egg size profiles are outcomes of the interaction between physiological and environmental factors rather than the result of ultimate factors. Nest desertion was much higher in 1992, probably because some females were under a critical body condit~on threshold and they deserted after laying the first egg. Hatching success also decreased significantly during 1992, probably related to changes in the size and the quality of the eggs. Chick growth and survivorship was h~g h e r in 1992, when trawler discards were normally available, than in 1993. Trawler discard availability during chick rearing was more important than egg size in determining chick survival. However, differences were not always significant, since In 1993 the n~oratorium did not start until the chicks were 1 wk old, and, further, chicks had h~g h e r skeletal size and body condition (in order of Importance) than In 1992. In fact, chicks had similar fledgling condition in both years at the end of the reanng stage. In 1993, the overall breeding success decreased by 4 8 % Nevertheless, gulls' breed~ng performance for the 2 years was much lower than before the establishment of the flrst moratorium In 1991, and the situation suggests that clupeo~ds, the prey on wh.ich Audouin's gull feed, are scarce. It seems that gulls were not able to compensate for the lack of trawler discards during egg production, since the avallablllty of secondary feeding resources was much lower in March and April (pre-laying and laying stages) than in June (chick-rearing stage). Although in long-lived species a decrease in food supply should affect fecundity before affecting adult survival, results confirm the importance of the trawler fleet activity for the conservat~on of this threatened species.

In recent decades, the Yellow-legged Gull (Larus michahellis) has become a problematic species in many Mediterranean countries, mainly because it interferes with human interests. However, this gull also has a negative impact on several other bird species, many of which are classified as endangered. Two different European Union Action Plans are currently under development with the aim of decreasing the availability of food derived from human activities, such as garbage and fishery discards, which are considered to be the main causes of the superpopulations of this gull. Here, we describe the diet of Yellow-legged Gull chicks, with particular emphasis on establishing the dependence of each population on refuse dumps, in order to forecast changes in gull population dynamics in response to the management decisions being implemented. We sampled four colonies along the Western Mediterranean in Spain: the Medes Islands, the Ebro Delta, the Columbretes Islands, and Mazarrón Island. To elucidate their feeding ecology and to avoid obtaining a discrete estimation from a single sampling, we collected regurgitates from each colony three times throughout the chick-rearing period. Slightly differential feeding habits were observed between chick age classes. Younger chicks in all four colonies tended to be consistently provisioned with smaller prey such as invertebrates. Distinct uses of several foraging habitats among localities were observed. In particular, the use of refuse dumps was common and abundant in two of the colonies: the Medes and Mazarrón Islands. As a consequence of current management strategies, generalized reductions in Yellow-legged Gull populations and increases in the consumption of alternative food resources to those of fishery discards and refuse scraps are expected. Finally, we predict that decreased food availability will force some gulleries to increase predation on endangered species, thereby raising a conservation concern.

Accurate determination of feeding habits using only conventional dietary analysis is usually difficult, due to both biases caused by prey digestibility differences and to the discrete aspect of the sampling. However, combining conventional methods with stable isotope analysis provides an integrated view of the assimilated diet. Here, we measured stable isotope ratios of carbon (δ 13 C), nitrogen (δ 15 N) and sulphur (δ 34 S) in feathers of yellow-legged gull Larus michahellis nestlings from 4 colonies along the western Mediterranean coast, where the gulls consume different proportions of marine, terrestrial and garbage resources. We collected and analysed chicks' regurgitates in each colony, thereby determining the isotopic signatures in a significant sample of prey. By applying a mixing model to our isotopic values, we compared the information provided by the 2 methodologies. According to direct prey sampling and the particular conditions of the breeding areas, populations with more enriched δ 34 S values consumed a diet richer in marine prey, with lower δ 15 N signatures indicating garbage consumption. The main pattern resulting from the mixing model agreed with direct diet sampling. However, estimated proportions for small-sized prey were controversial, which indicates that small, soft prey items might be underestimated by regurgitate analysis. We conclude that stable isotope signatures and the use of mixing models are useful tools for the rapid assessment of feeding ecology in certain populations. Solving analytical biases should be considered in future feeding studies, thereby saving time and minimising bird disturbance by using an isotopic methodology.KEY WORDS: Carbon-13 · Mixing models · Nitrogen-15 · Regurgitate · Sulphur-34 Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 377: [289][290][291][292][293][294][295][296][297] 2009 because of the intrinsic characteristics of some food types (e.g. pieces of meat and garbage taken at refuse dumps). These methodologies provide only an episodic view of an individual's diet, since each sample represents only a specific feeding event (Votier et al. 2001). Thus, to obtain reliable information about a population's feeding habits, exhaustive monitoring over time is needed (Jordan 2005). However, conventional approaches are still required when we need to identify the taxonomic group of prey consumed. Regurgitated food appears as the least biased method for describing diet composition (González-Solís et al. 1997).The use of stable isotope analysis (SIA) in animal ecology has increased enormously in recent years and has become an important tool to study trophic ecology (Hodum & Hobson 2000, Post 2002. Although SIA does not provide the taxonomic detail achieved by conventional dietary analysis, it avoids prey digestibility biases because it only takes into account assimilated food. Moreover, it provides a ready-made integrated estimate because the assimilated diet for a certain period of time is summarised, dep...