Bulletin 105 December 2004 50 Wader Study Group Bulletin 50 INTRODUCTION The past century has seen massive alteration and loss of natu- ral intertidal habitats that are of prime importance to large numbers of migrant shorebirds during the nonbreeding sea- son (Masero & Pérez-Hurtado 2001). However, worldwide, it has been shown that man-made and man-modified wet- lands, such as saltworks, can provide alternative foraging and/or roosting habitats (Velasquez & Hockey 1991, Masero et al. 2000). The relative importance of saltworks in relation to mud- flats has been studied by various authors (Múrias et al. 1997, Masero et al. 2000, Masero & Pérez-Hurtado 2001) includ- ing Rufino et al. (1984) and Biljsma et al. (1985) who dem- onstrated the importance of the saltworks in Ria Formosa, Portugal, as feeding and/or roosting areas for shorebirds. Batty (1992) also studied the wader communities using a small saltpan site and a nearby intertidal site in Ria Formosa, but concluded that his study areas were too small in relation to the habitats they represent for broad conclusions to be drawn; nevertheless his results serve as good reference points for further work. Moreover there have been no studies of migrant shorebird communities of intertidal and saltwork habitats on the Ria Formosa during spring that can give us an accurate picture of the relative value of each habitat for shorebirds in that season. If we consider communities of shorebirds instead of single species, we can identify those estuaries that support similar communities. This information can be used in further studies that allow us to map changes in community composition over time (Hill et al. 1993). This study compares the use of natural intertidal mudflats and adjacent manmade supratidal saltworks during the pre- and post-breeding periods by shorebirds and other important waterbird populations. We present data on species associa- tions and discuss possible causes, as well as distribution pat- terns of waterbirds on both intertidal mudflats and nearby saltworks at the Tavira estuary. We also explore the relative importance of these habitats for birds during April–August and consider management recommendations for the area studied, especially during spring migration. METHODS This study was conducted at the saltworks and nearby inter- tidal mudflats of Tavira (37°02'N 7°38'W), near the Gilão estuary, a part of the lagoon system of the Ria Formosa Natu- ral Park, Algarve, southern Portugal. Ria Formosa is a tidal lagoon system, comprising a narrow formation of wetland habitats separated from the sea by a range of barrier islands (Marcelo & Cancela da Fonseca 1998). The saltworks study area comprises 132 ha divided into nine separate saltpan systems (Fig. 1) each of which includes three sets of pans (for storage, evaporation and crystallization) connected via sluices. They differ mainly in their salinity, vegetation and water levels. The salinity of the first (storage) ponds is very similar to that of the marine environment (35–38‰), whereas in the last ponds, it reaches 250‰. The intertidal study area Patterns of association and habitat use by migrating shorebirds on intertidal mudflats and saltworks on the Tavira Estuary, Ria Formosa, southern Portugal VERA G. FONSECA 1 , NUNO GRADE 2 & LUÍS CANCELA DA FONSECA 1,3 1 Universidade do Algarve, FCMA, Gambelas-8000 Faro Portugal. veragfonseca@yahoo.com 2 Parque Natural da Ria Formosa (ICN), 8700 - 305 Olhão, Portugal 3 IMAR/Laboratório da Guia Estrada do Guincho, 2750-642 Cascais, Portugal Fonseca, V.G., Grade, N. & Cancela da Fonseca, L. 2004. Patterns of association and habitat use by migrating shorebirds on intertidal mudflats and saltworks on the Tavira Estuary, Ria Formosa, southern Portugal. Wader Study Group Bull. 105: 50–55. Key words: waders – shorebirds – spring and autumn migration – biodiversity – habitat selection – intertidal mudflats – species associations – saltworks. Patterns of association and habitat use were studied on intertidal mudflats and adjacent saltworks during spring and autumn migration in 1999 on the Tavira estuary, Ria Formosa, southern Portugal. Twenty of the thirty species studied, mainly Charadriiformes, winter in and/or migrate through Tavira. The species distribution patterns were highly influenced by habitat – mostly because of the vast diversity of habitats available such as intertidal mud, mud/sand flats and saltworks – and to a lesser degree by tide and month. Migrant species, all gregarious, were the most characteristic of the saltworks during low tide. Non-tidal habitats were used as alternative foraging sites at high tide in summer, but these were also the main foraging sites used during migration periods, significantly supporting several migrant shorebird populations. Overall, this study reinforces the idea that even small estuaries, like that at Tavira, play a crucial role for considerable numbers of shorebirds during migration.