Seasonal variation in the diet of migratory shorebirds wintering in the Bijagós Archipelago, Guinea-Bissau
Coelho, A.P.; Lopes, P.; Rocha, A.D.; Barros, A.R.; Piersma, T.; Alves, J.A. (2024). Seasonal variation in the diet of migratory shorebirds wintering in the Bijagós Archipelago, Guinea-Bissau. Ardea 112(1): 89-107. https://dx.doi.org/10.5253/arde.2023.a17
In: Ardea. Nederlandse Ornithologische Unie: Arnhem & Leiden. ISSN 0373-2266; e-ISSN 2213-1175, meer
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| Author keywords |
fuelling; macrozoobenthos; biomass; dropping analysis; non-breeding season |
| Auteurs | | Top |
- Coelho, A.P.
- Lopes, P.
- Rocha, A.D.
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- Barros, A.R.
- Piersma, T., meer
- Alves, J.A.
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| Abstract |
As on intertidal flats across the globe, the migratory shorebirds that spend most of their annual cycle in the Bijagós Archipelago, Guinea-Bissau, feed on macrozoobenthic prey buried in the intertidal soft sediments. Understanding the diet of shorebirds throughout the period of residence in the Bijagós can help to understand the degree of their trophic flexibility. In this study, we first reconstruct the diet of eight migratory shorebirds in the Bijagós, then investigate how their diet changes throughout the three main periods of the non-breeding season (arrival, mid-winter and fuelling periods) and finally explore the intraspecific dietary overlap between the three periods. We found significant changes in the diet of most shorebirds across the three periods, with some smaller species increasing the proportion of polychaetes in their diet in the fuelling period, while larger species increased the proportion of bivalves. The fuelling period showed the lowest overlap values with the other two periods, confirming that during this time most shorebirds considerably changed their diet, which may either reflect changes in prey availability or in prey selection.Shorebird populations across the East Atlantic Flyway (EAF) are undergoing rapid declines and for species with wintering populations in both Europe and West Africa, population trends tend to be less favourable in the tropics than in temperate zones (Oudman et al. 2020, Henriques et al. 2022a, van Roomen et al. 2022). Conditions in the breeding areas substantially influence demographic parameters and population trends (Gunnarsson et al. 2005, van Gils et al. 2016, Rakhimberdiev et al. 2018, Alves et al. 2019). However, the extent to which conditions in the African wintering areas may contribute towards such declines remains largely unknown (Oudman et al. 2020). A detailed understanding of shorebirds' diet and foraging plasticity allows the establishment of a fundamental baseline, upon which further feeding ecology and energetic studies can investigate if local food conditions may be constraining some of these populations. For example, local food resources may limit shorebirds' capacity to fuel their return migration in spring (Ens et al. 1994, Rakhimberdiev et al. 2015). Furthermore, analysing if and how shorebird diet varies through the wintering season can also help to evaluate their capacity to respond to changes in local food availability (Rakhimberdiev et al. 2015, Coelho et al. 2022).Although shorebirds are specialized in their prey and known to segregate into dietary niches while foraging on the same habitat (Prater 1981, Mazzochi et al. 2022), their diet can vary substantially between sites (Alves et al. 2013, Duijns et al. 2013, Penning et al. 2022), periods (Sánchez et al. 2005) and habitats (Masero & Pérez-Hurtado 2001, Alves et al. 2010). Understanding the causes and consequences of such dietary range and its variation can help identify how diet may influence individual condition and scale up to affect population level processes. For example, Dunlins Calidris alpina foraging in the Tagus estuary show a marked dietary shift from winter to spring, by changing from using a tactile technique feeding upon juvenile bivalves to a visual foraging technique targeting polychaetes, bivalve syphons and shrimps, which resulted in a 65% increase in intake rates (Martins et al. 2013). Another example of how dietary choices can scale up to influence survival and population trends is provided by the Red Knot Calidris canutus, a bivalve specialist, which in the Banc d'Arguin, Mauritania, consumes a highly available but toxic bivalve species, the lucinid Loripes lucinalis. Red Knots are able to adjust their diet to avoid poisoning by that widely available prey, by also consuming non-toxic but less profitable prey, the bivalve Dosinia isocardia. However, when these alternative prey are not available in sufficient densities, annual survival rates are lower (van Gils et al. 2013).Along the East Atlantic Flyway, two tropical sites host the majority of the shorebirds during the northern winter: the Banc d'Arguin in Mauritania and the Bijagós Archipelago in Guinea-Bissau (van Roomen et al. 2022). While detailed research on movement ecology, predation, diet and food availability of these species has been ongoing on Banc d'Arguin for several decades (Zwarts & Dirksen 1990, Wolff et al. 1993, Leyrer et al. 2006, Ahmedou Salem et al. 2014, Lourenço et al. 2016b, El-Hacen et al. 2020), much less is currently known for the Bijagós Archipelago, particularly regarding shorebirds' diet (but see: Zwarts 1985, Lourenço et al. 2017, 2018, Carneiro et al. 2021, Correia et al. 2023). This is a clear knowledge gap that needs to be urgently addressed in order to work towards reversing current population trends.Shorebirds arrive at the Bijagós Archipelago in September/October, coinciding with the end of the local wet season, and remain in the archipelago throughout the local dry season, leaving in April/May (Salvig et al. 1997, Coelho et al. 2022). During their stay in the Bijagós, shorebirds rely on benthic macroinvertebrates as a source of food and energy, foraging on the extensive intertidal flats when these are exposed during low tide and retiring to beaches or mangroves when these are submerged (Zwarts 1988). Despite an apparent low overall biomass of macroinvertebrates in the mudflats of the Bijagós, and in particular a low harvestable biomass for shorebirds (Zwarts 1985, Lourenço et al. 2018, Coelho et al. 2022), seasonal predation by shorebirds does not appear to deplete the benthic community (Coelho et al. 2022). In fact, macroinvertebrate density increases towards the shorebird's fuelling period (in March/April; Coelho et al. 2022), contrary to what is described for the Banc d'Arguin in Mauritania (Ahmedou Salem et al. 2014). Despite some earlier studies on the diet of shorebirds in the Bijagós in the middle of the nonbreeding season (Zwarts 1985, Lourenço et al. 2017, 2018), spatial and temporal representativeness has been limited and therefore much remains unknown with respect to their diet in this important West African site, currently with no information regarding diet variation throughout the entire season for any shorebird. In this study we use dropping analysis to (1) reconstruct the diet of eight migratory shorebird species in the Bijagós and assess for each of them (2) the seasonal variation in diet (as proportion of prey group and its biomass), as well as (3) the level of dietary overlap across three periods: arrival, midwinter and fuelling. Finally, we discuss if species-specific dietary variation correlates with seasonal variation in shorebird prey abundance (Coelho et al. 2022). |
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