Tracing shorebird movements along the East Atlantic Flyway
Many shorebird species perform long-distance migrations and while moving between breeding and wintering grounds,they depend on a network of intermediate wetlands (stopover sites) where populations of different origins extensively overlap. The difficulty to discriminate such populations represents a serious obstacle to the identification of the links between breeding or wintering areas and stopover sites, and also precludes the estimation of demographic parameters for each population. Moreover, identifying the origins of individuals sharing migratory routes and stopover sites is crucial to interpret inter-individual behavioural variability and further determine how changes driven by rapid habitat and/or climatic alterations in particular areas will impact populations.
Although tracking technology has received a development boost during the last years, most devices are still highly expensive and too heavy for small shorebird species. In this scenario, biochemical markers such as stable isotopes are still among the most promising tracers to study migratory connectivity in birds.
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Why and how to use stable isotopes to study migratory connectivity?
Ratios of isotopes naturally occurring in a particular environment are passed across food webs and integrated in tissues of birds foraging in that environment according to well understood processes. Hence, birds migrating from one isotopically distinct food web to another will carry the signature of the first in the second, and can thus be assigned to their origin if reference values are known for the areas included in their migratory route.
This methodology requires, however, a careful choice of the tissue to be sampled as the isotopic signature in a tissue can give information for different periods. Blood might be inappropriate to trace origins and migration patterns of birds due to its fast turnover rate. In fact, this metabolic active tissue typically yields an isotopic record of just days to few weeks prior to collection date, requiring tissue sampling within a short period since arrival date at the newgrounds. Feathers have successfully been used in migratory studies, including studies with shorebirds, given that isotopic information is integrated during feather growth (in moulting areas), and remains unchanged after synthesis. However, the use of feathers might be problematic if moulting patterns are variable or not understood in detail, that is, when it is unknown which feathers are grown when and where. This applies to several shorebird species, showing variable and complex moulting cycles, with some individuals starting moults at the breeding areas while others replace both body and primary feathers during migration,in moulting areas or at the wintering grounds.
Trying to find an alternative solution for shorebird studies, our team started using toenails (claws) as the key tissue used to establish the wintering origins of birds sampled during spring migration at their stopover sites (in particular the Tagus estuary, Portugal).
Investigating growth and isotopic turnover rates of shorebirds tissues
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Despite their potential as a target structure for stable isotope (SI) work on avian migration, very few studies attempted to estimate SI turnover rates in bird toenails. In order to enhance our ability to interpret SI data derived from bird toenails, we studied the change in carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of captive dunlins (Calidris alpina) after a controlled diet switch, aiming to (1) describe the turnover rates of δ13C and δ15N in toenails; 2) compare the turnover rates of δ13C and δ15N in toenails, plasma and red blood cells; and 3) calculate the isotopic discrimination values between diet and toenails.
As expected, toenails showed much lower SI turnover rates than blood components, with half-lives (the time required for a isotopic value to reduce to half its initial value) of 27 days and 35 days for δ13C and δ15N, respectively. Isotopic ratios in toenails reached equilibrium with the new diet after 100–120 days, roughly coinciding with the duration of toenail replacement.
Our experimental study highlighted the potential role of toenails to determine geographic origin of birds migrating between isotopically distinct environments. Indeed, their long turnover rates might allow for the detection of SI signals from the wintering or breeding ranges, by sampling individuals at their staging sites.
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Collecting reference isotopic fingerprints from key sites of the East Atlantic Flyway (EAF)
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To know and understand isotopic signatures of shorebirds from relevant wintering areas across the EAF is a critical point to assign the origin of on-route individuals during migratory periods. Thus, we have first used stable isotopes to described the structure and functioning of food webs and shorebird communities in several tidal environments across the East Atlantic Flyway: Bjagós archipelago in Guinea Bissau, Banc d'Arguin in Mauritania, Sidi Moussa in Morocco and Tagus estuary in Portugal. This research identified considerable differences among geographic areas in the isotopic signatures of most components of food webs, including shorebirds.
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Unveiling wintering origins of on-route migrants
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Building on the differences in isotopic signatures of shorebirds along the flyway, we assessed the wintering origin of individuals during spring migration at stopover areas in Portugal and France with a high successful rate (see Figure 1 ).
Figure 1. Carbon (13C) and nitrogen (15N) stable isotope signatures of shorebirds sampled at reference wintering sites in Portugal, Morocco,Mauritania and Guinea-Bissau (mean ± SD; large symbols), and during spring migration (March–May) at the Tejo estuary, Portugal (small symbols). Individuals captured in spring and assigned to a particular wintering area with linear discriminant analysis are represented by the symbol and colour of the corresponding wintering population, whereas all others are represented by a “plus” sign. Lines within each plot represent the discriminant functions obtained from a training data set and shades represent the approximate boundaries of group membership (from 50% to 95%).
Use of SIA to infer migratory phenological patterns of shorebirds
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Once we could assign the different wintering origins of shorebirds overlapping at stopover sites with SIA, we tested the ability of this approach to infer migratory phenological patterns of shorebirds. We took again the case study of the Tagus estuary, where passage migrant birds strongly overlap in space and time with local wintering populations of the same species during spring migration. By performing regular capture events during spring, we could indeed identify the arrival of the first African migrants and describe their phenology in this important stopover area.
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Long-term datasets of population composition (in terms of winter origin) and phenological patterns of migrating populations at important stopover areas may provide an invaluable contribution to assess changes in population dynamics along the flyway. In particular, we might be able to monitor the responses of birds to habitat loss and to expected climate-driven changes , as well as to identify which populations and areas are in emergent risk.
Figure 2. Predicted probability of occurrence of passage migrant dunlins (Calidris alpina) at the Tejo estuary, in Portugal, fitted by a logistic regression.