Submitted by editor on 16 February 2021. Get the paper!
By Camilo Carneiro
Many migrants distribute over wide regions during the non-breeding season, with individuals that breed a few hundred meters apart, wintering thousands of kilometres away from each other. Wintering at different sites (usually varying considerably in latitude) can mean that the weather conditions, habitat and food availability and quality will vary, resulting in distinct costs and benefits through differences in energy intake and expenditure (Castro et al. 1992, Piersma et al. 1993, Marra and Holmes 2001, Mathot et al. 2007, Alves et al. 2013). However, to reach the wintering site the requirements for migration are also likely to differ and individuals might tradeoff a longer migration distance with more favourable conditions at the wintering area (Alves et al. 2013).
Looking at Icelandic whimbrels (Numenius phaeopus islandicus), we investigated if these birds tradeoff wintering energetic balance with migration costs. They are long‐distance migrants that breed mainly in Iceland and spend the wintering season along more than 6000 km of coastline, from temperate regions in south‐west Europe to tropical areas in Benin and Togo (Gunnarsson and Guðmundsson 2016, Carneiro et al. 2019a). Whimbrels are long lived (Robinson 2018), philopatric to the breeding and wintering site (Mallory 1982, Zwarts 1990), and consistent on their schedule (Carneiro et al. 2019b).
We selected three wintering sites along the wintering distribution, each located in a distinct climatic region at different distances from Iceland: the Tejo Estuary in Portugal (temperate), Banc d’Arguin in Mauritania (arid) and Bijagós Archipelago in Guinea‐Bissau (tropical; Fig. 1). For each site, we calculated the energetic balance achieved by whimbrels and estimated the migration flight costs to/from Iceland. Please see the paper for details on methods.
Figure 1. Map depicting the sampled wintering areas (the Bijagós Archipelago in Guinea‐Bissau, Banc d'Arguin in Mauritania and Tejo Estuary in Portugal). It also shows a site used for stopover during migration in south Ireland (Cork Harbour; Carneiro et al. 2020) and the breeding study site in Iceland. Great circle distance to the breeding site is shown in parentheses for each non‐breeding location. Whimbrel photo by Tómas G. Gunnarsson. Photos of study sites by Camilo Carneiro.
Unsurprisingly, whimbrels migrating further require more energy than those wintering closer to Iceland, but we also found that birds migrating further tend to winter at places where the energetic balance over the winter is more positive (Fig. 2), suggesting a tradeoff between higher costs of migration with more favourable wintering conditions.
Figure 2. Daily (mean ± SE) energetic balance possible to achieve at each wintering area during one tidal cycle.
Another question raises: if conditions during winter differ, will that influence the individuals later when they return to Iceland to breed? To investigate that, we determined the wintering region (temperate, arid, or tropical) of 91 breeding females through tracking (geolocators) and stable isotope analysis, and recorded their laying date and size of eggs laid. Additionally, we collected data on breeding success (i.e., fledging success) to assess how relevant is laying date in this population.
First, laying date is important: the later the eggs are laid, the lower the probability of fledging young. Second, the region where females wintered had no apparent influence on laying date nor the size of eggs laid (Fig. 3). Hence, wintering site quality did not appear to carry‐over to the breeding season. Note, however, that only one female was assigned to winter where the energetic balance was the lowest, limiting analysis and interpretation.
Figure 3. (A) Laying date and (B) relative egg volume (mean egg volume, in mm3, divided by tarsus‐toe length, in mm) of female whimbrels that spend the winter in a tropical (A: n = 103, from 75 individuals; B: n = 102, from 74 individuals), arid (A and B: n = 13, from 10 individuals) or temperate climate region (A and B: n = 3, from one individual); (C) logistic curve of fledging success in function of laying date, with histograms of successful and unsuccessful events shown.
The abundance of wintering whimbrel through the range seems to match the variation in apparent wintering site quality, with more individuals in the tropical than arid and temperate regions (Alves et al. 2012, van Roomen et al. 2015, Oudman et al. 2017), where the energetic balance is more positive. Although we did not find a link between the wintering conditions and timing of laying or egg sizes, carry-over effects may exist in other ways. For example, through an effect on migratory schedule which could lead to a late arrival, divorce, and fail to breed (Gunnarsson et al. 2004).
I invite you to read the paper for a thorough discussion and many other details.
If you are interested in learning more about our research, please follow @Camilo_Carneiro on twitter.
Curiosity: during the fieldwork in the Banc d’Arguin we discovered that whimbrels are consuming West African Bloody Cockles, a novel (they were not seen eating it during intensive fieldwork in the 1980’s) and important food item in their diet (Carneiro et al. 2017).
Whimbrel pulling the flesh out of a West African Bloody Cockle in Banc d’Arguin, Mauritania (9 February 2017). Photo by Camilo Carneiro.
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