Spotlight: Two decades of monitoring in Mauritania

by Deborah Buehler originally published in Wader Study 131(2)

Imagine a place where the wind feels like a hair dryer – hot and dry. A place where faces are covered to protect against sand and sun. Now imagine a coastline, the sea brilliant blue and separated from salt flats and desert by a low line of dunes. And at the interface of land and sea, shallows and intertidal flats covered with birds. This is Banc d’Arguin, Mauritania.

Banc d’Arguin is one link in a chain of important habitats that migratory shorebirds depend on for their survival. It is part of the East Atlantic Flyway, one of eight major migratory routes used by birds to move between breeding and wintering grounds. In human terms, a flyway is like a string of connecting flights. A problem early in the itinerary (say an outage that shuts down check-in) could cause a series of delays and the potential of missing the purpose of the travel (say a wedding or a funeral). It is similar for the birds, but with much higher stakes.

Migratory birds from the north use Banc d’Arguin as a crucial non-breeding site while resident waterbirds and migrants from the south use the area for breeding. In 1973, during a British expedition, recoveries of ringed shorebirds and terns began to establish the significance of the area and its connectivity with coastal wetlands in Europe and with tundra breeding grounds beyond. Banc d’Arguin is now recognized as a site of international importance and the entire area was designated as a National Park by the Mauritanian government in 1976.

The monitoring of waterbird populations in Parc National du Banc d’Arguin (PNBA) is also decades old with the first shorebird counts conducted in January/February 1980 and six further winter counts conducted in 1997, 2000, 2001, 2006, 2014 and 2017¹. Additionally, teams from the Royal Netherlands Institute for Sea Research (NIOZ) have conducted winter counts within the Iwik region of PNBA every year since December 2003. In this issue of Wader Study, El-Hacen and colleagues report on these two decades of annual monitoring complementing and updating larger-scale studies on long-term waterbird population trends².

From December 2003 onward, the researchers conducted counts, after high spring tide in the last week of November, or in December or January. Counts were performed on a single day 1–2 hours before the predicted high tide and usually on a day when high tide fell in the early afternoon to standardize tide height and time of day as much as possible. The study area around the village of Iwik was subdivided into six counting units that covered all the roost sites (see Fig. 1 in the paper) and teams of one to two people covered each of the six units in the study area.

The Iwik region supports different waterbird groups: shellfish-eating shorebirds that breed to the north and spend the non-breeding season in PNBA, fish-eating waterbirds that breed in PNBA and are either resident or spend the non-breeding season to the south, and finally gulls and terns. The researchers analysed overall trends for these three waterbird groups and also selected the most common species within each group to assess individual trends over time (eight shorebird species, six species of large-bodied waterbirds, and six gull and tern species). In the analyses, they combined species that were difficult to distinguish in the field (e.g., small herons). To statistically analyze waterbird numbers over time, El Hacen and colleagues used generalized additive models (GAMs), which can estimate non-linear trends, identify periods and magnitudes of change, and account for the fact that the numbers one year are affected by what they were the year before (lack of independence among consecutive counts).

Over two decades of study, the researchers found that waterbird numbers decreased from 120,000 to 80,000 birds in the Iwik study area. The models indicated that the decreases were non-linear and not the same across waterbird types. Shorebirds, which made up 90% of total waterbirds, declined in the first half of the study period from 2003 to 2012 and then stabilized from 2013 to 2023. Large-bodied waterbird numbers (cormorants, herons, spoonbills, flamingos and pelicans) were stable until 2012, increased to 2019 and then decreased from 2020–2023. Gull and tern numbers remained stable across both decades with no statistically significant variation.

Looking at trends in the eight most common shorebirds, Red Knot Calidris canutus numbers followed the general trend, first decreasing, then stabilising after just over a decade. The decline of Red Knots in the Iwik area from 2003 to 2010 has been explained by decreases in the density of their preferred bivalve prey, which led the birds to consume a somewhat toxic alternative³. Bar-tailed Godwit numbers dropped continuously across the study period from 2003 to 2016, mirroring declines elsewhere in the flyway suggesting that factors beyond the Banc d’Arguin region, including climate change related shifts in food peaks in the Arctic during the breeding season, contributed to the decline. On the other hand, Whimbrels showed a steady and significant local increase indicating that conditions are favourable for Whimbrels in PNBA, at least in comparison to the rest of the flyway. The remaining five most common shorebird species showed year-to-year variations, but no statistically significant trends (Common Ringed Plover Charadrius hiaticula, Grey Plover Pluvialis squatarola, Dunlin Calidris alpina, Sanderling Calidris alba and Eurasian Curlew Numenius arquata).

Amongst the six most common locally breeding waterbirds, three groups showed significant trends. Reed Cormorants Microcarbo africanus and small herons increased over the entire study period. Grey Heron Ardea cinerea monicae numbers increased strongly between 2003 and 2019, but then declined until the end of the study period. Other large waterbird species such as White-breasted Cormorant Phalacrocorax lucidus, Eurasian Spoonbill Platalea leucorodia, and Greater Flamingo Phoenicopterus roseus did not show statistically significant treads over the two decades, nor did the six most common species of gulls and terns. However apparent declines over the last 2–3 years may be ecologically, if not statistically, significant. For example, recent declines of the endemic subspecies of Grey Heron, Caspian Tern Hydroprogne caspia and West African Crested Tern Thalasseus albididorsalis coincide with the incidence of Avian Influenza in breeding colonies in West Africa.⁴

El Hacen and colleagues demonstrated the utility of annual and standardized monitoring of waterbirds near Iwik. They recommend that this annual monitoring be maintained, and that similar monitoring be established in the south of the PNBA. Such continued monitoring will increase knowledge about one link in a chain of critical sites along the East Atlantic Flyway.

The other day, I felt a breeze as hot as the Mauritanian wind on my face. It reminded me of my own experiences working with shorebirds near Iwik in December 2006. I benefitted from a long-term collaboration between NIOZ and PNBA that has supported far more than winter counts. My study looked at the effect of age and environment on immune function in Red Knots and found that young birds at the lower quality Baie d’Aouatif roost site had higher white blood cell counts than adults or young birds at the higher quality Ebelk Aiznay site⁵. During my recent encounter with hair dryer hot wind, I was not in Mauritania, but rather far to the north, and the air was humid as well as hot. The heat and humidity created storms that unleashed a torrent of rain causing flooding in the subway. Due to delays, I nearly missed my regional train, a delay which would have caused me to miss the last regional bus at the next interchange, leaving me stranded. On long travels, success at each interchange depends on the last and each link is of paramount importance. In a world where climate change is making the weather unpredictable and violent, I feel solidarity with the birds, never quite sure what the next stop will bring, or if they’ll make it on time, on their tenuous journey from link to link.

 

¹ Oudman, T., H. Schekkerman, A. Kidé, M. van Roomen, M. Camara, C. Smit, J. ten Horn, T. Piersma & E.-H.M. El-Hacen. 2020. Changes in the waterbird community of the Parc National du Banc d’Arguin, Mauritania, 1980–2017. Bird Conservation International 30: 618–633.

² El-Hacen. E.-H.M., J. ten Horn, A. Dekinga, B. Loos & T. Piersma. 2024. Two decades of change in nonbreeding population sizes of shorebirds and other waterbirds in the Iwik area of Parc National du Banc d’Arguin, Mauritania. Wader Study 131(2): 112–121.

³ van Gils, J.A., M. van der Geest, J. Leyrer, T. Oudman, T. Lok, J. Onrust, J. de Fouw, T. van der Heide, P.J. van den Hout, B. Spaans, A. Dekinga, M. Brugge & T. Piersma. 2013. Toxin constraint explains diet choice, survival and population dynamics in a molluscivore shorebird. Proceedings of the Royal Society B 280: S 20130861.

⁴ Davis, J. (2023). Bird flu outbreak spreads across West African migratory route. Natural History Museum. Accessed 31 Oct 2023. https://www.nhm.ac.uk/discover/news/2023/april/bird-flu-outbreak-spreads-across-west-african-migratory-route.html

⁵ Buehler, D.M., B.I. Tieleman & T. Piersma. 2009 Age and environment affect constitutive immune function in red knots (Calidris canutus). Journal of Ornithology 150: 815–825.

 

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Featured image: A large flock of shorebirds and six flamingos in Banc d’Arguin, Mauritania (photo: Jan van de Kam).