Blog 1 | How to adjust travel plans in a changing world?

April 6 - by Eva Kok

The Earth is warming most rapidly around the North Pole. The birds that breed there spend the winter much further south, as far as the tropics. Using miniature satellite transmitters, a team of NIOZ researchers, led by prof. dr. Jan van Gils, are tracking 40 individual red knots “from equator to Pole” this spring to investigate whether, and how, migratory birds can adjust their travel schedule to global warming.

Global warming especially strong at the Arctic
Due to global warming, which is especially strong at higher latitudes, the snow in the Arctic melts ever earlier each year. As a result, the emergence of plants and insects (the main prey for shorebird chicks) has also advanced and this seems to induce a so-called ‘tropic mismatch’, with shorebird chicks growing up after the peak in food supply. This phenological shift may have major consequences for migratory shorebirds that fly from their tropical wintering grounds to the tundra each spring to raise their chicks. The red knot (Calidris canutus) is such a long-distance migratory shorebird. In fact, the population of red knots of the canutus subspecies is in steep decline since it was first counted in 1980.

A red knot in winter plumage feeding on bivalves, the main prey in their overwintering areas. Photo: Jan van de Kam.

Red knots from the equator to the pole
The red knots of the C.c. canutus subspecies spend most of the year on the intertidal mudflats in the Banc d'Arguin in Mauritania (West Africa). In spring, the red knots migrate from the Banc d'Arguin, via the Wadden Sea, to their tundra breeding grounds on the Taimyr Peninsula in far northern Russia. During the short arctic summer red knots depend on the abundance of insects to feed and raise their chicks.

The Banc d’Arguin in Mauritania is a national park consisting of over 10.000 km2 of intertidal mudflats and shallow inshore waters in West-Africa where the Atlantic Ocean meets the Sahara.

Are red knots able to adapt to Arctic warming?
Due to global warming, the snow on the Taimyr Peninsula melts on average more than two weeks earlier today than it did some 30 years ago. With that also the availability of insects has advanced considerably. One potential mechanism to deal with the advanced Arctic phenology for red knots is to advance their own breeding season. Contrary to expectations, recent observations of migrating red knots suggest that they do not (sufficiently) advance the timing of their northward migration accordingly. One of the hypotheses is that red knots are limited in their ability to advance the timing of their northward migration due to restricted fueling rates at their tropical wintering grounds.

Satellite transmitters reveal migration strategies of red knots
To assess whether timing and duration of migration is flexible, and indeed limited by food availability, we will equip 40 red knots in Mauritania with satellite transmitters and follow their subsequent migrations. Before the start of migration, red knots physically prepare for the effort by doubling their body weight by means of energy rich fat stores that serves as fuel for the long flight. To build up so much fat, red knots have to eat an awful lot in the weeks before departure. However, in the wild their food intake is limited by the amount of accessible prey and the trade off with other activities such as predator avoidance.

To investigate if red knots are able to advance their northward migration when foraging restrictions are released, we will provide a group of 10 red knots with ad libitum food in a safe environment. This treatment enables the red knots to quickly build up enormous fat reserves. These fat reserves should, in theory, allow the red knots to begin their northward migration earlier than the control group that is required to gain their energy stores independently on the intertidal mudflats.

In the first few days after release, we will follow the red knots on the intertidal mudflats in the Banc d’Arguin to record their food intake rate, collect their droppings and to take a benthos sample at their preferred foraging locations. With these additional measures we will be able to place individual differences in migratory behaviour observed through the satellites in a broader ecological context.

By the end of April, the red knots depart on their long journey north. Starting then we will keep track of the red knots behind our desk. We are primarily interested in the difference in timing between the ‘control’ group (only equipped with a satellite transmitter) in comparison to the treatment group that on top of the transmitter received unlimited amounts of food. However, the transmitters are even so accurate that we will be able to observe when the red knots start incubating their eggs on the Russian tundra. Consequently, we will be able to investigate whether individual red knots are flexible in the timing of their migration when conditions allow, and what impact a possible early start to migration has on the timing and success of breeding. In this blog series we keep you updated about our experiences.

A satellite tag that is used to track red knots on their migration. Photo: NIOZ.