Free as a bird – or on a tight schedule? A study of migration strategies in five Tringa wader species

Abstract

Migration does have a time, energy or mortality cost, and the strategy to minimise the costs shapes the migration behaviour. Depending on the relative influence of these costs in shaping the migration, an animal is expected to undertake these journeys in different ways, depending on their morphology and migration ecology. I investigated if five Tringa-type waders minimise their migration time, and compared observed behaviours to the predicted behaviours under a time minimisation strategy. We tracked 21 birds for at least a part of their autumn migration, and estimated their energy balances during migration. We did find long flights in the three largest species and two of them skipped sites. The stopover and flight times were significantly correlated, and all species seemed to rely on a few key fuelling sites either for long flights or for overloading. The larger species showed more time minimisation behaviours, though maybe due to other factors. My study has limitations and uncertainties such as lost birds, rather simple energy estimates and possible effects of age or the tracking devices. In conclusion, despite the limitations, our results do indicate that our waders minimise time during migration.

Popular Abstract

Free as a bird – or on a tight schedule? A study of migration strategies in five Tringa wader species

Every year birds make astonishing migrations as they fly across vast distances, even between continents. Migration is costly in terms of energy, time and may even cost a bird its life. Therefore, birds can use different migration strategies to try to minimise one or several of the costs. The strategy shapes the bird’s migration behaviour, for example where, how much and how often to store energy (fuel), as fuelling takes more time than to fly. For example, to minimise time a bird is expected to make long stops at a few good sites followed by long flights, i.e. to maximise fuelling at the sites providing plentiful resources.

I studied the migration of five Tringa-type wader species of different size: the smallest Common sandpiper, Green sandpiper, Redshank, Spotted redshank and the largest Greenshank, which all potentially migrate to Africa. My aim was to find which migration strategy these birds adopt. I hypothesised that they all minimise time and more clearly so in the larger the species. I expected our birds to skip (poor) sites and make the most of the best sites. Hence, the duration of a stopover and the following flight should be positively correlated, and either successively increase or they should overload (i.e. store enough fuel for several flights at good sites) depending on food resources. So, did they?

To find out, we tracked the migrations of birds of these five species using GPS-loggers that transmit data via the GSM network. The data showed where and for how long a bird stopped and how far, for how long and which way it flew. I also calculated if each stopover was long enough to fuel the following flight or if overloading was required.

The data showed that the two smallest species (Common and Green sandpipers) made many flights with single or few days between across mainland Europe. The Redshanks and some Greenshanks first made shorter flights, which were followed by longer flights from the Waddenzee area and southward from there. The Spotted redshanks and other Greenshanks made few, long flights. Redshanks and Spotted redshanks only used coastal stopovers, while Greenshanks used both coastal and inland sites. I did find examples of overloading in all species except in the Spotted redshanks, so all species seem to rely on a few key fuelling sites. I also observed a few examples of skipped sites, most clearly two pairs of Greenshanks that embarked on flights together, and then stopped at different sites - even in different countries. Overall, the stopover and flight durations did correlate, indicating a time minimising strategy. These behaviours were more prominent in our larger species.

Of course, migration is complex and there are both limitations and uncertainties in my study. One interesting aspect is how the mortality risk affected the migration. Three of our tracked birds were shot by hunters, one predated and a handful died of other or unknown causes. A handful made “U-turns” that seemed suboptimal, unless they were caused by a combination of barriers and bad weather and the bird seemed to choose the safe option. Another important aspect is how capture, handling and tagging affect the birds (and the results), with possible severe consequences for the bird.

In conclusion, all of our birds showed at least some behaviours that are expected if they primarily use a time minimising strategy during migration. The larger species showed all the expected behaviours, while the smaller species only showed some. Despite the limitations and uncertainties, I conclude that our five wader species seem to aim to migrate as fast as they can, i.e. to be on a tight schedule, as they set out to their winter quarters.

• author: Storckenfeldt, Erica
• supervisor: Anders Hedenström
• organization: Degree Projects in Biology
• course: BION03 20241
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212880
• date added to LUP: 2025-09-19 14:52:05
• date last changed: 2025-09-19 14:52:05