Clock-shift experiments with blackcaps, Sylvia atricapilla

Abstract

The ability to navigate from breeding sites and wintering grounds has fascinated numerous avian studies examining the bird orientation performances. The migratory avian navigational toolbox consist of route- and map-based orientation, non-compass, vector and compass orientation. The compass orientation consist of celestial and geomagnetic cues, which are divided into the three main compasses known as the magnetic compass, stellar compass and sun compass respectively. However, the use of the sun compass as the primary compass, in nocturnal migrants has so far been difficult to show. Here I show that Eurasian blackcaps (Sylvia atricapilla) do not use the sun compass as the main compass to define their migratory orientation. I found that applying a 4-hour fast clock-shift, with the expectations of a 60° counter-clockwise (CCW) shift, the experimental blackcaps did not show this shift, since their shift of 16° CCW was smaller and not significantly different from random, with a 6 days delay of responding time after the exposure. My results demonstrate that the blackcaps orient towards north-west suggesting that either they are widening their standard migratory route to winter in Scandinavia or the British Isles or they at least partly reverse migration. In addition, they also showed an attraction towards celestial bodies such as the sun and the moon, with only the moon showed an influence on their orientation. I anticipate that these results add to the knowledge of blackcaps behaviour while migrating as well as showing the difficulties testing the sun compass in nocturnal bird migrants. To investigate further the lack of response to the 4h fast clock-shift, one could expand the number of testing days after the exposure and apply for instance stable isotope analysis of the individuals to get a clearer viewpoint on their site of origin and preferred migratory direction. Ringing recovery analyses would further reveal the distribution of preferred migratory directions.

Popular Abstract

I don’t need a compass to tell me which way the wind shines!

Exploration of the world has always been part of the human nature, but how to navigate and travel from A to B is the real question. It is believed that navigation by sense or intuition is something anyone can do, but must be learnt and passed down from generation to generation. The Vikings were pioneers in exploring the opportunities of navigations using easily recognizable landmarks and the sun. Hence, the path of the sun across the sky and its rising and setting point changes with the seasons and latitudinal location, but at midday the direction to the south can easily be found. To use the sun for navigation, the Vikings utilized sunstones, which worked as a compass that always showed the correct direction. Birds, on the other hand, are able to navigate by landmarks and possess a compass sense to determine their direction much like the Vikings, in addition, they may have something like a biological GPS systems. The way it works is that the birds make use of either a route- or a map-based navigation and their compass sense to follow the chosen route or map. The compasses birds are able to use are; a magnetic compass based on the Earth’s magnetic field, a stellar compass based on single stars or star patterns and a sun compass, using the position of the sun. In this experiment, I examine if Eurasian blackcaps (Sylvia atricapilla) use the sun as their main compass to navigate during autumn migration.

The Eurasian blackcaps is a nocturnal migrant, a common and widespread warbler, known as an astonishing singer during the Scandinavia summer. It breeds in the southern half of the Scandinavian Peninsula and depending on its place of birth, when winter starts to arrive, they will head for different wintering grounds. Blackcaps born in Scandinavia would primarily migrate to southeast Europe and East Africa, whereas those born in continental Europe would migrate to the western Mediterranean. In recent years, however more and more blackcaps have been observed staying in Scandinavia or migrate towards the British Isle for wintering. Now that we know the direction the blackcaps are heading in, one may wonder how they navigate to these places.

Do I have the answer? Well… yes and no. To test the preferred orientation of the blackcaps in my experiment, I used orientation cages lined with thermal paper. On this type of paper, the birds would leave scratches, which I counted and used to estimate their preferred direction. But, to really test whether or not they use the sun for navigation, I performed a clock-shift experiment. A clock-shift experiment is a method, where the time of sunrise and sunset for the individual animal is shifted in relation to the local time. Doing so, the internal clock of the animal should be altered to the newly shifted subjective time. This would lead to the expectation that the shifted birds would orient in a direction that is appropriate to the shift. When exposing the birds to the real local time, they should be confused over the miss-match between its environment and its own subjective time and therefore, set priority to one of the cues to select its migratory route. This method has been use in several studies showing that birds use the sun, as their main compass, but it is not always easy to show, since other factors might affect the prioritizing of the compass.

I applied a 4-hour fast clock-shift, which means that the sun would rise 4 hours earlier than the locale sunrise and the sun would set 4 hours earlier than the locale sunset. We know that the sun moves across the sky with a velocity of 15° per hour and therefore shifting the birds internal clock by 4 hours would lead to the expectation that the time shifted birds would deviate by 60° from the preferred orientation of the birds experiencing the local time. In this project, I found that the blackcaps orient towards the northwest and to answer the question of whether they use the sun or not as a compass is very difficult to do. I found that the blackcaps did not show the expected deviation, but instead they seemed to be attracted towards the sun. Furthermore, I discovered the blackcaps not only were attracted to the sun, but also to the moon when visible. The amount of cloud cover that were present did not seem to matter. The results from this study, and others like mine, show the difficulties of proving that birds navigates primarily by the sun. Therefore, I must conclude that they might use a single compass or a combination of other compasses to find the correct way while navigating the world.

Master’s Degree Project in Biology 45 credits 2019
Department of Biology, Lund University

Advisor: Susanne Åkesson
Evolutionary ecology, Department of Biology

• author: Hansen, Mathias Hieronymus
• supervisor: Susanne Åkesson
• organization: Degree Projects in Biology
• course: BION02 20182
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8981989
• date added to LUP: 2019-06-11 15:37:56
• date last changed: 2019-06-11 15:37:56