(Pat)Terns in space and time: Movement, activity, and habitat preference in breeding Caspian Terns (Hydroprogne caspia)

Abstract

In this study, Global Positioning System (GPS)-derived tracking data were used to explore aspects of the breeding season movement ecology of an at-risk seabird, the Caspian Tern, in the Baltic Sea. In particular, the foraging effort, daily activity, and foraging habitat use of the terns were investigated at two breeding colonies. Data were collected during two seasons at the colony on Stenarna island in Sweden, and during one season at the colony on Gubbstenen island in Finland. Foraging movements, with respect to the maximum distance, total duration, and number of trips taken per day differed between the colonies, and in some cases between years at Stenarna. This suggests that breeding season foraging effort is variable across time and space in this species. Tern activity was shown to follow a diurnal pattern, with significantly more flight activity during the daytime and higher colony attendance at night. In general, terns selected shallow water (0-10m) as foraging habitat disproportionate to its availability. However, the colony-level pattern of habitat use was more diverse at Stenarna compared to Gubbstenen, implicating differences in prey conditions around the colonies. Individual-level habitat selection patterns suggest a degree of fidelity with respect to foraging habitat type, indicating that foraging-site fidelity and individual specialization may play a role in determining resource segregation in this species.

Popular Abstract

The Movement and Fish-Finding Habits of a Seabird

“Free as a bird,” they say. Humans have long associated birds’ ability to fly with a perception of freedom of movement. But are birds truly free to move wherever and whenever they wish? Seabirds, or birds whose existence relies on large bodies of water, may not be fully free.

For instance, a pair of breeding seabirds have a nest filled with eggs or chicks that they must tend to, if they are to successfully pass on their genes. This responsibility may in fact limit the freedom they have to move about. The chicks need to be fed, as well as protected from predators and this compels the parents to balance how much time and energy they spend looking for food with how much time they spend at the nest. Therefore, by investigating how the parent birds move, we can begin to understand how much work they do in order to breed successfully. Additionally, we can explore where the parents go when they leave the nest, to see if there is any pattern among the places they visit in search of prey.

In this thesis, I investigated the movement patterns of a species of seabird, the Caspian Tern, during its breeding season in the Baltic Sea. In this region, the Caspian Tern is considered ‘Near-Threatened,’ and therefore gaining insight into how they move and where they go can prove invaluable for the management of the species. To accomplish this, I fitted breeding terns with tracking devices (GPS-loggers) at two breeding colonies; one in Finland and one in Sweden. These tracking data allowed me to analyze and compare how far, how long, and how often the birds took trips away from their nests. In doing so, I could get an idea of the amount of effort the birds invested in order to feed themselves and their chicks at the different colonies. In addition, I analyzed what types of water bodies the terns visited while searching for their fish prey.

So then, if these birds were truly ‘free’ we might expect little to no pattern in their movement at all, but this is not what we see! In my study, I found that birds at the Finnish colony took more trips per day, which were shorter in distance and duration, compared to the birds at the Swedish colony. This indicates that the conditions at, or around each colony differed in some way impacting how the birds moved. By analyzing what type of water the terns visited while foraging, I was able to show that they prefer searching in water shallower than 10 meters. However, at the Swedish colony the terns used water up to 40 meters in depth, which suggests that the amount or types of fish available may differ between the colony locations.

Finally, these results provide basic information on how Caspian Terns move and what type of water bodies they prefer to visit while supporting nests. This is important as it gives us an idea of what Caspian Terns need in order to persist in a world where no lunch comes free, be you bird or otherwise.

Supervisor: Susanne Åkesson
Department of Biology, Lund University

• author: Beal, Martin
• supervisor: Susanne Åkesson
• organization: Degree Projects in Biology
• course: BIOP01 20162
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8937990
• date added to LUP: 2018-03-23 15:48:58
• date last changed: 2018-03-23 15:48:58