Migratory strategy and behavioral flexibility in response to available habitat for a declining seabird, the Caspian Tern (Hydroprogne caspia)

Abstract (Swedish)

To reduce the costs and survive the challenges of migration, animals should flexibly respond to changing environmental conditions along the route. In long-distance migratory birds, available habitat can vary substantially and so individual flexibility is needed to optimally exploit foraging opportunities, particularly for species that have specialized diets. However, flexible behavioral responses to habitat during migration remain poorly understood. I present the first study on the migration of juvenile and adult Caspian Terns (Hydroprogne caspia) using high resolution GPS tracking data. The population of this seabird breeding in the Baltic Sea is of local conservation interest, and the terns migrate more than 6000 km over Europe and Africa although they feed exclusively on fish. I identified key migratory routes, stopovers and wintering areas and characterized individual flexibility in travelling behaviors in relation to available foraging habitat, weather, days at stopover and intrinsic biological factors. In Autumn the population migrates using coastal and freshwater stopovers along six major routes to reach its wintering areas across the Sahel, the Nile River Basin and the Southern Iberian Peninsula. In the Spring, adults return to the breeding grounds in the Baltic using a time optimizing strategy whereas most subadults remain sedentary and some perform only partial return migrations. I found highly flexible migratory behavior that was mainly explained by the variation in available aquatic habitats and experienced wind conditions. Suggesting a fly-and-forage strategy, in both seasons terns reduced travelling speeds, flew fewer hours of the day and switched from nocturnal to diurnal migratory flight with increasing overlap with water bodies. Also, they selected foraging habitats during flight and stationary periods within travelling segments, but avoided foraging detours. Instead, greater route tortuosity was associated with lower wind support and cloudier skies, revealing the possible use of celestial cues and landmarks for orientation and navigation. My findings show how properties of the landscape play a major role in shaping individual flexibility in avian migratory behavior and give a better understanding of the annual distribution for this species. Conservation plans focused on wintering areas, management plans in stopover sites that are not protected or had not been previously recognized, and the inclusion of the species in important migratory flyways across Europe and Africa will be important to prevent further population declines.

Popular Abstract

Seabirds adjust their behavior to migrate over changing landscapes

Tracking birds with GPS technology, like the one that lets us consult Google Maps in our phones, has given exciting insight into how they are able to fly thousands of kilometers in challenging conditions over oceans and deserts during migration. Yet there are many species with fascinating migratory strategies that have not been tracked, so to infer their migratory behaviors we usually rely on information from direct observations done by people on the ground or recoveries of rings that are placed around bird’s legs with unique identification numbers.

This was the case for the Caspian Terns breeding on rocky islands of the Baltic Sea, a population that decreased sharply in the 1970s and is currently protected in Sweden. In this study, GPS devices revealed that young and adult terns from different breeding colonies fly to sites in the Iberian Peninsula and the Nile Delta in Egypt, as well as more distant areas in the Sahel of sub-Saharan Africa. Even though terns depend on water bodies to find fish for food, they crossed a variety of landscapes, including the harsh conditions of the Sahara Desert, where they were aided by favorable winds and reached extreme flight altitudes over 6000 meters above sea level! To have enough energy to complete migration, Caspian Terns used a combined strategy in which they stopped at coastal sites for several days to feed but they also followed aquatic habitats such as rivers, wetlands and coasts to forage while they advanced in their journey. They changed their behavior to fly during the day instead of in the night, flew slower and closer to the ground, and interrupted flight in areas where they could take advantage of finding food. However, they avoided going off the route to find suitable feeding areas, possibly because it would take too much time from their tight migratory schedules that have to fit travelling across continents twice during the year.

These findings highlight how birds can flexibly adjust to the variation in landscape and weather encountered during migration, and they also indicate that, to effectively protect the population from threats to its survival, aquatic habitats along its entire migratory route must be taken care of. In addition, particular attention must be given to protecting Caspian Terns in their wintering areas because juveniles stay there during the first three years of their lives whereas in Spring all adults migrate back to the Baltic Sea. The description of migratory routes and areas that they use during the entire year also implies that future conservation efforts must be developed through partnerships formed across Europe and Africa.

Master’s Degree Project in Biology 60 credits 2021
Department of Biology, Lund University

Advisor: Susanne Åkesson
Evolutionary Ecology, Department of Biology

• author: Rueda Uribe, Cristina
• supervisor: Susanne Åkesson
• organization: Degree Projects in Biology
• course: BION03 20201
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9042105
• date added to LUP: 2021-03-18 10:32:32
• date last changed: 2021-03-18 10:32:32