Effect of environmental factors on juvenile Caspian tern movements in non-breeding grounds

Abstract

1. Migratory species are particularly sensitive to climate change and face inadequate protection across their range. However, the behaviour of species and individual spatiotemporal variation in habitat use during their non-breeding periods are comparatively less well understood than breeding and migratory periods.

2. I combined five years’ worth of GPS tracking data on juvenile Caspian terns with two sources of environmental data to describe their movements in their non-breeding range. I used Normalised Differential Vegetation Index (NDVI) data across the Sahel to investigate the relationship between tern movement and NDVI values, and water depth data from the Inner Niger Delta (IND) to investigate the relationship between water depth and the presence of terns in the IND.

3. Most individuals made long-distance movements within the Sahel during the non-breeding period. I found no relationship between water depth of the IND and juvenile Caspian tern presence.

4. I found a positive relationship between NDVI values on departure from one area and NDVI values on arrival in another area. However, the relationship did not suggest directional movement to areas of higher NDVI.

5. My findings suggest that Caspian terns can track environmental conditions to some degree. However, the extent to which this is possible may be constrained by available habitat. With the threat of climate change, understanding how species use their environment is imperative to developing adequate conservation plans, particular with regards to migrating birds.

Popular Abstract

No Terning Back: What do terns do during the non-breeding season?

Migration is one of the clearest examples of animals travelling to find the best resources. However, what happens when species have reached their destination? Do they stay where they are? Do they keep moving, searching for even better habitats? Are their choices based on the environmental conditions they experience? To answer these questions, I used GPS data from 50 juvenile Caspian terns (Hydroprogne caspia), satellite data on how lush vegetation is across the Sahel region (sub-Sahara Desert) and on-the-ground measures of water depth in the Inner Niger Delta (Mali). The Baltic population of Caspian terns spends the non-breeding season in the Sahel region of sub-Saharan Africa. This is also a region highly sensitive to the effects of climate change. By studying what Caspian terns do in response to changing environmental conditions, we can learn more about how this species, and others, may respond under more extreme climate change-induced effects.

First, I investigated the non-breeding distribution of Caspian terns and found five main areas across the Sahel where birds stayed. Most were found along the west coast of Africa and in the Inner Niger Delta in Mali; others were in Chad, along the Gulf of Guinea, and scattered across Sudan, Ethiopia, and Eritrea. Then, I calculated individual’s home ranges – the specific area they occupied in a given time period. Essentially, if a bird travelled elsewhere for an extended period, this was assigned as a new home range. I could then see if there were differences in the vegetation lushness (NDVI) of each sequential home range. I found that if a given bird leaves a relatively lush area, it tends to travel to a relatively lush area. However, it does not necessarily travel to a lusher area. This is shown more clearly in the discrepancy between the difference in lushness (colour scale), and the trend line in the above figure. Of the 50 birds studied, 13 spent time in the Inner Niger Delta. I found no relationship between the daily water depth measures of the delta and the presence of birds. Based on previous study, I expected a higher probability of terns being found in the delta at around 3 m.

So what does this all mean? Lush vegetation and water depth measures explained very little bird behaviour. This may either be because these measures are not good enough proxies, or that the high behavioural variability between individuals means that accurately describing behaviour isn’t possible. For the delta specifically, few other water sources outside of the delta may mean that birds must stay at the delta to forage even if the conditions are not ideal. In terms of conservation implications, it is likely anthropogenic activities will lead to increasing climatic variability in the Sahel. This study suggests Caspian terns can track environmental conditions, but that there may be a limit to how much they can behaviourally adapt to climatic changes.

Master’s Degree Project in Conservation Biology, 45 credits, 2023
Department of Biology, Lund University

Advisor: Susanne Åkesson
Department of Biology/Animal Navigation Lab

• author: Wiersma, Ellis
• supervisor: Susanne Åkesson
• organization: Degree Projects in Biology
• course: BION02 20222
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• keywords: Hydroprogne caspia, habitat quality, non-breeding, utilisation distribution, satellite data, seabirds
• language: English
• id: 9140069
• date added to LUP: 2023-10-16 14:03:12
• date last changed: 2023-10-16 14:03:12