The Choice and Consequence of Nutrition During Stress: A study of fatty acids in migratory and urban birds

Abstract

Changing environmental conditions can generate physiological stress in birds, which subsequently can lead to the development of an altered diet. In addition to their energetic properties, dietary fatty acids (FAs) have been linked to several important physiological processes in birds, but still the knowledge of their effect on avian ecology is limited. In this study, I examine the fatty acid composition in plasma of two bird species, each in ecologically relevant and stressful contexts: migratory blackbirds (Turdus merula) and urban red-winged starlings (Onychognathus morio) with the aims of investigating whether: (1) migratory and sedentary birds differ in FA composition, and (2) exposure to anthropogenic food sources over the course of the week is reflected in plasma FA composition. Blackbirds were sampled on the island of Helgoland during the autumn migration, when both migratory and sedentary birds are present. Urban red-winged starlings were sampled on the campus of University of Cape Town, where a fluctuation in anthropogenic food abundance has been observed to alter the birds’ diets. Using gas chromatography–mass spectrometry (GC-MS) to quantify their FA profiles, blackbird migrants were found to have higher proportions of polyunsaturated fatty acids (PUFAs) than sedentary birds. Higher levels of the essential α-linolenic acid in migrants indicate that this difference originates from diet, possibly from a selective feeding strategy similar to what has been found for other nutritional compounds in migratory birds. Assuming that this is the case, a lack of difference in the physiologically important ω-6:ω-3 ratio could indicate that PUFAs might only be used metabolically, rather than to enhance other physiological processes. In the urban starlings, no evidence was found to support the hypothesis that the birds’ FA proportions vary with the temporal fluctuations. Instead, based on the large variation found, one could speculate that the birds could be supplementing their diet with anthropogenic food depending on their overall foraging success. Together, these findings show that birds under stress vary in their FA proportions, which could have important physiological and ecological implications in their response to how they meet these conditions.

Popular Abstract

The eating habits of stressed birds

Do you ever feel like you are too stressed to eat healthy, and that whatever is in the fridge will have to do? We might think that when our schedule is too hectic there is no time to consider diet, but birds beg to differ. Their stress is not due to schedules colliding but the physical challenges of long flights or living in our not-so-bird-friendly cities. To deal with these conditions, birds migrating might strategically eat food containing more polyunsaturated fats, since these fats can be performance enhancing. Urban birds on the other hand, are surrounded by a superabundance of anthropogenic food which most often is not the healthiest option, regarding fats. The challenge here is whether, and to what extent, the urban birds will resist the temptation of feeding on this unhealthy, easy to find food compared to their natural food sources.



What makes a bird stressed? Biological stress is not the same as that we humans often encounter in our everyday life. When a biologist talks of stress it is usually a response to a condition which is challenging to an animal. Migration could be one such situation, where the birds need to cover great distances over a short period of time. Another situation is city environments, which in many ways are different to the surroundings that birds have evolved to.

In the present study, I looked at both of these situations – migration and urbanization. For migration I studied blackbirds, a species of bird where only a part of the population will undertake migration while the others stay behind during winter. This creates a unique situation where the physiology of migrants and residents can be compared at the same time to investigate if they differ. Secondly, I looked at urban red-winged starling, an African bird which is common in cities and resides on the campus of University of Cape Town, where they often steal food from students. Since the students, and their food, will not be present on weekends, I asked the question if the birds physiology differed between human-food rich weekdays compared to human-food poor weekends. In both cases, the physiological trait of focus was plasma fatty acids which are a large and important part of any animal’s diet and needed as a source for energy as well as for many cellular functions and processes.

The results revealed that migrating blackbirds had higher proportions of circulating polyunsaturated fatty acids, compared to resident birds. Most interestingly, the essential α-linolenic acid was higher in migratory birds, which suggests that migratory and residential birds may differ in what they eat. We know from other studies that certain polyunsaturated fatty acids relate to bodily functions such as muscle endurance and that it is more effective fuel than saturated fatty acids. Thus, the present result suggests that migrating birds optimize the composition of fatty acids for the physiological challenge of migration.

The urban starlings did not show as pronounced differences across the groups (week-day versus week-end) as blackbirds above. Instead individual birds differed wildly in their circulating fatty acid compositions, independent of the weekday. This suggests that the need to complement their diet with anthropogenic sources varies from bird to bird. Variation in fatty acid levels are known to have large implications for the individual’s performance, such as immune response and reproduction, and a variation in diet could therefore affect the birds’ health.

The diet chosen is important and there is a lot we can learn from birds. Still much is unknown when it comes to how fat, different on a molecular level, will affect both animals and humans. Therefore, there are many questions left to be answered about the nature of birds’ stress-eating.

Supervisors: Caroline Isaksson (LU), Martin N. Andersson (LU) and Petra Sumasgutner (UCT)
Department of Biology, Lund University and FitzPatrick Institute of African Ornithology, University of Cape Town

• author: Kjellberg Jensen, Johan
• supervisor: Caroline Isaksson
• organization: Degree Projects in Biology
• course: BIOM02 20182
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8969020
• date added to LUP: 2019-01-31 12:39:47
• date last changed: 2019-01-31 12:39:47