Trace element analysis as a tool for the study of migratory connectivity in birds

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Julkaisun nimi: Trace element analysis as a tool for the study of migratory connectivity in birds
Tekijä: Ordax Sommer, Nicolás
Muu tekijä: Helsingin yliopisto, Bio- ja ympäristötieteellinen tiedekunta
University of Helsinki, Faculty of Biological and Environmental Sciences
Helsingfors universitet, Bio- och miljövetenskapliga fakulteten
Julkaisija: Helsingin yliopisto
Päiväys: 2021
Kieli: eng
Opinnäytteen taso: pro gradu -tutkielmat
Koulutusohjelma: Ekologian ja evoluutiobiologian maisteriohjelma
Master's Programme in Ecology and Evolutionary Biology
Magisterprogrammet i ekologi och evolutionsbiologi
Opintosuunta: ei opintosuuntaa
no specialization
ingen studieinriktning
Tiivistelmä: Trace element analysis is a useful tool for the study of migration and migratory connectivity in birds. Trace elements are present in the environment and, through the food chain, can be incorporated into tissues such as growing feathers. Since the concentrations of elements remain stable after the feather has stopped growing, and trace element abundances can vary at very small geographical scales, the concentration of trace elements in feathers can provide information on the location where a feather was moulted. Trace element analysis is still rarely used and there are important gaps in our understanding of how trace elements can vary at different organizational levels such as within a feather, between individuals or even between species. It is also not clear if large-scale geographical patterns can be detected by the method, as trace element concentrations can vary a lot even at small scales, which could make it impossible to see larger-scale patterns. To address that, my objectives were (1) analysing the variability of trace element concentrations within feathers, between individuals and between species and (2) determining whether trace element levels differed in feathers grown in Africa compared to feathers grown in Europe. This would shed insight on the suitability of trace element analysis for the study of migration and migratory connectivity. I analysed the concentration of 18 trace elements in the rachis of feathers from willow warblers (Phylloscopus trochilus) and barn swallows (Hirundo rustica) collected in Finland. I plucked three belly feathers from willow warblers collected in spring, whose feathers had grown in Africa. These feathers were used to analyse variability of trace element concentrations within feathers and between individuals. They were also compared to feathers plucked from barn swallows collected in spring (two feathers per bird) to analyse variability between the feathers of two species that winter in the same region. Finally, African-grown feathers of willow warblers were compared to European-grown feathers of willow warblers collected in autumn (two feathers per bird) to look for differences in trace element concentrations in feathers grown on two different continents. Trace element concentrations were analysed using Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS), which allowed to measure concentration at hundreds to thousands of points along the feather rachis. The concentration of each of the 18 elements was used as the response variable in linear mixed models (LMM). To model variation in concentration within the feather I used location along the feather rachis as the explanatory variable and explored how well it predicted concentration of each element. To compare variation between feathers and individuals I fit models including and excluding the feather and individual that each measurement belonged to as random effects and compared them using AIC. To compare between willow warbler and barn swallow feathers grown in Africa I included species identity as the explanatory variable and looked at how the concentration of the 18 elements differed between them. Finally, I followed the same approach to compare willow warbler feathers moulted in Africa and in Europe. For most elements there was little variation along the feather rachis, with concentration remaining stable from feather base to tip. Zn and S showed an increase in concentration starting at the feather base until the central part of the feather and then remained constant toward the tip. Feathers belonging to the same individual showed mostly similar trace element concentrations, although there were exceptions and differences between feathers of different willow warbler individuals were also little. 10 out of 18 elements showed significant differences in feathers of willow warblers and barn swallows grown in Africa. Eight of those elements were more abundant in willow warbler feathers, while only two were more abundant in barn swallow feathers.12 out of 18 elements showed significant differences between their level in African-grown feathers and European-grown feathers. Of those, 10 elements showed higher levels in African-grown feathers, while only two were higher in European-grown feathers. My results suggest that trace elements can show variation at different organizational levels. Variability within feathers was important in at least two elements, which could be caused by physiological processes. This means that when designing sample collection for trace element analysis, unless we know that an element does not vary along a feather, it is important to consider which part of feathers we are sampling. Variability between feathers and individuals was lower than within feather variability, but still significant. Future studies should account for possible within and between individual differences in their design. Differences between barn swallows and willow warblers were large, which was expected based on the literature. It is still unknown what drives these differences between species: some explanations suggested have been physiological and dietary differences or differences in their habitats. I also found clear differences between feathers of willow warblers grown in Europe and Africa. While the exact cause is still not known, this means that at least in willow warbler feathers it is possible to study large scale geographical patterns by trace element analysis. LA-ICP-MS has potential to be a powerful tool to study migration and migratory connectivity in birds. It allows to detect variation in trace elements at continental scales while also allowing to control for different levels of variability in the study design. I encourage researchers to adopt its use in their research.
Avainsanat: trace elements
migratory connectivity
spatial variation


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