Today’s guest blog is by Dr Ewan Stenhouse, a recent PhD graduate from Cardiff University discussing his latest paper on Hawfinch diet.. While undertaking his PhD predominantly at Cardiff University, Ewan was jointly funded by the RSPB Centre for Conservation Science and worked closely with Paul Bellamy and Will Kirby. Ewan’s Cardiff University supervisory team were Dr Pablo Orozco-terWengel, Professor William Symondson, and Dr Ian Vaughan.
Understanding the diet of a species can help us see how they interact within the ecosystem in which they live. Having knowledge of what an organism is eating can tell us a lot about the survival, reproduction success, and overall fitness of that organism.
Focusing on birds more specifically, having a high level of knowledge of avian diet, especially from populations in many different places, can help our understanding about the many complex interactions birds have within their environment. But why is this important? Birds can experience different pressures depending on where they live, which can affect their energetic balance and subsequent survival. Therefore, investigating how diet differs spatially is a crucial steppingstone in understanding how populations are locally adapted to the species on which they feed. Despite the clear importance of having this dietary knowledge, currently little to no diet information exists for many woodland birds.
The reason for this “black hole” of knowledge is that many birds feed on small invertebrates, making it very difficult to identify in the field. Furthermore, this interaction is often happening high in the canopy and at such a speed that field-based observations are nigh-on impossible. Even for birds which are primarily or completely herbivorous, it is very difficult to capture detailed information on the full range of items being consumed. Identification of dietary items in the past has been done through microscopic analysis of faeces and gut content analysis. Both these methods are not without their faults. Any soft dietary items will be indetectable, while gut content analysis involves euthanising the birds and dissecting the gut. If that wasn’t bad enough, the identification resolution of these methods is low.
Study on Hawfinches
Hawfinches, despite undergoing declines within the UK, are faring rather well across continental Europe. The Pan European Common Bird Monitoring Scheme (PECBMS) revealed Hawfinch showed an overall “moderate increase” in both long-term (1980-2014) and short-term (2005-2014) datasets. Despite Hawfinches being common across Europe (and regular back garden visitors, much to the envy of us in the UK!), there is currently no dietary information whatsoever about European Hawfinch populations.
Our recently accepted manuscript in Journal of Field Ornithology aimed to shed some light on European Hawfinch diet. We wanted to give an unparalleled insight into Hawfinch diet and whether it differed across two European countries, Denmark, and Germany. We used genetic techniques (explained further below) to analyse plant and invertebrate taxa within Hawfinch diet, to try and paint the most complete picture we could.
Hawfinch in the hand. All birds were captured, handled and ringed by licensed ringers endorsed by the British Trust for Ornithology (BTO). Photo credit: Andy Stanbury: Hawfinch Ringing Group.
“Genius Genetics”
To unravel dietary mysteries, genetics can provide at least some of the answers. Faecal metabarcoding (genetically identifying taxa contained within faecal samples) is a non-invasive, non-destructive method allowing the identification of taxa to high resolution. This method has unlocked the door of allowing researchers to ask previously unanswerable questions about the ecology of rare, cryptic, or even common species.
So, what did the DNA metabarcoding results tell us? We used a total of six sites used by local bird ringers, two in Denmark and four in Germany to catch Hawfinches and collect their poo! To collect faecal samples without risk of contamination, we placed Hawfinches within a clean, unused paper bag, placed inside a normal cloth bird bag. We left Hawfinches in the bags for 10 minutes, or until the bird defecated. To avoid excessive stress, if birds had not defecated after this time we processed and released them.
Female Hawfinch in the hand © Colin Berry
Once I had tested all the faecal samples for plant and invertebrate DNA, I was left with 80 samples, all collected in just one field season! Hawfinches really are thriving in Europe it seems. Once we had cleaned the data (removing unwanted taxa), 35 plant and 37 invertebrate taxa remained. The most frequently detected plant dietary item was European beech, while the winter moth was the most frequently identified invertebrate taxa.
Hawfinch diet was found to differ significantly between Denmark and Germany. The German diet largely consisting of native trees and caterpillars which feed on them, whereas in Denmark the diet contained more non-native trees and artificial food supplied in gardens. This was expected, as lots of Denmark has undergone deforestation, with many of Denmark’s forests full of non-native species. Germany on the other hand, still retains many of its natural forests, with 73% of them containing a mixture of native tree species. This, in turn, provides a wide range of herbivorous food resources for Hawfinches, and plenty of prey taxa for Hawfinch to feed on as well.
Hawfinch populations in Denmark, however, can adapt to their environment. Landscape features are big drivers of food availability, influencing dietary composition and how diets differ spatially. As Denmark field sites were in more urban environments than the German sites, our study showed that Hawfinches can adapt to this, by having a generalist foraging strategy and incorporating alternative food resources into their diet.
Is a European menu good enough to live off? In the case of Hawfinches, it seems so.
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