Today’s guest blog is by Ewan Stenhouse, PhD student at Cardiff University in conjunction with the RSPB Centre for Conservation Science (case partner), supervised by Dr Pablo Orozco-terWengel, Professor William Symondson and Dr Ian Vaughan. The RSPB supervisor is Paul Bellamy.

Birds are ubiquitous throughout the world, especially within the UK, with many species regularly seen in gardens and other green spaces throughout Britain. However, even for very common, well-studied birds we still have gaps in our understanding when it comes to the ecology of many of these birds. One of the most frequently identified knowledge gaps is having a detailed understanding of a bird’s diet.

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, and 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 from field-based observations to capture detailed information on the full range of seeds/buds 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 ideal, as any soft dietary items will be indetectable, while gut content analysis involves euthanising the birds and dissecting the gut. Furthermore, the identification resolution of these methods is low. 

Female hawfinch in the hand © Colin Harvey

So, what about the less studied species? The hawfinch certainly fits this category. Shy and secretive, with the population showing a steady decline since the 1970’s, data from breeding bird atlases show a 76% reduction in the number of 10km occupied squares with only 4% of 10km squares in Britain occupied. There are now only a few local populations mainly within the Wye Valley, Snowdonia and the New Forest and smaller numbers elsewhere. The only dietary information we have comes from Guy Mountford in the 1950’s!

My PhD aims to shed some light on the why the hawfinch has been declining within the UK using genetic techniques to give a detailed insight into hawfinch diet and how it changes throughout the year, looking at both plant and invertebrate taxa. A comparison will be made between the diet of European populations, which are faring much better than their UK counterparts. Furthermore, I will be undertaking UK tree species surveys in areas of hawfinch population strongholds which will give information about whether hawfinch are utilising an abundant species because they are providing a large food resource, or they are seeking out rarer tree species for a specific food resource.

The Genetic Revolution

In order to answer the questions posed above, genetics can provide at least some of the answers. Faecal metabarcoding (genetically identifying prey contained within faecal samples) is a non-invasive, non-destructive method allowing the identification of taxa to excellent resolution across space and time. This method has “unlocked the door” of allowing researchers to study rare or cryptic species in much more depth than ever before.

So what has been happening so far in my PhD? Finding hawfinches was the first challenge, and in partnership with the Hawfinch Project, I had a total of 11 UK sites where hawfinch ringing took place. In order to collect faecal samples without risk of contamination, hawfinch were individually placed within a clean, unused paper bag which was then placed inside a normal bird bag and left for 10 minutes until the bird defecated. To avoid excessive stress, if birds had not defecated after this time they were processed and released. All hawfinch were ringed and biometric information taken (age, sex, wing length and weight)

The glamorous side of fieldwork! Collecting hawfinch faecal samples © Jens Muff Hansen

This process was repeated in Europe (Denmark and Germany) and once I had tested all the faecal samples for plant and invertebrate DNA I was left with 362 samples collected between 2017-2019 – who says hawfinches are rare!

The data analysis has only just started, but the initial results are very interesting. Once we had cleaned the data (removing unwanted taxa) 62 plant taxa remained in hawfinches sampled between 2017-2018 and 78 plant taxa from the 2019 dataset. In the European dataset, 59 plant taxa remained. For invertebrate taxa we identified 103 different prey items in the 2017/2019 UK hawfinches and 59 different prey items in the European individuals.

These initial results suggest that hawfinch diet is far more varied than originally thought, and are opportunistically taking advantage of many food resources rather than primarily focussing on particular items. This is supported by the average number of plant taxa per sample being 6, but with some hawfinches having eaten up to 16 different plant taxa. It is a similar story for invertebrate prey items, with the average number of prey in a sample being 10, and up to 26 different prey items per sample.

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