Oxwich Marsh: investigating diet and competition for prey in Oxwich warblers

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This post has been written by Sarah Davies, a PhD student in Cardiff University. Sarah has been ringing at Oxwich for approximately 14 months, and has been using it as one of the sites for data collection to inform her research on dietary composition and prey partitioning. An overview is below.

Reed warblers are a common sight at Oxwich Marsh in the
summer months, and have been the subject of many studies in ecology and
animal behaviour. I have always been fascinated by these warblers and was
lucky enough to be given the opportunity to study them for my PhD at Cardiff
Reed warblers in the autumn (a worn adult left and a pristine juvenile right)

Focus of Research
My research is about the dietary choices of reed warblers and how
this is affected by the availability of prey. Reed warblers are generalist
insectivores, so they can consume a wide variety of invertebrate prey from
snails, to tiny midges, to spiders. In short, they are not fussy eaters! They also
show dietary plasticity which means that they adjust their diet opportunistically,
depending on what is available in the environment. In reedbed habitats, the abundance
of different prey groups fluctuates with local emergences over the season. Thus,
unlike woodland habitats, wetlands such as reedbeds are non-seasonal and prey
is available throughout the summer.
Reed warblers may not always have their pick of the best
invertebrates due to dietary competition from other warbler species in the
marsh. Two other reedbed species are commonly ringed at Oxwich in the summer:
the Cetti’s warbler and the sedge warbler. Little is known about to what extent
these birds partition their diets (i.e. choose different prey) to avoid
competition and exclusion. In addition, overlaps in diet may change depending
on prey availability. If prey is very abundant, coexisting bird species may be
able to eat many different prey items and dietary overlap will be low, but if
prey is less abundant, birds may only be able to eat select prey groups and
their diets will overlap more. 
As well as the differences in diet between
different species I am also interested in whether age influences dietary
choice. Juvenile birds may not have access to the best foraging sites due to
dominance of older individuals; they may be inexperienced hunters or may have
different nutritional requirements.
Data Collection and Analysis
One way to analyse diet is to collect faecal samples from
birds in the field and use molecular techniques to extract DNA from the prey
remains in the droppings. Using high-throughput sequencing, identification of
prey to species level can be achieved. Once prey in the diet has been
identified for a given number of samples, it is possible to estimate dietary
overlap between different species.
Last summer, I worked with my fellow ringers at the Gower
Ringing Group to collect faecal samples from reed warblers, sedge warblers and
Cetti’s warblers ringed at Oxwich Marsh. Ringing took place at least once a
week between mid-April and early September, so there was plenty of opportunity
to catch warblers. Samples were collected non-invasively when birds defecated
into clean bird bags while waiting for processing. Generally, this was a very
straightforward task with most birds leaving a sample for us after a few minutes! Each
was assigned its own ID so that the sample could be traced back to the bird and
its biometrics. The samples are currently stored at Cardiff University for
analysis and I am in the process of extracting the prey DNA in the lab.
To determine prey availability, I monitored invertebrates in
the marsh three times over the summer to compare emergences of different prey groups.
I wanted to include invertebrates from the reedbeds and the
surrounding scrub, since warblers are known to use a variety of foraging
habitats in the marsh. Once I know what is available at each time period I can
compare this to what was eaten by the birds  which will allow me to
detect dietary preferences for each species over the summer.
Sedge warbler (Keith Vaughton)
Next Steps
My fieldwork with the Gower Ringing Group was a lot of fun
and great practice for my bird ringing! Following on from last summer I am
planning to do a similar, smaller scale study this summer to look at the between
year changes in diet at Oxwich Marsh.
Thanks to everyone at the Gower Ringing Group for their
invaluable help with this project!
Sarah Davies
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