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 University.
|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)|
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!