Kestrel Diet Project Update: It's getting...Eggciting...

In case you’ve missed our previous blog introducing our Kestrel diet project, you can read it here. The fieldwork for this project involves visiting Kestrel nesting boxes distributed throughout the harbour and surrounding farmland – huge thanks go to all of the landowners who have given us permission to visit boxes on their premises. Visits include collecting pellets (regurgitated parts of prey that haven’t been digested), looking for prey remains and Kestrel feathers (these will be used for stable isotope analysis – more on that later in the blog!) and checking the boxes for breeding attempts using a GoPro camera attached to a telescoping pole. A full day of checks on Tuesday revealed exciting results – eggs!


Kestrel pellets and a female primary feather from a farmland site. Image courtesy of Jason Fathers, Wildlife Windows.


“A full day of checks on Tuesday revealed exciting results – eggs!”


 Our first box of the day set a gold standard – we saw the male and female, collected a bag of pellets and moulted feathers and our trusty pole-cam revealed two eggs in the nest box!


Throughout the day we discovered boxes in various stages of use. Some have not been taken up this year, one had a nest scrape, and in others eggs numbered from two to six. There can be quite a lot of variation in the timing of Kestrel attempts, with some pairs laying eggs much sooner than others. The number of eggs laid by each pair can also vary, with some pairs producing only one egg and others up to seven, though most will lay five.


Our box with a nest scrape in it was only put up this year (many thanks to Wildlife Windows and Men’s Shed Charity for helping us to put our boxes together) and could have been adopted by a new pair, which would explain why they are a bit behind the other pairs in the area. It’s thought that one of the reasons that UK Kestrel numbers are in decline is a lack of nesting boxes, so increasing the availability of nest sites for our Kestrels is big bonus of this project.


 Some examples of signs of breeding and breeding stage from boxes on farmalnd sites, 14th May.


Knowing which breeding stage the different pairs have reached is important as it will determine when the chicks can be ringed and feather sampled. Chicks must be ringed at a certain age, getting the balance right between when they are big enough that they won’t outgrow their ring, and young enough that disturbance won’t cause them to fledge prematurely. We have a licence from the British Trust for Ornithology to snip little bits of feather from the mantle (back) area of chicks and adults for stable isotope analysis (again, more on this diet research technique later in the blog!), and have teamed up with raptor ringing expert Jason Fathers of Wildlife Windows for ringing activities.



What are stable isotopes?


Stable isotopes are heavy versions of naturally occurring elements that can found inside the tissues of living organisms. Their abundance in an animal’s tissue can tell you what it has been eating and where it has been feeding. The most commonly used stable isotopes used in studies of animal diet and habitat use are the isotopes of Nitrogen and Carbon, which are written as d15N and d13C respectively. 



Diagram of light and heavy (isotopes) Carbon atoms. 


In the case of our Kestrels, the environmental Carbon and Nitrogen stable isotopes could be illustrated like this:



Typical Kestrel food items and habitats and Nitrogen (d15N) and Carbon (d13C) stable isotopes.



Where will we get the stable isotope data from?


We’ve mentioned that these isotopes are found in animal tissues. In our case this will be feathers sampled from Kestrels, and small bits of muscle taken from the remains of prey that Kestrels have left around their nests. We have a licence from the British Trust for Ornithology to collect small bits of feather from the mantle (back) area of Kestrel chicks. These feathers will be snipped at the same time that the chicks are ringed, which will take place in partnership with local raptor ringing expert, Jason Fathers, of Wildlife Windows.

 Kestrel feather sampling diagram. 


How exactly will the feather and prey samples tell us what the Kestrels have been eating?


The feathers and prey remains will be processed in collaboration with the University of Exeter. The samples will be dried out, chopped up very small and run through a machine that compares the isotopes in the samples to standard values.


Once we have the isotope values for our Kestrels and prey items, we can put them into a type of statistical software called an isotope mixing model. This model will compare the Kestrel and prey values and tell us which prey, and in which relative amounts, the Kestrels have been eating.



Stable isotope mixing model diagram.



If you have an occupied Kestrel box on your land in Dorset, we’d love to know about it! Please email project Principal Investigator Dr. Georgia Jones at with any information.

We’ll be collecting samples through the summer and plan on processing them later in the year. We can’t wait to share the results with you!


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