Post provided by Davide Gaglio and Richard Sherley

Digital photography has revolutionised the way we view ourselves, each other and our environment. The use of automated cameras (including camera traps) in particular has provided remarkable opportunities for biological research. Although mostly used for recreational purposes, the development of user-friendly, versatile auto-focus digital single lens reflex (DSLR) cameras allows researchers to collect large numbers of high quality images at relatively little cost.

These cameras can help to answer questions such as ‘What does that species feed its young?’ or ‘How big is this population?’, and can provide researchers with glimpses of rare events or previously unknown behaviours. We used these powerful research tools to develop a non-invasive method to assess the diets of birds that bring visible prey (e.g. prey carried in the bill or feet) back to their chicks.

Obtaining detailed information about the diet of wild animals is challenging though. What little information is available typically only provides a rough estimate of the more common food items eaten and may exclude important diet variations according to habitat type or season. An accurate and comprehensive knowledge of a species’ diets, and an estimate of the size of prey caught, can contribute greatly to our understanding of trophic relationships.

Appreciating the inter-dependency of resources between and within species as they compete for food is becoming increasingly important for wildlife conservation and management. A better understanding of these interactions and habitat requirements helps to ensure sound conservation management. Dietary studies can be powerful indicators of anthropogenic impacts and environmental changes on food-webs.

Disturbance-free Sampling

One of the main problems with traditional diet studies is that most techniques are fairly invasive. Historically, the most dramatic method was to shoot the animal just to obtain its stomach contents! Less extreme methods for birds include induced regurgitation, use of emetics, stomach flushing, or the application of neck-collars to prevent chicks from swallowing. At the very least these raise ethical concerns about animal welfare, and at worst they are detrimental at the individual or population level. For obvious reasons, such methods are especially hard to justify when dealing with threatened species.

Our solution was to develop a non-invasive, accurate and user-friendly diet sampling protocol using digital photography which works for any animal that carries prey, but is particularly useful for colonial breeding species. We used the method described in our recent Methods in Ecology and Evolution article – Dietary studies in birds: testing a non-invasive method using digital photography in seabirds – to investigate the diet of breeding terns, which carry prey items in their bills.

Advantages of Non-Invasive Sampling

We tested the methodology on a large breeding colony of Greater Crested Terns breeding on Robben Island, famous for being home to the prison that detained Nelson Mandela for 18 years during the apartheid era. Using a DSLR camera with a telephoto zoom lens, we were able to compare the photographic method to a more traditional diet sampling method: collecting prey regurgitated by chicks during ringing activities.

Adult terns carrying prey in their bills to feed mates or chicks were photographed from far enough away to cause minimal disturbance, providing thousands of images. The approach had several key advantages:

1. There was virtually no limit to the number of pictures that could be taken
2. It is easy to share images with experts so most prey items could be identified to species level
3. The size of prey can be measured fairly accurately and precisely relative to the birds’ bill length
4. Images can be easily archived and re-analysed without loss of data quality (i.e. samples do not deteriorate)

Also, by using other indirect methods – such as measuring stable isotope ratios in the blood and feathers of adults (amongst other things) – along with photo-sampling we would be able to estimate trophic discrimination factors in wild animals.

How Big a Fish?

The ecological information provided by prey size is almost as important as prey identification. It can  give us information on the prey cohorts targeted and even estimates of predator energetics. However, the estimation of prey size from photographs presents some challenges.

Terns typically carry prey crosswise in their bills (see image to the right) – this complicates the direct comparison of fish length to bill length, due to the curvature of the prey. To overcome this problem, we used allometric regressions between fish features that we could measure in most photographs, such as eye diameter and fish standard length.

By sampling this relationship in 50 dead anchovy – the species most often eaten by the terns – and cross-validating the results with another sample of fish, we were able to estimate fish length in an image with high accuracy and precision (> 97%). Fortunately, the length of the tern’s culmen (upper mandible) varies little in this species, so we were then able to estimate the length of the terns’ main prey from thousands of photographs in the field. Finding a similar body part that can be measured easily and varies little between individuals is the key to applying this method to other species.

Spreading the Word

Another advantage of this method is that it can involve citizen scientists. As a hobby, bird photography is growing in leaps and bounds, and this kind of project can harness the expanding army of amateur photographers. Digital images can be shared easily using digital platforms (e.g. I-spot, Google Images, Virtual Museums, Blogs) to facilitate global collaborations, and develop data archives to answer as yet unforeseen questions. Given the growing need to assess environmental changes and human impacts on natural ecosystems, our methodology offers a novel tool for collaborative efforts in conservation.

To find out more about using digital photography for dietary studies, read our Methods in Ecology and Evolution article ‘Dietary studies in birds: testing a non-invasive method using digital photography in seabirds’.