The fitness of an individual bird is a very important measure in the research of behaviour, evolution and the protection of species. But how do we quantify fitness? Is it just the body weight of an animal, or do we need to measure more variables? And how do you do this in a good manner using the appropriate statistical tools? Magali Frauendorf (NIOO-KNAW) and her co-researchers provide the answers to these questions in a recent publication in the Journal of Animal Ecology, using the oystercatcher as an example.