Niko Tinbergen and the red patch on the herring gull's beak

One of the classic studies in animal behaviour is that by Niko Tinbergen on the stimuli that elicit begging behaviour in young herring gull, Larus argentatus, chicks. Tinbergen examined which features of the beak induced chicks to peck by using various cardboard models of herring gull heads in different shapes and colours. Leiden University, home university to Tinbergen at the time, still has a summarizing overview of the data of the initial experiment of the study. In that experiment, models with a yellow beak with patches in various colours and different positions were presented to young chicks. In this essay, I relate those earliest data to the various publications in which Tinbergen discussed them over the years. Subsequent publications became more and more detached from the original data, resulting in sizeable discrepancies between the original study and later descriptions of when and how the experiment was done and its outcome. I first sketch the scientific context of the herring gull study. Next I present the original data and document the subsequent changes. I discuss what might have led Tinbergen to modify his account of the study over the years, relating it both to its historical context and to the issue of ‘expectancy biases’.