Precise cache detection by olfaction in a scatter-hoarder bird

The sense of olfaction has been traditionally considered of low relevance in bird biology. In particular, the location of cached seeds by scatter-hoarder corvids is assumed to depend on spatial memory, whereas no study has demonstrated the use of olfaction to detect these seeds. In this study we tested whether olfaction might play a role in the precise location of cached nuts by magpies, Pica pica, a scatter-hoarder corvid species. We conducted an experiment under natural conditions where nuts belonging to four treatments were buried, simulating natural caches, and we monitored the rate of nut detection by magpies. Three of the treatments included nuts with natural odour or odour of a strong-smelling substance (untreated nuts, nutshells filled with soil and nuts soaked in diesel), whereas the fourth treatment consisted of 3D-printed fake nuts made of polypropylene. The experiment was replicated in two sites and in several breeding territories, using a total of 624 nuts. Motion-activated cameras were installed to identify the animals foraging in the plots. The results show that magpies detected untreated nuts, nutshells or nuts soaked in diesel ca. three times more than 3D-printed fake nuts. Among detected and unburied nuts, the 3D-printed nuts were largely abandoned on site, followed by nutshells without a kernel, natural nuts and nuts soaked in diesel. All this strongly supports the hypothesis that magpies use olfaction to pinpoint cached nuts, either for nut recovery or pilfering, but once a nut is detected they may use other senses such as vision or touch for selection. Given the strong changes in landscape features that may occur under natural conditions between the time of nut caching and nut retrieval, we suggest that the use of olfaction might be crucial to help spatial memory in determining the location of cached nuts in corvids. (c) 2020 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.