No evidence for memory interference across sessions in food hoarding marsh tits Poecile palustris under laboratory conditions.

Scatter hoarding birds are known for their accurate spatial memory. In a previous experiment, we tested the retrieval accuracy in marsh tits in a typical laboratory set-up for this species. We also tested the performance of humans in this experimental set-up. Somewhat unexpectedly, humans performed much better than marsh tits. In the first five attempts, humans relocated almost 90 % of the caches they had hidden 5 h earlier. Marsh tits only relocated 25 % in the first five attempts and just above 40 % in the first ten attempts. Typically, in this type of experiment, the birds will be caching and retrieving many times in the same sites in the same experimental room. This is very different from the conditions in nature where hoarding parids only cache once in a caching site. Hence, it is possible that memories from previous sessions will disturb the formation of new memories. If there is such proactive interference, the prediction is that success should decay over sessions. Here, we have designed an experiment to investigate whether there is such memory interference in this type of experiment. We allowed marsh tits and humans to cache and retrieve in three repeated sessions without prior experience of the arena. The performance did not change over sessions, and on average, marsh tits correctly visited around 25 % of the caches in the first five attempts. The corresponding success in humans was constant across sessions, and it was around 90 % on average. We conclude that the somewhat poor performance of the marsh tits did not depend on proactive memory interference. We also discuss other possible reasons for why marsh tits in general do not perform better in laboratory experiments.