Comparative studies of cognitive abilities in the Paridae : Evidence from laboratory studies

Sammanfattning: The Paridae (tits, titmice and chickadees) is a bird family consisting of both food hoarding and non-hoarding species. Many studies have been conducted on birds that are considered to be cognitively advanced, such as parrots and corvids, whereas much less is known about these small passerines. In this thesis, I have investigated the cognitive abilities of two members of the Paridae family; marsh tits and great tits. There is a clear dichotomy in wintering strategy across Paridae, with these two species representing either side. Marsh tits are scatter hoarders that cryptically cache food over large areas, whereas great tits are a non-hoarding generalist species that are known to be particularly behaviourally innovative. First, I tested the spatial memory of marsh tits in a laboratory specifically designed for food hoarding studies. In nature, spatial memory is essential for scatter food hoarders to retrieve cached food. Marsh tits displayed around 40% retrieval success after 10 retrieval attempts. These results were in accordance with the previous food hoarding studies conducted on captive marsh tits. I also tested humans in the same task and found they performed much better than marsh tits as after just five attempts they retrieved 80% of the hidden food. Since marsh tits have specialised spatial memory, one may have expected them to outperform humans. The fact that this was not the case, led us to consider whether the marsh tits were affected by proactive memory interference, whereby more recent memories deteriorate as a result of repeated testing. However, this was not the case, as marsh tits performed similarly in all repetitions of the test. Next, I turned my focus to great tits. It is likely to be advantageous for great tits to have access to food caches created by food hoarders during cold winter months. I investigated whether great tits are capable of memorising the locations of caches created by marsh tits. Great tits were allowed to observe marsh tits while they were hiding food items. They successfully memorised the location of caches made by marsh tits after one hour and 24 hours retention intervals. Although it is remarkable that great tits can memorise caches made by marsh tits, we do not know if this ability is found in other Paridae species. In paper IV, I therefore tested the observational spatial memorisation ability of marsh tits in the same experimental setup. Marsh tits were unable to retrieve the caches they observed being stored by other marsh tits. This implies that they do not use this strategy for foraging and that this ability is not common to all Paridae species.In the Paridae family, sex differences are most pronounced in great tits with clear differences in behaviour, morphology and social hierarchy. Hence, I investigated sex differences in cognition in great tits. In chapter V, I investigated whether males and females displayed differences in cognitive abilities using the observational spatial memory task. Female great tits out performed males in this task. In paper VI, I tested for sex differences in the motor self-regulation ability of great tits using the transparent cylinder task. In this task, a food reward is placed inside a transparent cylinder with openings at both ends. The animal must therefore inhibit its urge to reach directly for the visible food and instead take a detour to one of the open end of the cylinder. In addition, I separated the birds into two groups; one with experience of the cylinder and another with no prior experience. Cylinder-experienced birds had a similar transparent cylinder in their cage for three days prior to the experiment whereas cylinder-naïve birds first encountered the transparent cylinder during the test. Great tits were generally successful at this task. I found no overall sex difference in motor self-regulation ability, however the number of repeats that was required to master the task differed between females and males. Males quickly mastered the task, even with no prior experience. However, males with prior experience did not outperform naïve males. Although females took longer to learn the task, those with prior experience outperformed naïve females. In conclusion, I found cognitive differences between two species that differ in their foraging strategies within the same taxonomic family. I propose that these differences have developed due to the distinct winter foraging strategies of these species. I also found evidence of cognitive sex differences in great tits, which I suggest are the result of sex differences in social hierarchy.