Spatial memory test

In this study we replicated a design previously used to assess spatial memory in free-living hummingbirds^38–42. Spatial memory was measured on free-living long-billed hermits at 3 leks. We placed 900 ml commercial hummingbird feeders (Perky Pet #209B) at 1–2 locations surrounding leks. Feeders were modified to have a single opening for accessing “nectar”. Three feeders arranged in a row (one next to the other; Video S1) were made available at a consistent location for each lek. A single feeder was filled with clear sugar water (the rewarding feeder; ~100 ml of water with 25% sucrose concentration) while the other two contained only water. The experiment trial consisted in two phases: a search phase in which visiting individuals identified the rewarding feeder and a return phase (the first visit after identifying the rewarding feeder) in which the ability to recall the position of the rewarding feeder was evaluated (Video S1). Spatial memory was tested when the position of the rewarding feeder was the same in both phases. Only the first visit after the search phase (i.e. the first return) was evaluated. Each feeder configuration lasted in average 52 min (+/−27 S.D.) and birds visited the feeders in average every 35 min (+/−9 S.D.). Performance during the return phase was coded as a binary variable (0 = fail, 1 = success) in which successful visits required visiting the rewarding feeder first. As long-billed hermits do not defend feeding territories, many individuals could be observed visiting the feeders in a short period of time, Therefore, several individuals were tested simultaneously (i.e. with the same feeder setup during the same days). The position of the rewarding feeder was changed after most visiting individuals have completed the return phase and the experiment was run until most visiting marked individuals had completed at least 10 trials. However, we also included individuals with at least 5 trials in the analyses given sample size limitations. The individual spatial memory score was calculated as the average of the performance scores (range: 0–1; 1 means perfect performance). There was considerable variation in time intervals between visits, mean number of daily trials and total number of trials. Hence, we also evaluated whether these factors influenced the spatial memory performance.