Sunbirds increase foraging success by using color as a cue for nectar quality

Nectar concentration and composition vary widely between plant species. Nectarivorous birds that associate floral characteristics with nectar quality may be able to avoid less rewarding flowers and therefore forage more efficiently. We assessed the abilities of amethyst sunbirds (Chalcomitra amethystina) to utilize color cues to discriminate between concentrated (1M) and dilute (0.25M) sucrose solutions. In an outdoor aviary, birds were presented with 3 rich feeders among 6 poor feeders on a feeding board. Following the assessment of baseline performance with optically identical feeders, color cues were added to the feeders for a 6-h training period. To assess the retention of learnt visual cues, birds were tested 1 and 7 days after the training. Observations lasted for 3h, with feeders being rearranged every 30min to minimize spatial learning. In the absence of color cues, birds selected feeders randomly, but when color cues were available, they visited more rich than poor feeders. This more rapid identification of rich feeders resulted in a decrease in feeding duration and feeding frequency compared with the baseline performance. Energy uptake from rich feeders and therefore the rate of energy gain increased when birds foraged with color cues. No differences were found between Days 1 and 7 after training. Total energy intake decreased with visual cues, which may indicate a reduction in foraging costs when cues allow for location of rewarding feeders. Our findings demonstrate that sunbirds forage more efficiently with cues, which may lead to increased fitness.