The fruits of selectivity: how birds forage on Goupia glabra fruits of different ripeness

Although many studies have been published on avian fruit selection, few have addressed the effects of fruit scarcity on the patterns of fruit choice. Here, we compared the consumption of seven bird species for six simultaneously present maturation stages of Goupia glabra fruits. Ripe G. glabra fruits contain more lipids, carbohydrates and energy, and fewer phenols, than unripe fruits. All bird species selected from among ripening stages and removed a higher proportion of ripe fruits than of intermediate or unripe fruits. Importantly, however, fruit choice was flexible in all species. Whether birds preferred or avoided fruits of intermediate ripeness depended on the overall fruit supply. When ripe fruits were scarce, birds showed a higher acceptance of fruits of intermediate ripeness, but still rejected the least ripe fruit stages. In a foraging bout, most birds fed on fruits of the same ripeness. By doing so, birds maximised instantaneous energy gain per time, because search time was longer for riper fruits while energy intake was lower for less ripe fruits. The results suggest that birds select fruits based on fine-scale differences in profitability, but accept less profitable fruits during low fruit abundance. If environmental factors such as overall fruit availability influence avian fruit choice, we suggest that the potential for directional selective pressures on fruit compounds is restricted.