Specialization by territorial hummingbirds on experimentally enriched patches of flowers: energetic profitability and learning

Abstract

On unmanipulated hummingbird feeding territories, time spent foraging in patches was directly proportional to the number of flowers they contained. After we added nectar to the flowers in some patches, foraging increased in those patches, decreased in other patches, and decreased overall relative to control conditions. Specialization on enriched patches and reduction in foraging effort quantitatively reflected the energy we added, but the reduced effort had little effect on overall energy expenditure. Nevertheless, rates of net energy intake increased under enrichment, because gross intake rates increased. These experiments provide evidence that hummingbirds allocate effort among patches, using memory of relative patch quality when they initiate bouts of foraging. The strongest evidence is that individuals begin each day of foraging as if it were the previous day. On mornings after control days, birds foraged among patches as they had the day before, even though there was an (undiscovered) bonanza on their territories. On mornings after days of enrichment, however, whether or not a patch was enriched again, birds specialized on it at first. If the patch was enriched again, they continued to exploit it; if not, they reverted to normal patterns of allocation.