WHY HUMMINGBIRDS HAVE SUCH LARGE CROPS

Male Anna's Hummingbirds (Calypte anna) defend territories that contain a predictable food source, floral nectar. For such a hummingbird, the meal size that maximizes long-term net energy intake is less than the maximal crop volume. Smaller meals must be consumed more frequently, but larger meals increase body mass and therefore flight cost. Individuals without territories or with inadequate territories do not have easy access to nectar and intrude on territories owned by other C. anna, where they may be chased at any time. It was predicted that these intruders should minimize the number of potentially risky intrusions necessary for maintenance by ingesting as much nectar as possible whenever they manage to feed without being chased (usually when owners are temporarily absent). Therefore, relative to uninterrupted feeding by owners, uninterrupted intruders should feed longer and take larger meals. Field observations supported these predictions. Intruders apparently filled their crops in all seasons, whereas owners ingested smaller amounts (0.21-0.22 ml) and fed for lengths of time consistent with the prediction of an optimization model (0.21 ml). Thus, owners may energetically optimize meal size whereas intruders fill their crops whenever they are not chased. Under most conditions, hummingbirds only fill their crops one-tenth to one-third full, leading to the question why hummingbirds have such large crops. This study demonstrates that a large crop volume may be of survival value when an individual lacks a territory or has inadequate access to resources and must poach on others' territories.