Optimal diet strategy of a large-bodied psittacine: food resource abundance and nutritional content enable facultative dietary specialization by the Military Macaw

Background Dietary specialization should arise when there is a relatively high abundance of a particular resource, where animals may select food items to obtain an optimal diet that maximizes energy intake. Large-bodied psittacines frequently exhibit a narrow dietary niche with specific habitat use, but few studies have determined whether psittacines select food resources, and how this influences habitat use.

Methods We established fruiting phenology transects to evaluate food resource availability for the large-bodied Military Macaw (Ara militaris) in semi-deciduous, deciduous, and pine-oak forest at two sites along the coast of Jalisco, during the dry season when macaws are nesting. We also determined Military Macaw diet by observations of foraging macaws along transect routes, and conducted bromatological analysis of the nutritional content of the most consumed resource.

Results Military Macaws used six plant species as food items during the dry season, and had a narrow dietary niche (Levins' B = 0.28), with 56% of foraging macaws consuming the seeds of Hura polyandra. No food resources were recorded in pine-oak forest during the dry season, with food resources and foraging by macaws concentrated in tropical deciduous and semi-deciduous forest, where H. polyandra was the most abundant fruiting tree species. When considering the proportional availability of food resources, we determined a broad Hurlbert dietary niche breadth of H = 0.67, indicating that Military Macaws consumed food resources according to their availability. Furthermore, the seeds of H. polyandra were an important source of protein, carbohydrates, minerals and moisture, and the hard fruit-casing means that these seeds are exclusively available for macaws.

Conclusions By concentrating their diet on the most abundant resources, Military Macaws may increase foraging efficiency in the dry season. The high nutrient content also means that concentrating the diet on seeds of H. polyandra may be an optimal foraging strategy for Military Macaws to meet their energy requirements during the breeding season.