Cattle pastures in the tropics are surprisingly attractive to birds. The scattered oak and acacia trees provide insect food and cover despite the grazing underneath.
The acacia trees are the star of the show. Birds, particularly migratory birds, are much more often found in the acacia trees than the oak trees. The attractiveness of the acacias is easy to explain. There are three to four times as many insects in them than there are on oak trees.
Why are there more insects in acacias? It appears that acacias have geared their defenses against leaf-munching mammals. They have thorny spines and chemicals in their leaves that are distasteful to large herbivores. Oaks, on the other hand, have a variety of chemical defenses designed to thwart insects.
Oak trees do have more large insects than acacias. Resident birds, which are quite a bit larger than migratory birds and have broods to feed, favor large insects. Many resident birds will nest throughout the year.
For migratory birds, a lack of competition and abundant food make the acacia a favored tree.
This article summarizes the information in this publication:
Greenberg, Russell S. and Bichier, Peter 2005. Determinants of tree species preference of birds in oak-acacia woodlands of Central America. Journal of Tropical Ecology, 21: 57-66.
In mid-elevation areas of Mesoamerica, Acacia pennatula commonly occurs in mixed woods with various species of oak. During a 1-y study in Nicaragua, we found the abundance of birds in acacia was far higher than the representation of this species in the mix of trees, whether this is estimated by number of individual trees, canopy cover or foliage cover. This higher abundance was probably related to the fact that acacias supported approximately three to four times the abundance of arthropods that were found in oaks and twice the biomass. Although oak foliage supported fewer arthropods, relatively more of them were large (>1 cm). The greater preponderance of small arthropods in acacias versus oaks was probably related to the small leaflet size of acacias. However, it is likely that the higher abundance of arthropods in acacias, particularly herbivorous species, was related to the higher nutritional content of the acacia foliage (crude protein, minerals, non-structural carbohydrates) and lower content of digestion-inhibiting compounds (structural carbohydrates, total phenolics, condensed tannins). The major defensive mechanisms of acacia are mechanical (thorns) or qualitative-defence chemicals (cyanogenic glucosides) that are apparently more effective against vertebrate than invertebrate herbivores. These observations support the hypothesis that the anti-herbivore defences of acacia are primarily directed against large mammalian herbivores, rendering the foliage highly palatable to arthropods.
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