Forest Service researchers at the Institute of Tropical Forestry in Puerto Rico studied tree species preferences for foraging by insectivorous birds in a novel Prosopis-Leucaena woodland in Puerto Rico. The study testedt the role of foliage palatability as a factor in tree species preference. The foliage palatability hypothesis posits that tree species preferences for forging by avian insectivores are based on the abundance of their arthropod prey, which is associated with foliage nutrition and palatability. This hypothesis was tested in five alien and one native tree species in a dry secondary woodland, dominated by introduced tree species, by relating foraging preferences to foliage arthropod biomass and leaf chemistry. The most frequently preferred tree species for foraging were the alien Prosopis julifera (preferred by five bird species) and the alien Pithecellobium dulce. Both tree species had high foliage arthropod biomass and, as expected, both had high Nitrogen content, low lignin-nitrogen ratios, and low hemicellulose content. In contrast, the alien Tamarindus indica foliage had low nitrogen, high lignin and hemicellulose content, and low arthropod biomass and was avoided by all bird species. The native Buccida buceras had tough leaves with low nitrogen content, low arthropod biomass, and was avoided by four bird species. Compounds, previously known to affect herbivorous insect responses to Albizia lebbeck and Leucaena leucocephala, likely contributed to the low arthropod biomass despite high nitrogen content in Albizia and avoidance of Leucaena by four bird species despite its high arthropod biomass. Herbivorous insects were deterred by high hemicellulose content in leaves as evident in a negative correlation between hemicellulose content and herbivorous arthropod biomass. The foliage palatability hypothesis applied to alien tree species in this novel woodland where some alien tree species had highly palatable foliage with high arthropod biomass and hence were preferred by avian insectivores. Thus, high foliage palatability of some alien tree species may weaken the effect of enemy release, which occurs when introduction of a plant species allows the plant to escape its specialized enemies from its homeland and prosper in its new environment where its specialized enemies are absent, in some novel plant communities.