virtually all plant parts chosen by parrots contained phenolics and other substances known to be toxic to vertebrates; a significant proportion of their observed foods contained levels that are considered to be highly toxic. Although direct testing of toxicity of each of these food items in a bird or even a vertebrate was beyond the scope of this study, as well as ethically problematic, the brine-shrimp bioassay was useful in providing a rough measure of toxicity, partly because the toxicities of these parrot foods varied over three orders of magnitude (
cf. [39]). Indeed, the brine shrimp assay is widely used and accepted in human medicine and agriculture, as a quick and inexpensive proxy for toxicity experienced by vertebrates (see
Methods). Accordingly, from comparison with toxicities of well-known plant secondary compounds tested with the same assay, and because some of these seeds, e.g.,
Hura [60] and
Hevea [61], are known to be poisonous to vertebrates, we conclude that many of the plants consumed by parrots are toxic to vertebrates in general. Secondary compounds are well known to be produced by plants at often high cost to their own growth and reproduction (e.g.,
[62]) to deter herbivores. Plant secondary compounds are known to deter foraging in herbivorous birds and mammals, yet toxic foods were nevertheless consumed by these herbivores, presumably as a trade-off in obtaining higher nutrition or abundant available food
[54],
[57]. Parrots apparently try to avoid toxic food, but our study shows that their foods nonetheless contain measurable levels of toxins and thus avoidence of these compounds is lower priority than choosing foods with higher nutritional content. In other words, parrots are able to overlook the presence of toxins in their choice of nutritious foods. This ability separates parrots from many other avian herbivores targeting fruits of rainforest plants. Many if not most avian frugivores are unable to process toxic food and serve instead as animal dispersal agents and mutualistic partners by digesting only the fruit pulp
[63].
No one has yet studied the physiological effects of toxins or costs of detoxification in parrots. Studies of other herbivores reveal that these costs may be high, depending on the environmental demands. For example, processing of secondary plant compounds that serve as toxic deterrents to vertebrate herbivores may have significant effects on sodium and water balance or require high energy or other costs in producing enzymes or carrier proteins such as P-glycoprotein
[64]ā
[66]. Although we do not yet know of specific physiological adaptations of parrots to detoxify their food, consuming clay has been shown experimentally to function
in vivo in food detoxification in parrots
[30]. The variety of foods consumed by parrots in this study may be related to obtaining protein and lipids from toxic foods, supporting the hypothesis that herbivores should increase their dietary diversity when confronting a variety of chemically defended foods
[67]. An alternate but not mutually exclusive view is that plant secondary compounds provide herbivores with potential benefits, such as interacting with other molecules to cause foods to be more nutritious or to kill internal parasites
[68].
