The following is extracted and modified from my book, Beaks, Bones, and Birdsongs: Rice farmers lose millions of dollars each year in lost crops and expenses incurred trying to control pest birds and are looking at any solution that’s not too expensive or difficult. Chemical deterrents to make the rice taste bad have been tried but they have proven expensive, ineffective, harmful to the birds, or not safe for food crops. One chemical that has seen moderate success is methyl anthranilate which has been used to protect rice, fruit and corn crops against some avian pests and golf courses against the ravages of Canada Geese.
Lincoln and Jane Brower, colleagues, and students studied various examples of chemical defenses. The classic example of bird predation being stymied by bad taste is that of Blue Jays eating a Monarch butterfly and vomiting shortly afterwards. The caterpillars of the Monarch feed on milkweeds and ingest cardiac glycosides which makes jays ill. The response is so quick that the birds learn not to eat these butterflies or the Viceroy that mimics the Monarch’s appearance. Recent studies indicate that Viceroy butterfly caterpillars accumulate salicylic acid from eating willow and poplar leaves and that they may be unpalatable on their own and perhaps the Monarch and Viceroy are co-mimics, both protected from bird predation.
Compared to other vertebrates, birds have few taste buds. Humans have around 10,000, a rabbit 17,000, cats and lizards 500, a pig 15,000 and catfish 100,000. Among birds, Bullfinches have 46, European Starlings 200, the Japanese Quail 62, and the barnyard chicken only 24. Taste buds are distributed throughout the oral cavity but mainly on the back part of the tongue and pharynx.
Most birds show little interest in sugar-laced solutions but hummingbirds easily detect differences between the content of high and low sugar solutions and the amount of time they spend feeding is based upon the concentration of the solutions; foraging on higher sugar concentrations means less time feeding.
The information on bitter taste is mixed, but birds will avoid foods high in tannins, an important chemical component of plant defense, especially in oaks. Tannins have physiological effects, reducing the digestibility of plant material and are sometimes toxic in high concentrations. Bitterness seems to be common in toxic plants, the noxious compound giving it that quality, and is often a signal to birds that the plant will cause illness and is to be avoided. Variation in the sensitivity to bitter tastes among species is to be expected, but recent studies of the White-throated Sparrow indicate that there may also be differences among individuals within a species as their genetic makeup encodes for 18 different bitter taste receptors.
Chile peppers contain the chemical capsaicin, which, as you know if you have eaten one, cause mammalian pain receptors to produce mild to painful burning sensations. Mammals learn to avoid eating peppers with capsaicin in concentrations as low as 100-1000 ppm but birds can eat peppers with concentrations up to 200,000 ppm. This evolutionary strategy assures that pepper plants will not be eaten by mammals which would destroy the plants’ seeds in their guts, but by birds who, for the most part, will pass the seeds through their digestive system unscathed and disperse them.
There is still a good deal of information to be uncovered and elucidated about the taste of birds. Experimenting with birds and various substances in the lab tells us something but the information provided by ecological and behavioral observations of birds in the wild is needed to give us the complete picture.