In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here are questions from our June 2015 issue:
Monarch butterflies are said to be poisonous to birds because, as caterpillars, they accumulate toxins from milkweed. Do any birds become poisonous because of what they eat? — Michael Maxwell, Des Moines, Iowa
Only a few birds are toxic: at least three species of pitohui and the Blue-capped Ifrit, all found in New Guinea. Their skin and feathers contain one of the batrachotoxins, the same alkaloids found in poison dart frogs. Like the frogs, the birds do not produce the toxins themselves; they acquire it from food they eat. Their prey is thought to be blister-beetles.
A few other bird species, from New Guinea and elsewhere, have tested positive for the toxins but generally not at the same levels or as consistently as the pitohuis and ifrit. The relatively low concentration of toxins probably protects against ectoparasites rather than larger predators.
In the February 2015 issue, you said pigeons and doves regurgitate a milky substance to feed nestlings. What is in this substance, and can other birds produce it? — David Patrick, New York, New York
Male and female pigeons and doves produce the substance. It’s created by the sloughed cells that line the crop, the structure that many birds use to store food prior to digestion. The thick liquid is approximately two-thirds protein and one-third fat; it also includes antioxidants and other trace substances that help enhance chicks’ immunity. It is regurgitated and fed to the nestlings for about 10 days after hatching. Although the substance often goes by the name “pigeon milk,” flamingos and male Emperor Penguins produce similar liquids.
About Julie Craves
Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She writes about her research on the blog Net Results, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds.
A version of this article was published in the June 2015 issue of BirdWatching. Subscribe.