Flowers throw unexpected curves at Mrs. Gould’s Sunbirds – nectar spiked with bitter toxins.
By Rex Graham
The sunbird with a hook bill co-evolved with similarly shaped flowers. But more than half of the world’s flowers add toxic chemicals to their nectar and scientists are only beginning to understand how they affect the birds, insects and other animals that consume them.
These chemicals are part of the newly appreciated reality of sunbirds. These beauties can be enjoyed on spring trips to Bhutan and Myanmar. (I recommend a 2-week tour led by an experienced local guide working with an international birding tour company.)
Mrs. Gould Sunbird trips
On these trips, watch for Mrs. Gould’s Sunbirds at the corolla tubes of nectar-rich flowers. The length and shape of the tubes function as “exploitation barriers” to shorter-billed birds. However, some birds take nectar without providing pollination services. They either force their way into the flower opening or stab a hole in the base of the flower. Sunbirds chase them away.
Sunbirds and nectar robbers
Sunbirds guard pantries of red, orange, yellow and white. However, if they abandon them the nectar volume inside a flower tube can increase 10-fold. At some point the nectar robbers return.
Bird-flower co-evolution seen in the bills of sunbirds and hummingbirds and the flowers they feed on has captivated biologists since before Darwin. He didn’t realize that nectar often contained toxic compounds. These poisons are used in a type of finely tuned chemical warfare. Flowering plants must balance the production of nectar poisons with their need to attracting pollinators and discouraging non-pollinators.
Some of the first examples of “toxic nectars” were found in wild tobaccos. Some of them add nicotine to their nectars. Almost all tobacco species produce nicotine in their leaves, but nectar, too?
The wild tobaccos are nicotine specialists, but plants produce all kinds of pharmaceutically active ingredients in a dynamic process to influence birds and insects. Some of these bitter chemicals may deter poor pollinators and nectar robbers. Some have antibiotic properties and others rid bumble bees of parasites.
The nectar of the Common Milkweed of North America is a classic story of how a plant species uses nectar defensively. The strategy could be repeated in other flowering plants across the world.
The milkweed bases its defense strategy on cardenolides. These are heart-arresting toxins that kill insects and cause birds to vomit.
The story of milkweeds and cardenolides unfolded millions of years ago as a leaf-defense ingredient. Many insects adapted focus on non-toxic food, but many laid eggs on milkweed leaves not knowing that they were toxic. The caterpillars damaged leaves, particularly those with low levels of cardenolides.
Then milkweeds added cardenolides to their nectar. Winged insects that routinely sip nectar before laying their eggs caught on. Rather than risk laying eggs on plants with nasty nectar, the pregnant insects moved on.
With the new nectar strategy, milkweeds had gained the upper hand. Commensual Bumble bee pollinators visited enough milkweed flowers to enable efficient cross-fertilization of seeds. Milkweeds flourished.
But then the Monarch Butterfly came along. The butterflies sample milkweed nectar like other insect species, but the bitterness tastes fine to them. They don’t move on. They lay eggs. Not only are voracious Monarch caterpillars able to resist the leaf toxins, but they actually sequester cardenolides. Monarch adults metamorphose with wings, thorax and abdomen packed with the unpleasant chemicals. Birds avoid them.
This example has significance beyond milkweeds in North America. Toxic nectar and pollen are ubiquitous: more than 50% of plants surveyed worldwide contain toxic “non-protein amino acids” in their nectar, 36% contain toxic phenolics, and 9% contain alkaloids.
“Where nectar secondary compounds are produced and how they are added to the nectar remain mysteries in most cases,” researchers with Dartmouth College, the U.S. Department of Agriculture and the University of Alberta said in the Journal of Agricultural and Food Chemistry.
Just as intriguing as how individual plant species use nectar ingredients to dynamically interact with birds, insects, bats and other animals. “Examining the effects of nectar chemistry on mutualistic and antagonistic flower visitors illuminates the costs and benefits of toxic nectar compounds,” researchers at Cornell University said in a 2016 paper in the journal Ecology.
The toxins story doesn’t end with leaves and nectar. Many types of brightly colored fruit contain tannins, toxic compounds that have powerful astringent effects on taste buds. Plants add them to the fleshy outer coverings of immature seeds. Tannins discourage hungry animals, but once the fruit ripen a switch in tannins can entice animals to eat the fruit and scatter undigested seeds.
Fruit-eating birds vary greatly in their reactions to tannins. For example, Cape White-eyes and Speckled Mousebirds that were fed a diet of artificial fruit laced with varying amounts of tannins ate vigorously no matter how much was added. In the same experiments, Red-winged Starlings reluctantly ate high-tannin food, but preferred the researchers’ offerings without tannins.
“Frugivores respond differently to the presence of secondary compounds in the diet,” scientists with the University of KwaZulu-Natal in South Africa said in 2014 in the journal Behavioral Processes.
A bite of clay
Parrots, macaws and other birds eat clay or other soil to help detoxify the tannins in the seeds, nuts, fruits, flowers and other plant parts they ingest. The “clay licks” of Tambopata National Reserve in the Amazon Valley of Peru are popular destinations for birdwatchers, They watch large groups of Scarlet Macaws, Blue-headed Parrots, Dusky-headed Parrots, White-eyed Parakeets and related species. (Many South American birding tours include clay licks in their itineraries.)
Sparrows tend to eat significantly more tannin-containing seeds than finches. Seeds typically contain phenolic toxins. Researchers discovered that Rufous-collared Sparrows eat much more when fed tannin-containing seeds. They use the extra nutrition to increase their metabolic rates to detoxify toxins. On the other hand, Common Diuca-finches increased their intake of water to more rapidly flush the toxins from their bodies.
The Chilean and Argentine researchers who studied the sparrows and finches said in the Journal of Comparative Physiology B that a better understanding of all the foods consumed by birds may explain their sometimes unexpected preferences for food or habitats.