Mutualism of the Month: Nectar robbers and pollinating birds

The purple sunbird drills into desert teak flowers and steals nectar without pollinating the flower. Photo source.

The purple sunbird drills into desert teak flowers and steals nectar without pollinating the flower. Photo source.

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between the nectar-robbing purple sunbird Nectarina asiatica and a small flowering tree, the desert teak Tecomella undulata, particularly how the purple sunbird impacts the relationship between desert teak and its two pollinating birds: the red-vented bulbul Pycnonotus cafer and the white-eared bulbul P. leucotis.

Nectar robbing refers to the act of an animal removing nectar from a flowering plant without pollinating the flower. Typically, this phenomenon occurs with flowers that conceal their nectar in a tubular corolla (botanical term for all the petals) [picture]. The animal pierces through the corolla tube to remove the nectar, incidentally, never coming in contact with the reproductive parts of the flower. Pollination of these flowers usually requires that animals (typically birds or insects) touch the male anthers of the flowers, picking up pollen and then depositing the pollen grains on the female stigma of other flowers.

The tubular flowers of the desert teak. All the flowers together are called a corolla and the this is what the purple sunbird pierces through. Photo source.

The tubular flowers of the desert teak. All the flowers together are called a corolla and the this is what the purple sunbird pierces through. Photo source.

The term nectar-robbing confers a negative connotation: the nectar-robber steals nectar at the detriment of the plant; this isn’t always the case. The positive effects become obvious when fruit-set (the amount of fruit produced by a plant) increases in response to nectar-robbers: a sign that nectar-robbing increases reproductive success.

How could this be?

By damaging the floral parts of the tree and removing the nectar within the flowers, nectar robbers can indirectly alter the behavior of pollinators.

This is what happens in the three-way relationship between the purple sunbird, the desert teak, and the bulbuls. Sunbirds can pierce more than half of the flowers of a desert teak, removing the nectar from the flowers and frustrating the bulbuls. Although nectar is replenished regularly, insufficient amounts of nectar compel the bulbuls to visit additional trees in their search for nectar. This is great for the desert teak as they are self-incompatible; their pollen cannot fertilize their own flowers. Pollinators must bring the pollen to another tree.

The red-vented (vent = rear) bulbul. Photo source.

The red-vented (vent = rear) bulbul. Photo source.

The white-eared bulbul. Photo source.

The white-eared bulbul. Photo source.

Scientists discovered this gem of information out by mimicking the sunbird. They drained all the flowers in eight desert teaks (1296 flowers!) with a small syringe and protected another eight trees (1120 flowers!) from robbing by wrapping the flowers in cellophane tape. They recorded how many flowers were visited by each Bulbul and how many subsequent trees were visited. Bulbuls that visited nectar-drained teaks visited significantly less flowers per bout (5 vs 19), but visited significantly more trees per bout when they couldn’t find nectar (7 vs 2). This should increase pollination in a self-incompatible species like teak and it did! Robbed trees had a significantly higher fruit-set. So while bulbuls have to work harder, reducing their benefit in this mutualism, the sunbirds and teaks benefit greatly from this relationship.

To read the original scientific article from the journal PLOS One click here.

Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!

Read more mutualisms of the month!