Galapagos finches are known for their wondrous diversity of beak shapes. Their variety of bills are remarkably well adapted to their different feeding habits, which range from cracking seeds to sipping nectar to extracting insects from tiny cracks using cactus spines. Just last month, the genomes of all of Darwin’s finches were sequenced, allowing researchers to pinpoint the genetic basis of their beak variation. And now, a new study published in Nature Communications adds a previously overlooked item to their regular menu: flowers. Turns out, 83 percent of land birds in the Galapagos Islands have expanded their diet to now include nectar and pollen.
Compared to mainland habitats, isolated islands typically harbor a lower diversity of plant and insect species, since dispersing over vast watery expanses of ocean can be difficult for them. Birds, however, can fly to these different regions, but then they’re faced with a limited buffet of bugs and plant matter. As a result, insect- and seed-eaters begin to widen their diets to include floral resources. They end up interacting with more species than their mainland counterparts in a phenomenon called “interaction release.” Until now, it’s never been reported for an entire community before.
For four years, a team led by Anna Traveset of the Spanish Research Council measured flower visits and pollen transport by birds on 12 Galapagos islands during peak flowering season. Their study represents 19 out of the total 23 land bird species on the islands, and in addition to finches, these included warblers and flycatchers.
Every species they observed interacted directly with flowering plants, suggesting that birds act as key pollinators all across the archipelago. Such a large expansion of a feeding niche has never before been reported among vertebrates. To the right is a small ground finch (Geospiza fuliginosa) pollinating the flowers of the common shrub Bursera graveolens in the Galapagos.
Furthermore, when they compared the bird-flower interactions of this island ecosystem to that of the South America mainland, they found that the Galapagos network is more highly connected—suggesting a greater degree of diet generalization. That means that birds don’t discriminate between native or invasive plant species: Of the 106 plant species the birds visited, 29 percent were alien to the Galapagos. Expanding their diet may have helped their survival upon arrival, but it may have also led to the unwanted pollination of invasive plants.
Images: Ruben H. Heleno