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After watching migrant Swainson’s Thrushes refuel on fruiting shrubs in Michigan one fall, an observer felt a bit sorry for the plants. “After all that work producing fruit,” he said, “and the thrushes come in and wipe it out.” But he had it wrong. That’s exactly why shrubs produce fruit.
In nature, it is common to see interactions between species that can be beneficial to both. Birds need food to eat, and plants need to disperse their seeds. “Let’s make a deal,” said some ancestral bird. “You put some delicious pulp around your seeds, and we’ll disperse them for you.” Scientists call these interactions symbiotic or, if the interactions are required by the species, mutualistic. The relationships develop over time as a result of co-evolutionary changes. One common plant strategy for seed dispersal was to develop flowers that formed fruits, which contained and protected their precious seeds. Fruit-eating animals are called frugivores, and birds are the most important frugivores of all. More than 300 trees, shrubs, and vines in North America have small fruits that depend on birds for seed dispersal.
Most seeds are dispersed through defecation, while others are regurgitated. As a general rule, larger seeds relative to the size of the bird are regurgitated and smaller seeds defecated. As a bonus, defecated seeds are deposited with a good nitrogen fertilizer and, sometimes, have scoured seed coats from gizzard action, which seems to increase the rate of germination.
A plant’s production of high-energy fruits is a costly metabolic process, as it doesn’t use the products directly. But they are an important part of the bartering arrangement. The synthesized fatty lipids in most fruits, for example, are an important source of energy for migratory birds.
A key adaptation
Many bird species that eat fruits also eat insects, and their digestive tracts are designed as generalists, having well-muscled stomachs (gizzards) and long intestines to give more time and surface area to absorb digested food. Birds that feed primarily on fruits, on the other hand, have reduced musculature of the gizzards and shorter intestines. This physical adaptation seems to ensure food passes through the gut quickly — in minutes rather than hours — allowing the bird to forage rapidly when food is available. And some believe the faster the seeds move through the bird’s digestive tract, the less time there is for seed damage. A chemical laxative, for example, was discovered in the fruit of a shrub in Costa Rica that induced early defecation, thereby reducing the time the seeds spend in the bird’s digestive tract.
The most frugivorous birds in North America are the Cedar and Bohemian Waxwings and the Phainopepla. Waxwings feed largely on fruit during most of the year but add large numbers of insects during the breeding season. Outside of the breeding season, waxwings are usually in flocks and rather nomadic as they search for patchily distributed fruit trees.
The Phainopepla inhabits dry regions in the Southwest and has a preference for the berries of the parasitic mistletoe, often found on mesquite trees. The mistletoe seeds are defecated with a sticky substance that causes the seeds to stick on tree branches, where they germinate and start a new plant.
The timing of fruiting is generally related to the abundance of frugivores rather than the seasons. For example, one would expect plants in warmer regions to have ripe fruit earlier than fruits from cooler, northern regions. But that is not the case. In both North America and Europe, the peak of fruiting corresponds to the southern advance of the frugivores that disperse the seeds of the plants.
Frugivorous fall migrants include thrushes, catbirds and thrashers, tanagers, vireos, and warblers. One of the best-known relationships is between the Yellow-rumped Warbler and the flowering shrub, wax myrtle. This preference was so strong that the eastern birds were called Myrtle Warblers before they were lumped with the western Audubon’s Warbler as Yellow-rumped.
While frugivorous birds do a service for plants by dispersing their seeds, fruit-eating mammals tend to eat the seeds. To counteract that behavior, some plants synthesize toxic chemicals that cause mammals to become ill or die from feeding on the seeds. Because of different feeding behaviors and metabolism, however, birds are not affected.
Another plant strategy to prevent mammals from eating seeds is adding chemicals that are distasteful. Capsaicins, the chemical that gives chilies their fiery attribute, prevent woodrats from eating the seeds but don’t inhibit the thrashers that typically disperse chili seeds. This is likely due to birds having fewer taste buds and the thrasher eating style of gulping — quickly swallowing the fruit whole.
The adoption of a frugivore lifestyle provides birds with an enormous source of high-energy food. And it’s well worth the price of dispersing some seeds. The special relationship demonstrates the amazing adaptations that can result from natural selection on two unrelated species.
This article from Eldon Greij’s column “Amazing Birds” appeared in the January/February 2018 issue of BirdWatching.
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