Gray Catbirds can’t pass up ripe fall berries. In this case native American Beautyberry berries.
Gray Catbirds are not alone. Birds, especially migrants, use the high fat & nutrient content of berries they co-evolved with. These nutrients help resident species prepare for winter. They also aid migratory species successfully make their trips back to tropical wintering grounds.
It is the high concentration of carbohydrates and lipids stored as fat that sets native species apart from nonnative, invasive species. Nonnative berries are high in sugar content, but low in fat content, some as low as 1% by weight. Natives in contrast can be as high as 50% fat by weight.
Birds store this fat for its high energy to weight ratio. They use it to fuel their high calorie consumption, especially migrants as they travel each leg of their journeys south. In one of the most exquisite synchronized timings in nature, native fall berries of different species ripen just-in-time. They are ready for the migratory birds’ stopovers. Or, for some species, to overwinter in southern North America rather than going all the way back to the tropics.
In return and as an act of biological mutualism, the birds disperse the native plants seeds.
An example of coevolution. The birds have selected for the most energy packed foods for overwintering or during their strenuous journey. The plants in turn evolved bright colors and bird-bite sized packaging to attract the most dispersal.
This is finely tuned mutualism is breaking down. Native berry plants are being replaced intentionally for ornamentation. They are also being unintentionally replaced by invasion.
In an even more finely tuned example of mutualism, Red-eyed Vireos’ are an example of a species whose diet proportions between insects and fruits\berries change markedly during the year. The proportions shift significantly between different seasons.
In summer & late spring their diets consist almost entirely of insects with a high proportion of caterpillars in the summer. Seeds, fruits, & berries are eaten in very small amounts.
During migrations they eat both insects & fruit\berries. In the fall, the amount of fruits\berries rises. By the time they reach their wintering grounds, their diets are almost entirely fruits.
This varied diet is serving them well & one of the reasons that their population numbers have increased .6%\yr since the mid-1960s.
Plant Natives
