Island plants and animals often differ from their mainland relatives. Why? In general, isolated islands lack top predators and large herbivores, which can influence food chains and traits of island organisms. In addition, differences in human interactions and threats posed by pathogens and parasites can also contribute to variances in traits.

In a case study involving eastern bluebirds, (Sialia sialis) researchers show just how island life shapes the physiology and life history of a species.

Eastern bluebirds are familiar to many people living in the eastern United States, and also to residents and tourists in Bermuda, an archipelago that lies in the North Atlantic Ocean about 1,100 km off the East Coast of the United States. Although the current outlook for the bluebirds in the U.S. is good, their Bermuda relatives have been designated as threatened and vulnerable.

In an effort to determine the differences of this species, researchers compared island (Bermuda) and continental (Ohio, U.S.) populations of the Eastern bluebird.

First, researchers investigated how nestlings and adults differed in growth, size and shape, immune function, numbers of eggs and nestlings that pairs produce, and how frequently parents deliver food to their young.

Researchers also attempted to identify differences between continental and island birds that might intensify the risks of decline typically associated with small and geographically isolated populations, such as the Bermuda bluebirds.

The study showed that bluebirds in Bermuda were lighter weight and had longer wings than the Ohio birds.

Also, while parents fed their nestlings at equal rates throughout the season in both locations, island nestlings grew slower and, as the breeding season progressed, more chicks died in their nests in Bermuda, though no similar seasonal pattern was observed in Ohio.

Overall, the results suggest that the Bermuda bluebirds may be adjusted to certain aspects of the island environment but not to others.

As a result, the study provides insight on how conservationists in Bermuda can manage declining bluebird populations. For example, by removing any mammalian or avian predators and competitors, or by managing human-driven changes in populations of insects (which the bluebirds feed on), both changes in survival and mortality rates and changes regarding physiology and reproduction of the species may occur.

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Island Living Shapes Physiology and Lifestyle of Eastern Bluebirds