Orioles

A pair of Baltimore Orioles at their nest

Several species of orioles migrate north to breed in North America, but most oriole species are resident in the tropics. Recent research suggests that many tropical species have evolved from migratory ancestors.

■ The shape of birds’ eggs varies in different species from nearly round to more elongated. The non-round eggs can be either symmetrical (extended evenly into an ellipse) or asymmetrical (strongly pointed at one end). Many possible explanations for this variation have been proposed. A recent study comparing a large sample of eggs from fourteen hundred species found a surprising link between egg shape and flight habits. Species that spend more time flying, or are stronger fliers, tend to lay eggs that are less round. This suggests that egg shape has evolved, in part, in response to the requirements of flight, but the reason is still unknown. One possibility is that as birds’ bodies evolved to be light and streamlined for efficient flight, round eggs no longer fit. An elongated egg can hold the same volume as a round egg while also being narrower, and might be required in species that are more committed to flight.

The Song Sparrow flies less and lays less pointed eggs (left) than the Baltimore Oriole (right).

Baltimore Oriole Population Chart

The following chart shows just over one year in the population cycle of the Baltimore Oriole (other songbirds are similar). In this chart a breeding population of thirty adults (in blue) produce 100 eggs (yellow). From those eggs fifteen birds return the next spring and join the fifteen surviving adults from the previous year to keep the breeding population at thirty and produce another 100 eggs.

A population graph of Baltimore Orioles

Key points:

•Young birds outnumber adults during fall migration.

•Fully half of each year’s breeding population is made up of first-time breeders.

•The entire system is extremely fragile. If no young are produced one year, the population drops by half, and tiny disturbances can nudge the percentages and make the difference between an increasing or declining population.

How long do birds live?

Most individuals live less than one year. If they make it to their first breeding season, songbirds generally have about a 50 percent chance of surviving each year. Given the 50:50 odds, about one songbird out of a thousand would make it to ten years and one in thirty-three thousand would live to fifteen years, but that might be beyond the potential lifespan. Banding records show that the oldest Baltimore Oriole known was twelve years old, and the oldest songbird was an American Robin, at just under fourteen years. Larger birds generally live longer. For example a Bald Eagle reached thirty-eight years. And seabirds are particularly long-lived, with one Laysan Albatross at least sixty-seven years old and counting. These are remarkably long life spans compared with similar-sized mammals, especially considering the high metabolism of birds.

Do birds mate for life?

In songbirds, the answer is “Yes, but…” In a Baltimore Oriole pair, if both survive the winter, they will most likely find their way back to the same territory, recognize each other, and make another nesting attempt together. But the male and female each have a 50 percent chance of surviving the year, leaving only a 25 percent chance that both will make it back. So yes, they generally mate for life—but in most cases that only lasts for a single breeding season.