Reprinted from: Purple Martin Update 3(1): 14
James R. Hill, III
Purple Matin Conservation Association
The Purple Martin is the earliest tropical-wintering migrant to return to the North American continent. Species-wise, however, their migration is an extremely protracted affair, with 2 to 3 months separating the arrival of the first martins and the last martins at any given colony site. Most people don't realize this and panic if their martins are a few days late in returning, or if their numbers seem down early in the migration.
The oldest martins (i.e., the 5 to 7-year-olds) are the first ones back at their colony sites, followed a week or two later by the 3 to 5-year-olds. Two-year-olds are the next to return, followed finally by the "1-year-olds." These 1-year-olds (i.e., the subadults = yearlings = SY's) are actually only 10 to 11-months old when they arrive, but are ready to breed. They begin arriving about 4 to 6 weeks after the earliest adults (= ASY's) at any given location.
The staggered return schedule of martins at their colony sites results in several interesting consequences. For one thing, it results in a staggering of breeding activities. At PMCA headquarters, nest building begins as early as the second week of May and the earliest eggs are laid during the last week of May, long before many subadults have even arrived. An incidental effect of this staggered arrival is that late winter/early spring storms rarely have a chance to wipe out an entire colony site, although this is not how or why staggered migration evolved.
Why have martins evolved a migration schedule that gets them back at their colony sites as much as 4 to 5 weeks before nest building begins despite increased risks of starvation? Because they are secondary-cavity nesters and there is intense competition for the limited supply of suitable nesting cavities. Quite simply, if they don't get a cavity, they don't breed.
Finally, why is martin migration so spread out? Because older martins are more experienced than younger ones at navigating back to their previous nesting sites and because they are better at finding food during the foul weather so characteristic of early arrival. Yearling martins lag the farthest behind because they remain in Brazil to finish their first prenuptial molt and because they are less skilled at finding food during hard times. They also lag behind because they are so subordinate to adult birds and would have great difficulty competing, early on, for mates and cavities.
|A chart showing the 1990 arrival schedule of Purple Martins at the Purple Martin Conservation Association's research grounds in Edinboro, Pennsylvania. Note that martin arrival is spread over an 8-week period. This is typical for martins in the northern half of their breeding range. In the southern half, arrival is spread over an even longer period. The "scouts," in this particular case, two adult females, arrived on April 12th, 1990, right on schedule. The average date-of-first-observation for the Edinboro area, based on 40 years of data, is April 13th. The first subadult arrived on May 12th, exactly four weeks after the first adult. Approximately 100 pairs of martins bred at the PMCA research site in 1990. About 30-40% of these pairs had at least one subadult member. There were about a dozen subadult bachelors.|
Copyright 1991 by Purple Martin Conservation Association. All Rights Reserved.
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