Reprinted from: Purple Martin Update 8(1): 7
James R. Hill, III
Purple Martin Conservation Association
Range & Migration-timing Map: Purple Martins breed in Canada, the United States, and Mexico, and winter in South America. The purple area on the map corresponds to their breeding range. The horizontal lines mark the average first-arrival dates of older adult martins at established breeding sites. Landlords with younger or smaller colony sites typically experience slightly later return dates. Yearling martins (i.e., subadults = SY's), the age group that typically colonizes new breeding sites, begin arriving about 4-6 weeks after the earliest adults, and continue arriving for an additional 4-8 weeks in the northern half of the breeding range, 12-16 weeks in the southern half. Generally, it is during this later time period that martins colonize NEW sites. This is because most adults aren't looking for new breeding sites; they are on their way back to their breeding sites from the previous year. Martins show a high level of site fidelity and only switch breeding sites if they experienced reproductive failure the previous season, or if their colony site has deteriorated. During migration, adult martins will use housing as temporary "motels," but are unlikely to stay and nest. So, unless you have an active site yourself, or there are active colony sites within one mile of you, don't open your housing up until about 4 weeks after the dates on this map; you'll just attract nest-site competitors. If there is an active site within one mile of you, open your housing around the dates on this map. Your site may attract some colony-site switchers from nearby housing. If you have an active colony site (a site that had breeding martins the year before), make your housing available around the dates shown on this map.
Three Subspecies and Their Breeding Habits: There are three subspecies (races) of Purple Martins. East of the Rocky Mountains, Progne subis subis nests today only in supplied houses and gourds. Nests in natural cavities are extremely rare. West of the Rocky Mountains, Purple Martins have different nesting habits. In the Pacific Northwest, Progne subis arboricola still nests naturally, and also uses clusters of single boxes and gourds. Neither they nor southwestern martins (see below) nest in the multiple room houses. In the southwestern states, Progne subis hesperia nests in old woodpecker cavities in saguaro cacti.
Arrival Dates and Migration Routes: The timing of the Purple Martins' northward advance coincides with the warming temperatures that support the emergence of flying insects, their only food source. Martins follow at least three different pathways northward from South America: Some are circum-Gulf migrants, going around the Gulf of Mexico through Mexico. Others are trans-Gulf migrants, making the 600-mile water crossing from the Yucatan peninsula to Louisiana. Others hop the Carribean islands to Florida. Based on the February 1st arrival line (see map), the island-hopping route is shortest, the trans-Gulf route the next shortest, and the circum-Gulf route the longest.
Timing: Most 'would-be' martin landlords rush to get their martin housing opened up for the arrival of martin "scouts" in their particular area. As stated above, this is 4-6 weeks too early for new sites, and decreases chances for success. Contrary to folklore, "scouts" are not looking for new breeding sites for their flocks. "Scouts" are simply the first martins to arrive in, or pass through, an area on their way back to last year's nest sites. Scouts aren't likely to switch to housing at new sites.
When to Close Martin Housing: If martins don't find your housing right away, don't give up. Breeding martins can arrive, build nests, and begin to lay eggs well into late June. This is true even in the southern states. And DO NOT close or take down your housing if you fail to attract breeders. Fledgling martins wander in July and August looking for potential breeding sites where they will return and nest the following summer, so housing at new sites should be kept open and free of nest-site competitors until the end of August. After August, housing should be brought inside or closed.
Copyright 1997 by Purple Martin Conservation Association. All Rights Reserved.
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