Sex and Number of Scouts

Reprinted from: Update 10(2)
James R. Hill, III
Purple Martin Conservation Association

It has long been assumed that Purple Martin "scouts" were always adult male birds. For instance, in his book, "What You Should Know About the Purple Martin," (1966, Griggsville, IL) author J. L. Wade stated: "The scouts are older males ... who arrive early, investigate available food and housing conditions, and return [south?] to bring in other members of the flock when conditions are right." Wade goes on to say: "Males usually precede females in these migrations. In some instances, the females lag behind by as much as two weeks."

Further support for this male-first perception are the statements of R. B. Layton, author of the book, "The Purple Martin," (1969 Nature Books Publishers, Jackson, MS), who said: "The male birds, acting as scouts, usually arrive well in advance of the females, then use the intervening time to establish themselves at the potential breeding site .... In instances when the females accompany the males on their first visit, bad weather may have delayed the arrival of the male scouts."

Even Dr. Charles Brown in the Purple Martin species account for the Birds of North America series (1997, AOU, Washington, DC) writes: "First arrivals are often single adult males, perpetuating the myth in [the] popular literature of 'scouts' who return south to guide other birds to nest sites. At least in e. U.S., dark-bellied males usually outnumber females in [the] local population for several weeks after first arrival, indicating that older males migrate earlier than females ..."

Not everyone accepts this "male-first" perception, however. Here at PMCA headquarters, we've seen females among the first scouts several times (see photo, facing page) and PMCA scientific advisor, Dr. Eugene Morton of the Smithsonian Institution, has informed us that based on his decade of work with Purple Martins at his former colony site in Maryland, female Purple Martins arrived first, or simultaneously with males, approximately half the time. So, is there a differential arrival schedule between male and female Purple Martins?

Gathering Sex-of-Scout Data Continent-wide

To determine what really happens on a continent-wide scale, the PMCA modified its annual Scout-arrival Survey this year to include questions about the sex and number of martin scouts observed on the first encounter of the season. The survey was entirely web-based and an invitation to participate in the survey was sent via e-mail to over 9,000 martin enthusiasts. 1129 cooperators from 44 states and provinces supplied usable data in time for this write-up. Fig. 1 shows a summary of the results.

Sex of the Scout

1046 of 1129 sites (92.6%) reported male Purple Martins among the birds seen on the first encounter of the season; 415 of 1129 sites (36.8%) reported female martins among the first scouts. 714 of 1129 sites (63.2%) reported males only; 83 of 1129 sites (7.4%) reported females only; and 332 of 1129 sites (29.4%) reported both males and females together among the first scouts. 1544 males and 511 females were reported, which is 3.02 males reported for every female reported. So, contrary to Wade, Layton, and Brown, these results show that female Purple Martins are indeed among the first wave of arriving birds, and are present in substantial numbers. In fact, I believe the arriving wave of migratory Purple Martins continent-wide, has a 50/50 sex ratio, with an equal number of males and females. If this is true, then why would the male-to-female ratio be 3.02:1 in the earliest observed Purple Martins?

I think there are a couple explanations. First, there certainly is a positive bias in the observability (detectability) of males since they are the more vocal sex, and thus, are more likely to be heard and then seen. Second, I feel that the females are present, but just more concerned with feeding/survival than they are with cavity exploration, territory acquisition, and defense, which is why the males may be more strongly attracted to housing upon arrival than females.

How Many Scouts Did Landlords Observe?

The average number of martins first seen by each observer was just 1.82 birds (see bar graph, Fig. 1). This dramatically shows that martins are not typically seen in flocks on their northward journey unless weather factors force them to congregate at good feeding or resting sites (ships at sea; offshore oil drilling platforms; landfalls along the Gulf Coast; or warm bodies of water during foul weather). Landlords whose first observation of the season included larger groups of martins likely didn't catch them the first day of their arrival and their numbers had time to build. Indeed, many of our respondents who reported a male back alone, had additional martins (including females) within hours or a day of the first return.

James R. Hill, III, is Founder and Executive Director of the Purple Martin Conservation Association. He has been studying Purple Martins since 1981.

Copyright 1998 by Purple Martin Conservation Association. All Rights Reserved.

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