Sex/Age Differences in the Breeding Success and Mate Choice of Purple Martins

Reprinted from: Purple Martin Update 7(3): 28-29
James R. Hill, III
Purple Martin Conservation Association

 

Sexual Dimorphism
and Delayed Plumage Maturation

As most martin landlords know, Purple Martins are a sexually dimorphic species, meaning that the males and females sport different and distinguishable plumages throughout life (except, in the case of martins, in their juvenile plumage). They also exhibit delayed plumage maturation, meaning that they take more than one year to acquire their adult plumage. As 1-year-olds (i.e., yearlings) each sex wears a distinguishable subadult plumage while on the breeding grounds in North America. As 2-year-olds and older, each sex wears a distinguishable adult plumage. With a good pair of binoculars, or a tripod and spotting scope, these four sex/age classes (i.e., adult male, adult female, subadult male, and subadult female) are relatively easy to tell apart.

Telling the Sexes Apart

Adult Male: Adult male Purple Martin are entirely glossy purple-black (or steel-blue). They are unmistakable in appearance and give the species its name. Male martins do not acquire this plumage until their 3rd calendar year of life. In other words, a male martin hatched last summer, in 1996, will not have the characteristic, all purple-black plumage until next summer, in 1998, (e.g. 1996 = 1st calendar year; 1997 = 2nd calendar year; and 1998 = the 3rd calendar year). Male martins molt into this plumage during their second "wintering" stay in Brazil.

Subadult Male: Subadult male Purple Martins are often mistaken for females by the novice landlord. Subadult males differ, however, from all females by some subtle plumage differences; differences that can be difficult to see without optical equipment. First of all, subadult males have a sprinkling of at least one, but usually many, solid-purple feathers in one or all of the following areas: their chins, throats, breasts, flanks, bellies, or undertail coverts (crissum). The number of purple feathers in these areas is highly variable. Some individuals may only have one, tiny, purple feather in just one of these areas. Others have so many that they are almost half-purple on front. Female martins never have any purple feathering in these areas. Subadult males also have stronger and more extensive purple on their crowns and cheeks than females do. In addition, subadult males usually have a scattering of isolated purple feathering on their napes (hind neck), backs, and rumps.

Subadult Female: In spring and early summer, subadult females are easy to distinguish from adult females by the weak purple to brownish color of their upper surface (i.e., back side) as compared to the much brighter purple of the adult females. However, as the season advances and the feathers of adult females become worn and sun bleached, it can be a bit more difficult, on some individuals, to distinguish these two female age classes using back color alone. An alternative method, is to compare the colors of the undertail coverts. The undertail coverts of adult females are usually quite dusky all over the individual feather vanes. In contrast, the undertail coverts of subadult females are usually pure white or faintly dusky, with only the central quill a darker color. One caution, however, there is a lot of variation in crissum color and this field mark should be used in conjunction with back color. And finally, subadult females are also usually slightly lighter on their breasts and bellies than are adult females, which tend to be duskier in these areas.

Adult Female: Adult females are far more purple on their crown, nape, "shoulders," back, and rump than are subadult females, which tend to be far browner in these areas. In addition, adult females tend to have slightly (to greatly) darker breasts, bellies, and undertail coverts (i.e., crissums) than do subadult females.

ASY's and SY's

Martins fledged in 1996 are subadults this year in 1997. Even though they can begin breeding when only 11 months old, they are technically in their second calendar year of life (i.e., 1996 and 1997). In ornithological parlance, they are known as "SY's," for "second-year birds." SY-M means a subadult male. An SY-F is a subadult female. Martins fledged in 1995 or earlier are adults in 1997. They are technically in their third (or greater) calendar year of life (i.e., 1995, 1996, and 1997). In ornithological parlance, they are known as "ASY's," for "after second-year birds." ASY-M means an adult male. An ASY-F is an adult female. If you are a participant in the PMCA's Project Martinwatch, you'll need to understand and use this terminology. HY = "hatching year" bird and is a synonym for juvenile. (For front and back color photos of the four sex/age classes of breeding Purple Martins, refer to Update 3(4)).

Assortative Mating

In the past, martin researchers assumed that if the male bird in a mated pair was an ASY bird, so was the female. Likewise, they assumed that only SY's paired with other SY's. We now know this is not entirely true. As can be seen in Fig. 1, of the 834 breeding pairs observed in this 12-year study, 676 (81%) mated assortatively by age class (i.e., adult males with adult females and subadult males with subadult females). The remaining 158 breeding pairs (19%), however, paired nonassortatively by age class (i.e., adult males with subadult females and subadult males with adult females). So, in Purple Martins, we can say the species exhibits positive assortative mating by age class, but not total assortative mating by age class.

 

Fig. 1. This graph shows the relative breeding success of different aged Purple Martins as a function of the age of their mates. On average, ASY-M's paired with ASY-F's have the highest reproductive success. SY-M's paired with SY-F's have the lowest reproductive success

 

A possible explanation for the positive assortative mating by age class in martins is that most adult males and females arrive at their breeding sites about 4-6 weeks earlier than most subadults do. Since pair formation begins shortly after arrival, the majority of martins only have individuals of the same age class to pairbond with.

Results

This paper reports on the average clutch sizes, hatch sizes, and fledge sizes of Purple Martins in Edinboro, PA, during the 12 seasons from 1984 through 1995, as a function of parental age. As can be seen in Figs. 1 and 2, in Purple Martins there is a strong tendency for older parents to have higher reproductive success than younger parents. Adult females lay more eggs, and hatch and fledge more young, on average, than subadult females do. The most surprising results of this study, however, are how both SY and ASY females vary their clutch sizes depending on the age of the male they are mated to (see Fig. 1 above). SY females mated to SY males lay significantly smaller clutches than SY females mated to ASY males. Likewise, ASY females mated to SY males lay significantly smaller clutches than ASY females mated to ASY males.

Fig. 2. This graph shows the relative breeding success of Purple Martins independent of the age of their mates. On average, ASY-F's have the highest reproductive success, followed by ASY-M's and SY-F's. SY-M's have the lowest reproductive success.

 

Discussion

Why would subadult females lay smaller clutches on average than their older female counterparts? One explanation could be the differences in their levels of physiological maturity. Another could be experience - subadults are breeding for the first time - adults have at least one year of prior breeding experience. Presumably, older birds are also better foragers making them better able to provision their young with food. Subadults also tend to breed later in the season, about 10 days on average, and laying a smaller clutch enables them to fledge their young about a day sooner, maximizing the amount of time their young have to fatten up prior to the southward migration.

Why the increase in how many eggs both female age classes of martins lay when mated to ASY males as opposed to SY males (Fig. 1)? This phenomenon is likely the result of females making quality assessments of their mates prior to egg laying and predicting how well their mates will be able to provision (feed) her (their) young. Apparently, older males make better parents because of their age and experience, and can handle the extra burden of feeding additional young. This study would not have been possible without keeping "Project Martinwatch-like" data.

 

James R. Hill, III, is the 46-year-old Founder of the PMCA. He has a Biology degree from the Edinboro University of PA and an Ecology and Wildlife Management degree from Penn State University. He has been a martin landlord for 17 years.


Copyright 1997 by Purple Martin Conservation Association. All Rights Reserved.

Our members benefit from 4 issues annually, packed full of helpful and fascinating information like the article above. You can become a member and support the work of the PMCA by making a tax-deductable donation.

JOIN US TODAY!