Reprinted from: Purple Martin Update 8(4): 2-6
James R. Hill, III
Purple Martin Conservation Association
In his zeal to promote his company's aluminum Purple Martin houses, J. L. Wade, owner of Nature House, Inc., of Griggsville, IL, and manufacturer of the Trio and DuraCraft lines of martin houses, once sent press releases to the nation's newspapers imploring martin hobbyists everywhere to return to its place of purchase, any martin housing that wasn't manufactured by his company. He was especially adamant in his attacks against those who professed to care about Purple Martins, yet promoted gourds as martin housing. He claimed that gourds, as well as most martin housing made by other manufacturers, were so inadequate as martin domiciles that they rarely fledged any young since most would fall out prematurely and end up on the ground as "dog and cat food." Wade went on to say:
"Native Americans had to use gourds for martin housing. They had to use a lot of things that were not ideal. Because of technological advances, you'll rarely find Native Americans in teepees, using horses for basic transportation, or plowing a field with a wooden hoe. All of these things have been made obsolete. So has gourd housing for Purple Martins. Like the teepee, gourds are no longer the housing that should be erected for Purple Martins. The reasons are wide ranging and compelling." J. L. Wade (Nature Society News 29(7):5, 1994).
|Fig. 1. A 15-year study comparing the reproductive success of Purple Martins nesting in natural gourds, wooden houses, and unmodified Trio aluminum houses. This study was conducted by the PMCA at 22 different colony sites in northeastern Ohio, northwestern Pennsylvania, and western New York. These results clearly show that martins have the highest reproductive success in natural gourds, the lowest in unmodified Trio aluminum houses.|
Objectives of This Study
Because Mr. Wade's anti-gourd/pro-Trio martin house claims are not backed by any research data, I thought it would be interesting to put his claims regarding the relative merits of various types of martin housing to a scientific test. Therefore, I took the PMCA's 15-year database of 2729 active Purple Martin nests and statistically compared the relative reproductive success of martins nesting in wooden houses, unmodified Trio/DuraCraft aluminum houses, and natural gourds. If, as Mr. Wade claims, any of these three housing types differed in how good or bad they were for nesting Purple Martins, I would expect to find measurable differences in the breeding success of martins using them. Since the martins nesting in the three housing types used during this long-term study were all exposed to identical external (i.e., environmental) conditions, any observed differences in their nesting success would be entirely due to the relative strengths or weaknesses of the three housing-types tested. Such an analysis is also relevant from a conservation standpoint since all landlords should strive to use the type of housing that will maximize the reproductive success of the martins under their care.
Study Area and Methods
For the past 15 nesting seasons, I have offered the breeding Purple Martins at the Purple Martin Conservation Association's research site in northwestern Pennsylvania (and at 22 other sites in northwestern PA, western NY, and northeastern OH) nesting opportunities in up to 9 Trio aluminum Castle martin houses, 5 Trio Grandpa's, 3 Trio Mussleman's, 1 Trio-Wade, 12 LST-1 singles, 1 DuraCraft Hex, and 1 DuraCraft Box. Also offered were up to 34 wooden martin houses and up to 665 natural gourds. The Trio/DuraCraft aluminum houses were unmodified and were not equipped with either owl guards or porch dividers. The wooden houses were made from 3/4" western red cedar (or 3/4" plywood) and were vertical designs with only one hole per porch. The natural gourds were 1/4" to 1/2" thick and were painted with clear polyurethane during the first 7 years of the 15-year study, were painted white for the last 8, and were not equipped with rain canopies or access doors until the last 6 seasons. Cavity and compartment sizes of all the housing used are given in the "Results and Discussion" section. All housing systems raised and lowered vertically for nest inspections. All compartments were numbered and nest checks were conducted every 4-7 days all season long. Most importantly, all the martins nesting in each type of housing were exposed to identical weather extremes, predation pressures, parasite infestations, foraging opportunities, and management regimes.
To determine if there were differences in how martins performed reproductively when nesting in the three kinds of housing offered in this study, I examined 9 different measures of reproductive success (i.e., average clutch size, 7-egg clutch prevalence, 6-egg clutch prevalence, average hatch size, egg-failure rate, average fledge size, nestling survival rates, % of eggs laid that led to fledged young, and % of nesting attempts that ended in failure). I then compared these measures of reproductive success statistically among the martins using the different housing types.
For accurate results, a scientific study should have a large sample size and be repeated for at least three breeding seasons (preferably five). This ongoing study was conducted during the 15 breeding seasons spanning 1984 through 1998, and tracked the outcome of 2729 active Purple Martin nests containing 12,814 eggs. It was the longest, most comprehensive study of Purple Martin breeding success ever conducted. Five hundred and forty-seven nests were in unmodified Trio/DuraCraft aluminum houses, 968 nests were in wooden houses, and 1214 were in natural gourds. Since this was not a test of housing preference, no effort was made to offer equal numbers of cavities in each housing type.
Results and Discussion
Average Clutch Size: In this study Purple Martins laid larger clutches of eggs in natural gourds than they did in wooden houses (4.81 vs. 4.52), larger clutches of eggs in natural gourds than in unmodified aluminum Trio houses (4.81 vs. 4.23), and larger clutches of eggs in wooden houses than in unmodified Trio aluminum houses (4.52 vs. 4.23) (Fig. 1).
Why would there be measurable differences in the average number of eggs laid per clutch in each of the three kinds of martin housing investigated in this study? I propose that the differences are due almost entirely to variations in compartment sizes among the three types of housing. The natural gourds used in this study varied in diameter from about 10" to 13", having floor areas in the range of 78.5 - 132 square inches. The three styles of wooden house used had compartment floors that measured either 7" x 12" (= 84 square inches of floor area), 6.5" x 11" (= 71.5 square inches), or 7" x 7" (= 49 square inches). The Trio and DuraCraft aluminum houses had either square 6" x 6" compartments (= 36 square inches, i.e., Musselman, Grandpa, Trio-Wade, Box, and LST-1), or pie-shaped compartments with 35.35 square inch floor areas (Castle and Hex). The natural gourds in this study had floor areas that were 2.2 to 3.7 times larger than the aluminum houses tested.
|Fig. 2. A 6" x 6" square drawn to scale over the top of five nestling Purple Martins, demonstrating the floor size of the unmodified Trio aluminum houses used in this study. Imagine a brood of six martin nestlings (and their two parents = 8 martins) having to cram into a compartment this size. I believe that, ideally, martin house compartments should be at least twice this large, especially considering that a full-sized nestling is 7" long.|
Numerous studies of hole-nesting birds have found a positive correlation between clutch size and the area of the bottom of the nest cavity (see Korpimäki (1984) and Karlsson & Nilsson (1977)). Females of several cavity-nesting species respond to the size of the box by adjusting the size of their clutch. For instance, Rendell & Robertson (1993) found that female Tree Swallows preferred large nest boxes over small ones and laid significantly smaller clutches of eggs in small boxes. Likewise, both Mosman (1990) and Hill (unpublished) have found that Purple Martins show a strong preference for nesting in 6" x 12" compartments over 6" x 6" when given a choice. Mertens (1977) found that Great Tits with larger cavity nests not only laid more eggs and hatched more young compared to tits in smaller cavity nests, but the larger nests also had better insulation and less heat loss during incubation and brooding. Van Balen (1984) was able to get female Great Tits to lay smaller clutches of eggs by offering smaller nest boxes. In Sweden, Gustafsson & Nilsson (1985) were able to get individually-marked Pied Flycatchers to lay smaller clutches from one year to the next if they only offered them smaller nest boxes, and larger clutches from one year to the next if they only offered larger nest boxes. Clearly then, the clutch-size differences observed in Purple Martins nesting in the three types of housing in this study were almost entirely related to the floor areas of the nesting compartments.
7-egg Clutch Prevalence: Further evidence supporting my proposition that cavity size affects clutch size in Purple Martins is the frequency of 7-egg clutches in the three types of housing examined. Seven-egg clutches are very rare in Purple Martins. During this study, only 37 (1.3%) of 2729 nesting attempts had 7-egg clutches. Twenty-five of these were observed in the 1214 natural gourd nests (2.05% of gourd nests had 7-egg clutches); ten were observed in the 968 wooden house nests (1.03% of wooden house nests had 7-egg clutches); and two were observed in the 547 Trio aluminum house nests (0.36% of Trio house nests had 7-egg clutches). Seven-egg clutches were 5.69 times more likely to be laid in natural gourds than in Trio houses, 2 times more likely to be laid in natural gourds than in the wooden houses used in this study, and 3 times more frequent in the wooden houses than in the Trio aluminum houses.
6-egg Clutch Prevalence: Additional evidence supporting a relationship between floor area and clutch size in Purple Martins was the relative frequency of 6-egg clutches among the three types of martin housing investigated. During this study, 475 six-egg clutches were laid among the 2729 clutches examined (17.4% of all clutches contained 6 eggs). 241 (19.9%) of the 1214 clutches in natural gourds were 6-egg clutches; 187 (19.3%) of the 968 clutches in wooden houses were 6-egg clutches; whereas 47 (8.59%) of the 547 clutches in unmodified Trio aluminum houses were 6-egg clutches. Six-egg clutches were over twice as frequent in natural gourds and wooden houses than in the Trio aluminum houses. Again, these results suggest a strong correlation between larger floor areas and larger clutch sizes.
Average Hatch Size and Egg-failure Rates: In this study, Purple Martins hatched, on average, more eggs per nesting attempt in natural gourds than in wooden houses (3.93 vs. 3.54), more eggs per nesting attempt in natural gourds than in Trio aluminum houses (3.93 vs. 2.71), and more eggs per nesting attempt in wooden houses than in Trio aluminum houses (3.54 vs. 2.71) (see Fig. 1). Purple Martins hatched, on average, 1.22 more eggs per nesting attempt in natural gourds than in Trio aluminum houses. There were also differences in the percentages of eggs laid that failed to hatch in each of the three housing types. In the unmodified Trio aluminum houses, 36% of the eggs laid failed to hatch, whereas only 21.7% and 18.3% of the eggs laid in the wooden houses and natural gourds, respectively, failed to hatch.
|Fig. 3. The relative nest-failure rates of Purple Martins breeding in Trio aluminum houses, wooden houses, and natural gourds. A failed nesting attempt is any active nest (a nest that had at least one egg) that failed to fledge at least one young. In this study, martins had the highest nest-failure rates in Trio aluminum houses, the lowest rates in natural gourds.|
Why would there be differences in these measures of productivity among the three types of housing? Some of the observed differences were due to different starting points (i.e., since the three housing categories began with different average clutch sizes: 4.81, 4.52, and 4.23, you would expect different average hatch sizes), but why would a larger percentage of eggs fail to hatch in unmodified Trio aluminum houses than in gourds or wooden houses? I believe there are several reasons.
First of all, when exposed to the same rainy weather, nests in the unmodified Trio aluminum houses got substantially wetter than nests in either the gourds or the wooden houses. One reason for this is the design of these houses. In Trio houses, the porches and compartment floors are the same, continuous, piece of metal so that any rain falling on the porches just runs under the lift-up doors into the nesting compartments. The "dri-nest subfloors" do little to elevate the nests above the puddles that form in the nest compartments during a driving rain, so the nests end up absorbing water like a sponge. It's also much easier for blowing rain and rain splashing off a porch to travel the short distance into a nest bowl of a 6-inch compartment. It's not uncommon to find nests in Trio houses completely saturated with water after a heavy rain. Unfortunately, when martin nests get wet, their eggs are much more subject to chilling, and chilled eggs are abandoned at a much higher rate than non-chilled eggs. The nest bowls in gourds stayed drier than Trio nests, even in the early years of the study, before the use of rain canopies on gourds. Nests in gourds are usually built high against the back wall, several inches above the low point of the gourd. The cup-shaped bottoms of gourds tend to channel any rain that enters down the drain holes. Nests in the wooden house styles used in this study rarely, if ever, got wet.
Vulnerability to both aerial and pole-climbing predators also played a role in causing a higher egg-failure rate in Trio aluminum houses. Both the incubating parents and their eggs are more vulnerable to all forms of predation in the shallower, 6-inch-deep compartments of the Trio houses, compared to the larger gourds (10- to 13-inches deep) and homemade wooden houses (7, 11, or 12-inches deep) used in this study. Brown (1978) found that egg loss to House Sparrows, starlings, and accidental "brush out" (i.e., being knocked out by a parent that is scared from the compartment) was higher in Trio 6" x 6" compartments than in wooden 7" x 7" compartments.
Finally, insulation: there is no question that 3/4" of wood or 1/4" to 1/2" of porous gourd wall is a far better layer of insulation than a thin sheet of aluminum. Metals aren't insulators; they are conductors of both heat and cold. Eggs and young would likely stay cooler in warm temperatures and warmer in cool temperatures in gourds and wooden houses, relative to those in an uninsulated metal house. A more stable compartment temperature requires less time and energy expenditure on the part of parents needing to regulate egg and nestling temperatures during both hot and cold periods. Parents nesting in cavities possessing more temperature stability can convert their time and energy savings into increased foraging time, increasing not only their own survival chances during bad weather, but also that of their eggs and young.
Average Fledge Size and Nestling Survival Rates: In this study, any nestling that disappeared before it reached 24 days of age (the minimum age for successful fledging in Purple Martins) was considered not to have fledged successfully. The martins nesting in natural gourds fledged an average of 3.01 young per nesting attempt, those nesting in wooden houses 2.63 young per nesting attempt, and those nesting in unmodified Trio aluminum houses 1.8 young per nesting attempt. Purple Martins fledged, on average, 1.21 fledglings more per nesting attempt in natural gourds than in unmodified Trio aluminum houses.
Again, some of these differences mirrored the different average clutch-size starting points, but there were some measurable differences in nestling survival rates. In the unmodified Trio aluminum houses, 33.6% of the young that hatched failed to survive to fledging age, whereas 25.7% and 23.4% of the young in the wooden houses and natural gourds, respectively, failed to fledge.
Why would there be higher mortality rates among nestling martins in unmodified Trio/DuraCraft aluminum houses relative to wooden houses and natural gourds? For many of the same reasons that there was a higher egg mortality rate in Trio aluminum houses (i.e., lack of insulation, wetter nests, and increased vulnerability to predation). During this study, Great Horned Owls preyed on parents and young from all three classes of housing, but showed a disproportionate attraction to Purple Martins nesting in Trio houses (see Figs. 4 and 5). Martins in Trio aluminum houses are easier prey due to their shallow compartments and easy-open doors. Based on the predation rates I've observed these past 15 years at our 22 research sites, I feel that Trio houses, unless equipped with the company's optional owl guards, are little more than expensive owl feeders.
|Fig. 4. A Trio Castle martin house after its occupants were eaten by a Great Horned Owl. Trio houses are particularly vulnerable to owl predation because of their shallow, 6-inch compartments and easy-open doors. These powerful owls can easily pull open the doors as they extract martins through the entrance holes.|
|Fig. 5. A road-killed Great Horned Owl used to demonstrate the 8-12 inch reach these birds have. Note how far beyond the back wall of the 6" x 6" Trio aluminum LST-1 martin single this small male owl can reach in search of a meal. These nocturnal predators take a large number of martins each summer from colony sites throughout North America. Since all owls are Federally-protected, martin landlords need to make sure all of their housing has been owl-proofed with guards.|
Another common mortality factor in this study unique to the Trio houses was their shared porches that allowed older nestlings to wander out of their compartments and into nearby cavities where they either stole incoming food from their younger neighbors, or got lost. The former can result in the starvation death of the kleptoparasitized young, the latter their own. In Maryland, Morton (1991) lost 17% of his broods to these "porch invaders" in his three Trio castles. See also Rogillio (1989). Porches, in general, increase martin mortality because they encourage nestlings to wander outside of their cavities before they are physically capable of flight, exposing them to increased predation and vulnerability to falling accidents. Porches also facilitate nest predation by House Sparrows, starlings, gulls, crows, owls and hawks. In contrast, gourds, because they are porchless, force nestlings to stay inside until they can fly, and because they swing when touched, make it more difficult and less attractive for nest predators to strike.
I also observed increased mortality in the Trio houses when the thin, flimsy nests of subadult females disintegrated, resulting in featherless nestling being reared on the floors. Bare metal floors, through thermal conduction, rapidly sap body heat away from featherless nestlings, not only increasing their vulnerability to hypothermia, but also increasing their chances of developing the fatal disorder known as "leg splaying" (see Dellinger, 1986) where the leg muscles of young martins become so permanently deformed from lack of friction on the slippery, flat floors that they can't ever perch or support their bodies.
Several other aspects of the small compartments of Trio houses also contributed to the higher mortality observed. The smaller, 6" x 6" size of the nest cavities affected the proportion of young falling (or being accidently pushed) out just due to overcrowding. In contrast, larger compartments mean less defecating and walking on top of siblings, less feather damage, and lower rates of premature fledging. Larger cavities also allow nestlings to separate completely from each other during periods of heat stress so they can thermoregulate more efficiently. In the Great Tit, Van Balen (1984) found that in smaller nest cavities, mortality from hyperthermia (overheating) was increased because there was no room for nestlings to escape physical contact with each other to maximize cooling during high temp-eratures. In Sweden, Gustafsson & Nilsson (1985) found that breeding failure in the Pied Flycatcher (Ficedula hypoleuca) due to predation and nest desertion was significantly higher in small boxes than in large.
I'm not the first researcher to call for larger nest compartments in Purple Martin houses. Allen and Nice (1952) recommended rooms 6" x 7" x 8," stating that many man-made martin houses "have rooms too small for the needs of the birds." Lowrey (1974) and Brown (1978) recommended 7" x 7" x 7" or larger. Imhof (1976) recommended 8" x 8" x 6". Based on tests I have conducted here at the PMCA, I recommend 7"(W) x 6"(H) x 12"(L).
Other reasons why martins may have had lower reproductive success in unmodified Trio houses and wooden houses relative to gourds could have been the spread of parasites and disease organisms. When the young fledge from one nest in a typical martin house, the fleas, nest mites, blowfly larvae, and bed bugs disperse to other nearby active nests in search of new hosts by crawling along the walls and shared porches (or under gaps in wall partitions) causing even higher parasite loads in those less advanced nests. Hill (1994) has shown that nest parasites decrease nestling survival in Purple Martins by 50%. In gourds, the nests are much more isolated and separated from each other and the pathways connecting the gourds are much narrower, or nonexistent, preventing most parasite dispersal.
Theoretically, contagious diseases are more easily spread in martin houses than in gourds due to the closer arrangement of house compartments compared to more widely-spaced gourd clusters. The more crowded nature of the house compartments in this study relative to roomier gourds could also have played a role in disease transmission. And as stated previously, nests and nestlings also seemed to get wetter in Trio houses compared to the gourds or wooden houses used in this study. Wet and chilled nestlings are more susceptible to respiratory diseases.
% of Eggs Laid that Led to Fledged Young: When examining overall nesting success, I found that 62.6% of the eggs laid in natural gourds led to fledged young, 58.2% of the eggs laid in wooden houses led to fledged young, and 42.6% of the eggs laid in unmodified Trio aluminum houses led to fledged young.
Nest Failure Rates: In the unmodified Trio aluminum houses there were 488 nesting attempts with known outcomes; 219 (44.8%) of these nesting attempts failed to fledge at least one young (see Fig. 3). In the wooden houses there were 949 nesting attempts with known outcomes; 263 (27.7%) of these nesting attempts failed to fledge at least one young. In the natural gourds there were 1146 nesting attempts with known outcomes; 242 (21.1%) of these nesting attempts failed to fledge at least one young. The highest nest failure rates for Purple Martins were in Trio/DuraCraft aluminum houses, with nearly half of all attempts ending in failure. Martins had the lowest nest-failure rates when nesting in natural gourds. The nest-failure rate of Purple Martins in the Trio aluminum houses was more than twice as high as that observed in the natural gourds.
The results of this 15-year study strongly contradict the statements of J. L. Wade, owner of Nature House Inc., manufacturer of the Trio and DuraCraft aluminum martin houses used in this study. In this investigation, Purple Martins nesting in natural gourds fared the best in all 9 measures of reproductive success examined. Martins nesting in wooden houses did next best, while martins nesting in unmodified Trio/DuraCraft aluminum houses consistently did the poorest. Purple Martins fledged an average of 1.2 offspring more per nesting attempt when nesting in large natural gourds than when nesting in unmodified Trio aluminum martin houses. Viewed another way, these results suggest that a martin colony of 50 pairs nesting in large natural gourds would fledge 60 more offspring per summer than a colony of 50 pairs breeding in unmodified Trio aluminum houses! From a conservation standpoint, these differences are extremely significant.
Consumers need to know the facts in order to make informed decisions when choosing Purple Martin housing, especially in a market so saturated with incorrect information and inadequate products; the welfare of martins depends on it. However, the results of this study are not meant to be an indictment against Trio/DuraCraft martin houses. They just point out the strong need for design modification and the addition of accessories. Owners of conventional aluminum (or where appropriate, wooden) martin houses should add porch dividers (to prevent porch wandering), add insulation (to stabilize compartment temperatures), install floor meshing (to prevent splayed-leg syndrome), double the compartment sizes from 6" x 6" to 6" x 12" (to increase clutch size), make short stilts for your subfloors to elevate them higher off the floors (to control wet nests), do nest replacements (to control parasites and wet nests), and (most importantly) add owl guards. If you do these things, the breeding success of your martins will greatly increase. In fact, preliminary research at the PMCA on Trio houses with compartments enlarged to 6" x 12", suggests, as expected, that female martins increase their clutch sizes significantly over those laid in 6" x 6" compartments. A managed aluminum house will always have higher productivity than an unmanaged wooden house or gourd, so management is also a key. In the past, only aluminum houses were easy to manage. But now with winch-up wooden houses and gourd racks (and gourds with access doors and canopies), aluminum houses have lost most of their advantages over other types of housing.
If the results of this 15-year, three-state study are representative of the relative reproductive success Purple Martins are experiencing continent-wide when using large natural gourds, wooden houses, and unmodified Trio aluminum houses, (and there is no reason to believe they're not) then martins have a very serious problem. The most popular and widely-used Purple Martin houses in North America are the Trio and DuraCraft lines of aluminum houses; millions of them have been sold during the past three decades. Unfortunately, as these data show, Trio houses, sold as they are with 6" x 6" compartments, appear to be seriously depressing the reproductive potential of Purple Martins. Also not helpful to the Purple Martin's plight is the continual, anti-gourd, anti-wooden-house rhetoric of Nature House, Inc.'s owner, Mr. J. L. Wade.
Since this study shows that the most popular commercial houses are the type in which martins have the poorest breeding success, the conclusion is obvious. Manufacturers need to combine the features that make aluminum houses so popular with landlords (lightweight, easy to manage, and low maintenance, i.e., landlord friendly) with the features that made the wooden houses and gourds in this study so martin friendly (i.e., larger nesting compartments, better insulation, better protection from weather and predators). For years, landlords have been buying Trio and other brands of aluminum housing and then modifying them to have double-sized compartments, nest inserts, insulation, starling-resistant entrances, and owl guards. Fortunately, some new martin house manufacturer's are listening to what landlords and researchers are saying, and are making aluminum housing that better meets the best interests of both the martin and the landlord (e.g., Lone Star and Trendsetter). Hopefully others will follow. Mr. Wade and other manufacturers can best demonstrate their continued commitment to the well-being and future welfare of the Purple Martin by following suit.
Allen, R. W. and M. M. Nice. 1952. A study of the breeding biology of the Purple Martin (Progne subis). American Midland Naturalist 47:600-665.
Brown, C. R. 1978. Inadequacies in the design of Purple Martin houses. Bird-Banding 49:321-325.
Dellinger, T. B. 1992. Bare floors can maim and kill baby Purple Martins. Purple Martin Update 3(4):14-15.
Gustafsson, L. & Nilsson, S. G. 1985. Clutch size and breeding success of Pied and Collared Flycatchers Ficedula spp. in nest-boxes of different sizes. Ibis 127:380-385.
Hill, III, J. R. 1994. Do parasites lower the nesting success of Purple Martins? Purple Martin Update 5(1):28-29.
Imhof, T. A. 1976. Alabama Birds. 2nd ed. University of Alabama, Univ. of Alabama Press.
Karlsson, J. & Nilsson, S. G. 1977. The influence of nest-box area on clutch size in some hole-nesting passerines. Ibis 119:207-211.
Korpimäki, E. 1984. Clutch size and breeding success of Tengmalm's Owl Aegolius funereus in natural cavities and nest-boxes. Ornis Fennica 61:80-83.
Lowrey, G. H., Jr. 1974. Louisiana Birds. 3rd ed. Baton Rouge, Louisiana State University Press.
Mertens, J. A. L. 1977. Thermal conditions for successful breeding in Great Tits (Parus major L.) Oecologia 28:1-29.
Morton, E. S. 1991. The fatal flaw of martin house designs with contiguous porches. Purple Martin Update 3(1):26-27.
Mosman, D. 1990. The martin research of Darwin Mosman. Purple Martin Update 2(4):6-9
Rendell, W. B. & Robertson, R. J. 1993. Cavity size, clutch size, and the breeding ecology of Tree Swallows Tachycineta bicolor. Ibis 135:305-310.
Rogillio, C. 1989. Porch dividers: A way to improve the productivity of house-nesting Purple Martins. Purple Martin Update 2(1):6-7.
Van Balen, J. H. 1984. The relationship between nest-box size, occupation and breeding parameters of the Great Tit Parus major and some other hole-nesting species. Ardea 72:163-175.
James R. Hill, III, is founder and Executive Director of the Purple Martin Conservation Association. He has been studying martins for 18 years.
Copyright 1999 by Purple Martin Conservation Association. All Rights Reserved.
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