New answers for an age-old mystery
Reprinted from: Purple Martin Update 2(1)
James R. Hill, III
Purple Martin Conservation Association
|An adult male Purple Martin that has just plucked a green leaf from the outermost branches of a plum tree to take back to his nest. Purple Martins instinctively line the cups of their nests with fresh green leaves. There are several theories why this behavior has evolved, but the exact reasons remain a mystery.|
An adult male Purple Martin that has just plucked a green leaf from the outermost branches of a plum tree to take back to his nest. Purple Martins instinctively line the cups of their nests with fresh green leaves. There are several theories why this behavior has evolved, but the exact reasons remain a mystery.
The Purple Martin (Progne subis) instinctively lines the upper surface of its nest cup with fresh green leaves and/or leaf fragments that it plucks or tears from the outer branches of trees near its nesting sites.1,11,13,16,22,28,32,40 This behavior is so universal within the species that even the saguaro-nesting Purple Martins of Arizona's Sonoran Desert, where green leaves are scarce, engage in this unusual behavior.36
Martins begin bringing green leaves to their nest cavities about the time the main nest structure is complete, just prior to egg-laying, and continue bringing them throughout the incubation period.4,23,37 Leaf-bringing behavior is reported to continue throughout the nesting season and to be even more frequent after the young hatch.33 Observations at the PMCA's research facility in Pennsylvania, however, don't support this latter point. Our PMCA birds begin bringing leaves as the main nest structure nears completion, increase the behavior during the egg-laying stage, continue it at a reduced rate during the incubation period, then cease the behavior at hatching time.17 Both sexes gather green leaves,4,23,33 but the male takes a more active role, especially during the incubation period,1 and it can constitute a major portion of his morning activity.12
Within a breeding aggregation of martins, leaf-pulling is a highly contagious activity and it is not uncommon to see several dozen martins all in the same tree at once, plucking leaves.1,23 In fact, on rare occasions, where colonies are quite large, martins have been known to do damage to pear trees by completely stripping certain branches of their leaves.11 Occasionally, martins will enter neighboring compartments to steal green leaves.38
Martins bring so many green leaves to their nests during the egg-laying stage that their incomplete clutches of eggs get deliberately (or incidentally) covered by these leaves.1,22,32,40 However, by the time the female completes her clutch and begins incubation, all of her eggs will be neatly arranged in the center of a shallow green bowl of interlacing leaves.17
Over time, some of these leaves become desiccated and brittle in the nest, and martins will fly off with them in their beaks, dropping them in the vicinity of their housing.33,37 During these leaf-removal episodes, the birds sometimes engage in unusual displays of avian "play," with several birds repeatedly dropping and recatching the same leaf before the last bird finally lets it hit the ground.17,24
Many types of leaves have reportedly been used by martins for lining their nest cups. Published references include: apple22,29,40, pear11,33, peach34, cherry9, willow6,, pecan8,14, oak33, California-laurel13, aspen/poplar2,6, maple27, elm15,25, camphor10, pine10, apricot3,34, and eucalyptus.10,27 The sidebar below shows the results of a survey I conducted on leaf use by the Purple Martin. It lists a total of 67 different leaf types that martins have used and gives an indication of their relative usage.
Why does the Purple Martin go to all the trouble of plucking fresh, green leaves for lining its nest cup? The primary function of this leaf-adornment behavior is not understood, but there has been a great deal of speculation. At least seven different theories have been proposed to explain the adaptive significance of this instinctive behavior.
Decreases nest-chamber temperature: The theory that has been around the longest is that through evaporation these green leaves serve to cool the nest chamber and eggs.6,11,40 Since martin houses are erected in open areas where they can become hot inside, and many nestling martins are known to perish from heat stress19,26,39, it's no wonder leaf bringing was regarded as an adaptation to help cool the nest. However, in regards to the cooling theory, one authority1 stated: "As to cooling, seems very unlikely to me. It seems doubtful if they would be very effective in this capacity, as no nest I have seen had more than 3-4 leaves that were really fresh. And why should they be carried in during late May and early June when the birds do not need a cooling system as much as a heating system?" And another investigator found no correlation between weather and the amount of leaf-bringing in the martin.12 Therefore, this theory seems to hold little substance.
Increases nest-chamber humidity: Another theory is that leaf adornment increases the humidity of the nest chamber, similar to the way poultry farmers add moisture to their incubators to increase the hatchability of their eggs.37 Whether or not green leaves actually increase humidity in martin nest cavities is unknown, but it certainly is a possibility that needs investigation. Related to this, it has been shown that the feeding rate of nest mites (Arachnida:Dermanyssidae) is indirectly related to the relative humidity of their nest environment. If martins are increasing the humidity of their nest chambers by adding green leaves, then leaf-bringing behavior might be a behavioral adaptation to reduce the feeding rate of one of their major parasites, the martin nest-mite, Dermanyssus prognephilus.18
Increases nest insulation: Another theory states that the leaves serve to increase the insulative value of the nest lining so that the eggs and young can more easily be kept at optimal temperatures during incubation and brooding.4 This also might be a possibility, but other substances, such as feathers (which are used as nest-lining material by the closely related Tree Swallow, Tachycineta bicolor and the Barn Swallow, Hirundo rustica), would be of much better insulative value.
Enhances egg concealment: Because the Purple Martin occasionally covers its eggs with the green leaves it brings to the nest it has been proposed that their function might be to conceal the eggs from potential predators in the female's absence.1,40 Perhaps there is some merit to this, since martins seem to cover their eggs with leaves while their clutches are incomplete (i.e., during the four to six days of egglaying)17, a time when both parents are frequently absent from the nest, leaving eggs unattended. However, once clutches are complete, a parent usually is in attendance at all times, making egg concealment unnecessary. And only about 25% of the martin nests at the PMCA's research site ever have leaf-covered eggs.17 Therefore, if leaf-bringing evolved as a form of egg-concealment, it has a very short span of effectiveness and is not used by all individuals. Something else to consider - the Purple Martin is a cavity-nester, which in itself is a highly-adaptive form of egg concealment. Most cavity nesters, including the Purple Martin, have pure white eggs, evidence that historically, egg predation rates must have been very low since camouflaged egg-patterning never evolved. Based on all of this, I feel that leaf-bringing probably didn't evolve as a form of egg-concealment.
Induces intrapair copulation: Because unforced copulation between mates in Purple Martins is believed to occur in the nest compartment when the male brings in a green leaf and presents it to the female, a recent theory suggests that leaf bringing is the male martin's way of inducing his mate to copulate with him, thus ensuring his paternity of her eggs.30 According to this theory, green-leaf carrying appears to function as a precoitional display (i.e., a behavioral stimulus that may lead directly to the copulation response). Support for this theory comes from two other swallow species, the Tree and Barn Swallow, who apparently use feathers in much the same way that martins may use green leaves. Tree and Barn Swallows are so powerfully attracted to feathers for lining their nest cups (and also perhaps for gaining copulatory access to their mates) that they will take feathers from human hands.17 Such an unusually powerful attraction of males to materials needed to complete their nests near the time of egglaying hints that these materials may be used as precoitional displays. A weakness in this theory as it applies to the Purple Martin, however, is that no one knows for sure whether martin pairs actually copulate in their nest compartment.
Facilitates ease of nest cleaning: My personal observations suggest that a nest lining of green leaves serves the martins as a removable bed liner of value before the nestlings are "toilet trained" (i.e, physiologically old enough to defecate out the entrance hole). When the nest cup gets soiled with droppings from the nestlings, parent martins just remove the soiled top leaves leaving an underlayer of relatively "clean sheets." It is common to see martins removing these feces-covered leaves.17,21 Since clean nests are healthier nests (i.e., they attract and harbor fewer nest parasites), having a removable bed liner could be an adaptation to keep the nest cleaner and thus freer of nest-dwelling parasites.
Decreases nest-dwelling parasites: One of the most plausible theories as to the adaptive value of leaf adornment in martins is that it helps to fumigate the nest by chemically repelling nest-dwelling parasites.4,10,13,23 The decay of green leaf material is known to release chemical compounds that act as repellents, contact toxicants, or natural fumigants against arthropod parasites and pathogenic bacteria.23,41 Since heavy infestations of nest-dwelling parasites can cause mortality in nestling Purple Martins31 and lead to parental desertion of nests, any adaptation in nest-building behavior that limited parasite populations (e.g., green leaf bringing) would be favored by natural selection, the very process of evolution itself.
Historically, martins nested in natural sites (i.e., old woodpecker cavities, natural tree cavities, and crevices in cliffs) and no doubt reused these old nest cavities from one year to the next. Such reuse of nest sites is known to increase a bird's risk of incurring large parasite loads,35 because viruses, fungi, bacteria, and many arthropods can lie dormant in nest debris and feces for several months, and can withstand freezing temperatures.5 Therefore, birds breeding at historically-active sites have an increased risk of being parasitized or getting infected.
Many species of birds incorporate fresh vegetation into their nests.16 A survey of 137 passerine birds breeding in eastern North America5 revealed a statistically significant relationship between the nesting mode and the use of green vegetation for nest construction. Passerines nesting in enclosed spaces (e.g., secondary cavity-nesters or species using crevices) were more likely to incorporate green plants into their nests than passerines nesting in open cup nests or nests characterized by infrequent reuse. One of the most likely explanations for these results is that green vegetation use is meant to counteract the effects of parasites and pathogens in nests that are frequently reused. Studies on the European Starling (a secondary cavity nester that reuses old nests and also uses green leaves) reveal that they choose a small subset of available leaves from their environment, and the leaves they do choose contain more volatile compounds at higher concentrations than the leaves they don't choose.5 All of these results help support the theory that green leaf use by Purple Martins evolved as an adaptation to reduce parasite and pathogen loads in their nests.
Several of the theories just presented have merit in explaining the many possible benefits martins derive from their instinctive habit of lining their nest cups with green leaves. It's quite possible that the adaptive value of this unusual behavior is a combination of several of the above functions. Before any definitive answer is put forth, however, much more research will be needed.
1Allen, R. W., and M. M. Nice. l952. A study of the breeding biology of the Purple Martin (Progne subis). Am. Midl. Nat. 47:606-665.
2Barker, M. R. l965. Air-conditioned martins. Audubon Magazine 67:342
3Bauer,T. E. l984. Letter to the editor. Nature Society News l9(4):l9.
4Bryens, O. M. l942. Purple Martins using leaves in nest-building. Condor 44:75-76.
5Clark, L., and J. R. Mason. 1985. Use of nest material as insecticidal and anti-pathogenic agent by the European Starling. Oecologia. 67:169-176.
6Clement, R. C. l965. Comment. Audubon Magazine 67:342.
7Coulson, T., and J. R. Hill, III. 1984. The Nature Society's 1984 Purple Martin survey. 4pp. Trio Manufacturing Co., Griggsville, IL.
8Creech, N. M. M. l982. Letter to the editor. Nature Society News l7(9):l7.
9Danner, M. S. l9l6. Martin problems. Bird-Lore l8:l09.
10Edgar, C. l984. Letter to the editor. Nature Society News l9(2):22.
11Forbush, E. H. l929. Birds of Massachusetts and other New England States Vol. 3. Mass. Dept. of Agric., Boston.
12Gaunt, A.S. l959. Behavior in the Purple Martin. Kansas Ornithol. Society Bull. l0:l4-l6.
13Grinnell, H.W. l935. Minutes of Cooper Club Meetings. Condor 37:29l-292.
14Grzeskowiak, J. l983. Letter to the editor. Nature Society News l8(2):26.
15Harrington, R. l982. Observations of Purple Martins. Nature Society News l7(l0):4.
16Harrison, H. H. l975. A Field Guide to Birds' Nests in the United States east of the Mississippi River. Houghton Mifflin Co., Boston.
17Hill, J. R., III. Personal observations.
18Hill, J. R., III. l983. Here's an introduction to the common parasites of martins. Nature Society News l8(4):l-6.
19Hill, J. R., III. l988. The summer of 88: A severe drought and heat wave take their toll on martins. Purple Martin Update 1(3):14-15.
20Hill, J. R., III., and T. Coulson. 1985. The Nature Society's 1985 Purple Martin survey. 2pp. Trio Manufacturing Co., Griggsville, IL.
21Holley, R. A. l985. Letter to the editor. Nature Society News 20(10):22.
22Jacobs, J. W. l903. The story of a martin colony. Gleanings No.2:l-24. Waynesburg, Pa.
23Johnston, R. F., and J. W. Hardy. l962. Behavior of the Purple Martin. Wilson Bull. 74:243-262.
24Krantz, C. l984. Notes on migration, feeding, and more. Nature Society News l9(l):22-23.
25Kruse, H. l982. Letter to the editor. Nature Society News l7(l2):23.
26Layton, R. B. l969. The Purple Martin. Nature Books Publishers, Jackson, Mississippi.
27Mabry, R. L. l970. Eucalyptus: Is its use purposeful or coincidental? Purple Martin Capital News 5(ll):24.
28McAtee, W. L. l940. An experiment in songbird management. Auk 57:333-348.
29McLean, R. l982. Letter to the editor. Nature Society News l7(6):25.
30Morton, E. S. 1987. Variation in mate-guarding intensity by male Purple Martins. Behaviour 101:211-224.
31Moss, W.W., and J. H. Camin. l970. Nest parasitism, productivity, and clutch size in Purple Martins. Science l68:l000-l003.
32Olmstead, R. l955. Observations on Purple Martins. Kansas Ornithol. Society Bull. 6:8-l0.
33Raynor, G. S. l959. Leaf pulling by the Purple Martin. Auk 76:362.
34Reiss, E. W. l97l. Green leaves. Purple Martin Capital News 6(l0):2l.
35Stoner, D. 1936. Studies on the Bank Swallow Riparia riparia (Linneaus) in the Oneida Lake Region. Roosevelt Wildlife Annual 4:126-233.
36Stutchbury, B. Personal communication.
37Taverner, P. A. l933. Purple Martins gathering leaves. Auk 50:ll0-lll.
38Updegraff, D. l984. Martin house has been a treasure. Nature Society News l9(12):25.
39Wade, J. L. l966. What You Should Know about the Purple Martin, Trio Manufacturing Co., Griggsville, Illinois.
40Widmann, O. l922. Extracts from the diary of Otto Widmann. Trans. Acad. Sci. St. Louis 24(3):l-77.
41Wimberger, P. H. l984. The use of green plant material in bird nests to avoid ectoparasites. Auk l0l:6l5-6l8.
A Survey of Green Leaf Use
in the Purple Martin
James R. Hill, III
Over the last several years, I have been surveying the types of leaves Purple Martins use for lining their nest cups. A majority of the data for my study came from questionnaires I ran in 1984 and 1985 (see Coulson and Hill 1984; Hill and Coulson 1985), but data also came from the scientific literature, from personal observations, from letters received by the PMCA, and from letters published in the Nature Society News. From these sources, 67 different leaf types were reported as used by martins. Samples came from 595 martin colony sites covering 46 states and provinces. The results (listed below) may show the martin's preference for certain leaf types, but may also be biased by what tree types were locally available and/or abundant.
Leaf-type---------- # colony sites
Pine needles 21
Chinese Tallow 8
Red Bud 7
Black Walnut 3
Cherry Laurel 3
Russian Olive 3
California Laurel (bay) 2
Autumn Olive 2
Green Pepper 1
Forsythia Bush 1
Rose of Sharon Bush 1
Digger Pine 1
Cypress needles 1
Privet Hedge 1
Lobbuli Bay 1
Pinto Bean 1
Bois d' arc 1
Garden Bean 1
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