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Why Does A Hawk Build With Green Nesting Material?
Bernd Heinrich

Northeastern Naturalist, Volume 20, Issue 2 (2013): 209–218

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2013 NORTHEASTERN NATURALIST 20(2):209–218 Why Does A Hawk Build With Green Nesting Material? Bernd Heinrich* Abstract - Most hawks construct the frame of their nest from coarse dry branches and line it with finer materials before the eggs are laid. However, Buteo platypterus (Broadwinged Hawk) and some other hawk species resume to build on the nest lining in the nestling stage, by adding fresh sprigs of green vegetation. Various hypotheses for the latter behavior have been suggested. I examined the contents of a Broad-winged Hawk nest in western Maine repeatedly to gain insights into the function of the green sprigs. The birds added on average two large fresh green fronds (each 15–42 cm in length) per day to the nest during the first 18 days after their young hatched, and they continued to add about 1 frond/twig with fresh leaves per day during the last 17 days the chick was in the nest. These fresh greens consisted of ten species of plants, including five species of ferns and two species of conifer. Of my nine examinations of the nest, in all but the last one the nest mold was lined with either ferns or Thuja occidentalis (Northern White Cedar). I compared the percentages of these greens with their local availability near the nest, and conclude that the birds selected for a flat but feathery leaf structure. The literature suggests various possible functions of greenery in nests. I distinguish between greens added onto the nest, and greens added later on post-hatching into the nest lining as a layer, and conclude that nest hygiene is the most plausible explanation for green vegetation in hawk nest linings. Introduction Birds use a great variety of nest materials to make a nest platform. These materials are highly functional: long sticks used by herons anchor the nest by bridging between limbs, mud used by some swallows attaches the nest to solid vertical surfaces, fibers used by orioles and vireos permits hanging nests from twigs, etc. (Hansell 2000). The nest-linings are soft, dry materials, such as plant down, moss, fine rootlets, hair, and feathers. However, some perching birds occasionally incorporate fresh moist plants into the nest platform, counter to the expectation of the standard function of cushioning and insulating, and some raptors incorporate green vegetation onto the nest periphery and into the nest lining. The incorporation of green vegetation into a nest could reflect the available material near the nest, or it may be used preferentially for specific adaptations. For example, male Sturnus vulgaris L. (European Starling) during the time prior to egg-laying incorporate green vegetation into nests so as to help attract a mate (Brouwer and Komdeur 2004, Gwinner 1997). Green vegetation could also be valuable for protection from ectoparasites (Wimberger 1984), stimulation of the immune system (see review of literature in Gwinner and Berger 2005), camouflage (Welty1962), maintaining nest structure (Lyons et al. 1986), retaining heat *Department of Biology, University of Vermont, Burlington, VT 05405; bernd.heinrich@ 210 Northeastern Naturalist Vol. 20, No. 2 and reducing incubation costs (Rodgers et al. 1988), nest hygiene (Orians and Kuhlman 1956), or advertising that the nest is in current use. Early naturalists and egg collectors noted green vegetation in hawk nests (Bent 1937). However, due to limitations on both access and ease of experimentation, we know essentially nothing about the use and extent of greens in the large, open nests of raptors. Based on nest descriptions associated with museum egg collections, Wimberger (1984) excluded camouflage, nest hygiene, and mate attraction and deduced that green vegetation function in raptor nests to deter ectoparasites because raptors reuse nests. His study highlights one of the major problems with interpretation of the use of green nesting material: there are almost no details of individual nests, nor differentiation between stages of nest construction and presence of eggs versus chicks. To my knowledge, only one study explored this behavior in a raptor, Buteo platypterus (Vieillot) (Broad-winged Hawk), in which the birds were examined through spotting scopes, and it was tentatively suggested that the green vegetation serve to strengthen the nest for re-use (Lyons et al. 1986). Details such as when in the nesting cycle the green vegetation is incorporated and what kinds are used relative to what is available are necessary in order to rank hypotheses that explain hawks use of green nesting material. For example, as in Mycteria americana L. (Wood Stork), could the use of green plants in nest lining material from “within the surroundings of the colony” (Rodgers et al. 1988) be an incidental by-product of indiscriminate use of readily-available material? Study Area and Methods The observations described here were of a Broad-winged Hawk nest near Weld, Franklin County, in western Maine. It was located 12 m up in the triple fork of an Acer saccharum Marsh. (Sugar Maple). The nest had not been used in at least the last two previous years, nor was it used in the subsequent year, although Broad-winged Hawks were present in the area every year. Several branches had leaves hanging directly over and by the nest, which was in a forest of Sugar Maple, Acer rubrum L. (Red Maple), Acer spicatum Lam. (Mountain Maple), Acer pensylvanicum L. (Striped Maple), Fraxinus americana L. (White Ash), Fagus grandifolia Ehrh. (American Beech), Prunus serotina Ehrh. (Black Cherry), Betula alleghaniensis Britt. (Yellow Birch), Betula papyrifera Marsh. (White Birch), Quercus rubra L. (Northern Red Oak), Populus tremuloides Michx. (Quaking Poplar), Populus grandidentata Michx. (Big-toothed Poplar), and a few stems of Pinus strobus L. (White Pine), Picea rubens Sarg. (Red Spruce), and Abies balsamea (L.) P. Mill. (Balsam Fir). A count of trees with a diameter at breast high of at least 10 cm, whose crowns reached the canopy, and were located within a circle 100 m around the nest, consisted of 1 Red Spruce and 33 deciduous trees (25 Sugar Maples, 3 Red Maples, 2 White Ash, 2 Paper Birch, and 1 Black Cherry). Samples of four similar-sized plots at 50 m in the four cardinal 2013 B. Heinrich 211 directions beyond 100 m from the nest consisted of 3 conifers (2 Balsam Fir and 1 Red Spruce) and 152 deciduous trees (58 Red Maples, 45 Sugar Maples, 17 Birch spp., 11 Black Cherry, 9 White Ash, 8 poplar spp., and 4 Malus pumila P. Mill [Cultivated Apple]). There were no Thuja occidentalis L. (Northern White Cedar) trees within a half a kilometer of the nest, but at that distance there were scattered stems of Picea glauca (Moench) Voss (White Spruce) as well as stands of numerous Balsam Fir, Red Spruce, and White Pine. To minimize disturbance of the nest, I restricted direct nest inspection to the post-hatching period of from 19 June to 22 July 2011. Before hatching, approximate nest contents were seen using binoculars from a nea rby tree. Based on four previous nests examined in Maine and Vermont, nests before egg hatching often contained a small sprig of Balsam Fir, Tsuga canadensis (L.) Carr. (Eastern Hemlock), or Spruce, but there were no linings of greens within the nest cavities; the nest lining prior to the appearance of the young always consisted of bark chips. Table 1. Observations at a Broad-winged Hawk nest in Weld, ME, 2011. Date Observation 16 June The nest contained a 1–2-day-old chick and one egg directly upon one large fresh fern frond that could have been picked within the last hour, and another perhaps day-old wrinkled fern under the fresh one. Both fronds covered most of the typical Broadwinged Hawk nest-lining of bark chips (Fig. 1; Heinrich 2010) 26 June The nest was lined with a new fresh (a day old or less) fern frond, and a large fresh Sugar Maple sprig with several green leaves was located directly underneath it. Several dried fern fronds were at the nest periphery. The second egg still had not hatched and was later found to be infertile. 4 July The nest mold was entirely covered with fresh Northern White Cedar sprigs. In order to identify all the greens that had so far been brought into the nest, I removed the (now mostly withered and decaying) greens below the cedar lining in order to count and identify them. In relative order of abundance they consisted of the following sprigs/fronds: 11 Northern White Cedar, 10 fern, 6 Sugar Maple, 5 Balsam Fir, 2 White Ash, 1 Red Maple, and 1 Red Spruce (the maple sprigs each had 4 to 8 leaves, the fern fronds were 25–42 cm long, the spruce and fir sprigs measured 17–20 cm in length, and the cedar sprigs were 15–18 cm long). 5 July A freshly-killed young American Woodcock lay upon a solid spread of fresh cedar sprigs. 6 July Two new fern fronds, of two different species, had been added since yesterday, and they covered the entire nest mold, obliterating the green cedar of t he previous day. 7 July Two new fern fronds (one 45 cm and the other 38 cm long) had been added and they covered the nest mold. The nest contained a freshly regurgitated pellet consisting of tightly compacted down feathers, a small bone, and numerous small seeds (presumably from stomach contents). 9 July The nest lining was completely covered with two new Northern White Cedar sprigs, which were on top of a Red Spruce sprig. 10 July The nest lining was almost entirely lined with one large Sugar Maple sprig of eight green leaves. The nest also contained two wing feathers of an immature of either American Woodcock or Ruffed Grouse. 22 July The hawk had fledged, and dry Red Squirrel hair remained along with two fresh pellets and feather shafts of a Ruffed Grouse or an American Woodcock. It had rained the previous day, and since these nest contents were dry, I deduced that the bird had fledged within the last day. 212 Northeastern Naturalist Vol. 20, No. 2 Results During the entire 35 days of the chick’s nest occupancy (Table 1), I observed a total of 55 fresh green sprigs added to the nest: 20 Northern White Cedar, 14 ferns (of 5 species, Athyrium angustum (Willd.) C. Presl [Narrow Lady Fern], Parathelypteris noveboracensis (L.) Ching [New York Fern], Dryopteris clintoniana (C.C. Eat.) Dowell [Clinton’s Wood Fern], Osmundastrum cinnamomeum (L.) Presl [Cinnamon Fern], and Onoclea sensibilis L. [Sensitive Fern]), 11 Sugar Maple, 5 Balsam Fir, 2 Red Spruce, 2 White Ash, and 1 Red Maple. Throughout these observations, I observed no specks of feces in the nest. The birds had added, on average, 2.0 sprigs/day during the first 18 days, but only 0.8 sprigs/day during the last 17 days. Some of the prey remains in the nest had included at least 1 Microtus pennsylvanicus (Ord) (Meadow Vole), 1 Tamiasciurus hudsonicus (Erxleben) (Red Squirrel), 1 Scolopax minor Gmelin (American Woodcock) , and probably 2 young Bonasa umbellus (L.) (Ruffed Grouse). A recently-killed Lepus americanus Erxleben (Snowshoe Hare) was found on the ground nearby . Of my nine examinations of the nest, in all but the last one the nest mold was lined with either ferns or Northern White Cedar (Fig. 2). On the other hand, during the course of my observations, the most readily available green material was maple leaves. They surrounded the nest and dominated in the nearest forest and represented 69% of the crown-cover tree species within 100 m of the nest. Despite the dominance of Sugar Maple trees near the nest, maple sprigs accounted for only 13% of the total number of sprigs in the nest. Northern White Cedar sprigs accounted for 20% of all sprigs, but the only available Northern White Figure 1. A 1–2-day-old Broad-winged Hawk chick and egg in nest lined with fern frond and other green material. Photograph © Bernd Heinrich. 2013 B. Heinrich 213 Cedars were approximately 0.5 km downslope from the nest. This is not a long distance for a hawk to fly, but these trees were localized, whereas other greens were widespread around the nest. The next most common greens used in the nest, fern fronds, accounted for 18% of the fresh material brought into the nest, and grew as abundant ground cover in most of the woods near the nes t. Discussion Broad-winged Hawks, like other nest-building raptors, use long dry sticks to build their nest frame, and they line the nest with the flat outer chips of bark. They Figure 2. A Broad-winged Hawk nest in a deciduous forest in western Maine, showing the nest bowl lined with green material (primarily Northern White Cedar) during the fledgling stage. Photograph © Bernd Heinrich. 214 Northeastern Naturalist Vol. 20, No. 2 have long been reported to also add bits of greenery to their nest (Burns 1911, Lyons et al. 1986, Riley in Bent 1937, Rosenfield 1982). Bent (1937) reported “seldom more than one kind of leaf [is] used in the individual nest.” Lyons et al. (1986) found one nest with green sprigs of only Robinia pseudoacacia L. (Black Locust), and six species of tree sprigs in other nests. Matray (1974) reported sprigs from as many as five tree species per nest. Where the greens were placed in or on the nest was not documented. Based on my repeated direct observations of the same nest during the post-hatching phase, I suggest that previous reports of nests lined with only a single species of foliage were based on incidental observations when only the last layer was visible. The number of green sprigs added to the nest may not be an accurate measure of the use of green vegetation to replenish the nest lining, since their individual size varied greatly, but in composite they usually covered the entire nest mold. Another measure of variability is the species of leaves used for that purpose. The kinds of green vegetation added to the nest by the hawks indicate selectivity since the cedar sprigs they added to the nest must have come from a distance, and the ferns came from the ground whereas Broad-winged Hawks hunt from tree perches. Large Red Spruce, Balsam Firs, and White Pines were all much closer to the nest than the Northern White Cedars, yet they were used sparingly (13% for spruce and fir), or not at all (White Pine). Although the overall choice for continual replenishment of the nest lining was exclusively fresh greens, ferns and Northern White Cedar appeared to be selected for. Red Maple, birch, and cherry were under-used relative to local abundance, yet leaves of Sugar Maple (the nest tree) were used, perhaps suggesting a bias towards leaves that are directly adjacent. It is unlikely that my removal of all of the greens underneath the layer that had accumulated by 4 July stimulated later green deposition, because depositions on subsequent days remained relatively constant over the next six days. This finding suggests that only the last, topmost, layer is of significance to the nest-lining behavior. Some hypotheses regarding the adaptive benefit of adding distant greens such as Northern White Cedar to the nest, especially in such large amounts and outside of the bird’s nest-building phase, can be excluded. For example, the hawks regularly lined their nest during the nestling period. Purely incidental incorporation of greens is excluded, not only by the choice of materials used, but also because the greens were gathered and incorporated independent of nest-building. The incorporation of greens only throughout the nestling phase also excludes the hypothesis that the greens serve in mate attraction (courtship hypothesis of Gwinner 1997) and territorial advertisement. The behavior of lining the nest with green vegetation (as opposed to the inclusion of an occasional green sprig into the nest) is apparently largely a post-hatching phenomenon, suggesting that it is probably related to a function associated with nestlings. If fresh greens aid the incubating parents, then one would expect green sprigs to be incorporated before incubation, since the investment of finding the right green vegetation would then not compete with that of finding food for the young. 2013 B. Heinrich 215 The hypothesis that green vegetation added to the nest rids the nest of mites and other ectoparasites (Clark and Mason 1985, Gwinner 1997, Orians and Kuhlman 1956, Wimberger 1984) does not appear to apply to hawks. The use of fresh green vegetation to control ectoparasites by other birds shows a highly significant correlation with cavity as opposed to cup or open nesters (Clark and Mason 1985), where nest fumigation may be important for some species because of the costs associated with infestation by nest ectoparasites (Hansell 2000). However, even green vegetation brought to (hole-nesting) European Starling nests failed to reduce mite abundance (Gwinner and Berger 2005, Gwinner et al. 2000), and fumigatory properties of herbs should be even lower in open hawk nests, where vapors would dissipate into the air. Nest linings can sometimes function as camouflage, such as the white feathers used by Tachycineta bicolor (Vieillot) (Tree Swallows; Heinrich 2010) that may hide white eggs and protect the nest from parasitic egg dumping. However, green vegetation added to nests does not camouflage brown-spotted white eggs nor white nestlings. Additionally, if greens served mainly as camouflage, then using the readily available greens such as maple leaves exclusively would suffice. It has also been suggested that the young hide under the greens (Criddle 1917) and use them for protection from the sun. However, the hawk chicks never appeared covered by them. Specifically for the Broad-winged Hawk, it has been suggested that the green vegetation added to the nest during the nesting season aids in preserving the structure of the nest and thus saves work during rebuilding on subsequent nesting (Lyons et al. 1986). However, the green sprigs that were used in the nest observed in this paper offered no structural support. In fact, they would be expected to retain moisture in the nest, creating the equivalent conditions of a compost heap and thus accelerating the decay of stick supports. Even if greens were useful in nest maintenance, there is no reason to insert them specifically after the young hatch, and to replenish them on a regular basis throughout the nestling period. Any potential re-nesting in the same nest would, in any case, not be until a year later, and the few sticks required to repair a nest could be incorporated at little energetic cost. Inserting greens throughout a previous year would be a poor investment, especially since re-nesting in the same nest is never certain. It is not likely that green vegetation added to nests serve as cushioning or insulation because they are not consistently used in Accipiters, nor in falcons that build no nest. A partial function of green vegetation in Broad-winged Hawk nests may be illuminated by a comparative review of greens used and not used by other raptors. Bent (1937) mentions the use of bark chips in the nest linings of all the northeastern Buteo species, the Broad-winged Hawk, Buteo jamaicensis (Gmelin) (Red-tailed Hawk), and Buteo lineatus (Gmelin) (Red-shouldered Hawk). Bent also mentions that these species incorporate sprigs of green leaves into their nests, as do other raptors. Aquila chrysaetos (L.) (Golden Eagle), for example, used green leaves that “take pairs to secure from miles” and that “are 216 Northeastern Naturalist Vol. 20, No. 2 added after the young hatch”. Gary Clowers (Raven Research West, Madras, OR, pers. comm.), in a study of the nesting of Golden Eagles, examined 8 nests for 2 years and usually found Pinus ponderosa Douglas ex P. & C. Lawson (Ponderosa Pine) twigs or other green vegetation on the nest periphery shortly before mating and egg-laying, with other green plants added later. For the sympatric Accipiter species of the northern forest, Accipiter gentilis (L.) (Northern Goshawk), Accipiter cooperii (Bonaparte) (Cooper's Hawk), and Accipiter striatus Vieillot (Sharp-shinned Hawk), which also line their nests with bark chips, Bent (1937) makes no mention of any use of fresh green leaves. Wimberger (1984), relying on egg collector’s reports in museum collections, reports strong geographic variation in the use of green nesting material in some species of ha wks. The use of ferns and Northern White Cedars in nests may serve an anti-pathogenic function. For example, the use of greens by European Starlings promoted the growth of the young, not by suppressing mites, but by suppressing the growth of bacteria in the nest and boosting the young’s immune function (Berger et al. 2003, Gwinner and Berger 2005). The hawks’ use of aromatic conifers and ferns may indicate a complementary antiseptic function relating to food. The maple foliage, ferns, and cedar sprigs have one thing in common: their sprays flatten down into a surface in the nest mold. If replenished they provide a clean surface. Both Northern White Cedar (Hoffman 2003) and ferns (Gracelin et al. 2012, Srivastava 2007) contain various medicinal, antiseptic antibacterial compounds that have long been used in human medicine. Their function may be to provide an antiseptic surface (dependent not only on chemical content but also on freshness) upon which the parents deposit the meat that then prolongs its freshness. Other conifers such as spruce, Balsam Fir, and pine may also have antiseptic qualities, but they were sparingly used in the nest lining after the meat stock-piling could start, despite being locally abundant. Unlike cedar and ferns, these tree species do not provide a flat surface. A “clean-surface” hypothesis would not apply to passerines that ingest all of their food in small, fresh chunks that are delivered directly mouth-to-mouth. Hawk parents deposit the bodies of their prey directly into the nest mold. Providing a “clean plate” should help store fresh meat and would be beneficial especially at high temperatures. Temperatures in late June and July near the Broad-winged Hawk nest in this study were regularly above 30 °C. If the parents procured a surplus of meat that was partially torn apart, then the fresh substrate would serve as a relatively clean surface where the dismembered prey would be subjected to a reduced bacterial load that would retard spoilag e. I speculate that the apparent difference in the use of greens between Buteos and Accipiters and falcons, could be the nature of their primary prey. The latter specialize on birds, meals that could often be quickly disposed of. Greens have been reported in the nests of at least five species of eagles (Wimberger 1984) and numerous Buteos. Both eagles and Buteos specialize on large-sized mammalian prey such as hares, as well as on squirrels and mice that often live in a boom or bust prey schedule so that carcasses accumulate in the nest. Bent (1937) noted up 2013 B. Heinrich 217 to nine Red Squirrels found in a Red-tailed Hawk nest at one time and an eagle nest “always contained Jack Rabbits, Cotton Tails, mountain rats”—and the birds additionally prey on young deer and lambs of mountain sheep. Nest-lining with greens during the nestling stage is a behavior that presumably evolved as a carry-over from nest-building, and owls and most falcons (who build no nest) would not be expected to carry nest-lining material. Additionally, most northern owls and falcons nest early in the year and the young are fledged before high temperatures fostering decay occur. I conclude that nest hygiene is the most plausible hypothesis for the use of green vegetation i n the hawk nests. Acknowledgments I thank David Barrington for identifying the ferns, Gary Clowers for observations of Golden Eagles, and Rosalind Renfrew, Glen Mittelhauser, and two anonymous reviewers of the manuscript for their excellent and useful comments and criticisms that greatly improved the manuscript. Literature Cited Bent, A.C. 1937. History of the Broad-winged Hawk. In Birds of Prey (Part 1). Dover Publications, New York, NY. 425 pp. Berger, S., R. Disko, and H. Gwinner. 2003. Bacteria in Starling nests. Journal of Orniothology 144:317–322. Brouwer, L., and J. Komdeur. 2004. Green nesting material has a function in mate attraction in the European Starling. Animal Behaviour 67:539–548. Burns, F.L. 1911. A monograph of the Broad-winged Hawk (Buteo platypterus). Wilson Bulletin 23:139–320. Clark, L., and J.R. Mason. 1985. 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Saunders, Philadelphia, PA. 720 pp. Wimberger, P.H. 1984. The use of green plant material in bird nests to avoid ectoparasites. Auk 101:615–618.