Red-shouldered Hawks and toxic foliage

A great paper recently was published in the esteemed Wilson Journal of Ornithology by three Ohio researchers: Cheryl Dykstra, Jeff Hays, and Melinda Simon. They've been studying red-shouldereds for years in southern Ohio, and have unearthed some fascinating behavioral traits associated with nest construction. The title of the paper is: Selection of Fresh Vegetation for Nest Lining by Red-shouldered Hawks.

Dykstra, Hays, and Simon document the preferences of the various tree foliage that these hawks, arguably our most beautiful raptors, use to rejuvenate their nests at the onset of the breeding season.

Stunning Red-shouldered Hawk, Buteo lineatus, captured for banding. Photo courtesy of Bill Bosstic.

Young hawklets in the nest, northeast Ohio. Photo courtesy of Jim Dolan.

Of the twenty species of trees documented in the Dykstra et al paper as being added to nests, one stands head and shoulders above the rest in popularity amongst the red-shouldered crowd. Black Cherry, Prunus serotina, was found in over 80% of the ninety-eight nests studied in Hamilton and Hocking counties. And to me, this cherry/red-shouldered relationship is the most interesting facet of their research.

So enamored are the hawks with cherry that they'll go out of their way to get the plant. Dykstra and company meticulously quantified all trees within reasonable reach of the nests, and cherry were only found close at hand in a bit more than half the cases. Of course, powerful flyers like Red-shouldered Hawks are not going to be overly put out flying halfway across the county if they wish, but the research does suggest they covet this foliage enough to travel out of their way to get it. The ability of these raptors to recognize the plant means that they are also adept amateur botanists: rapto-botanists, or perhaps buteoflorists.

Why? The answer surely lies in the chemical composition of Prunus serotina. The foliage is rich in toxic nasties such as hydrogen cyanide, acetone, and a natural ant-repellant, benzaldehyde. By adorning their nests with sprigs of fresh cherry, the hawks are effectively playing the part of the Orkin Man. No pesky ants for Junior Red-shouldered! Very clever, these rust-shouldered beauties.

White-marked Tussock Moth, Orgyia leucostigma. These little bags of heavily furred goo are real stunners, but I wouldn't recommend eating one. Not that you would, but I'm just sayin'. Birds probably shun them to some degree, too. In part perhaps because these tussocks feast on... Black Cherry. Just as the Red-shouldered Hawks decorate their abodes with cherry foliage for pest management purposes, an enormous number of caterpillar species chow on cherry, probably because they have evolved to process the nasty chemicals. And thus, become unpalatable themselves. Douglas Tallamy, entomologist at the University of Delaware, has found that Black Cherry supports far more species of moth and butterfly larvae (caterpillars) than any other species of tree, including oaks.

It's a war out there. Plants don't like to be eaten. So they evolve toxins to ward off the insects. Insects evolve faster, and some cope with the poisons. They win, because then birds and other predators go "ugh", make faces and spit them out when they try for a snack. And theoretically don't try again. Then along comes some superior life form, like a Red-shouldered Hawk, and figures out how to put all of this to work. Nature is perennially fascinating.

The Dykstra, Hays, and Simon paper is a short, interesting read, and for those of you not subscribed to the Wilson Journal of Ornithology, I've posted it below (sans Table 1, which wouldn't translate from the PDF file).

THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 121, No. 1, January 2009

Selection of Fresh Vegetation for Nest Lining by Red-shouldered Hawks

Cheryl R. Dykstra, Jeffrey L. Hays, and Melinda M. Simon

ABSTRACT.—Red-shouldered Hawks (Buteo lineatus) typically line their nests with fresh branches of coniferous and deciduous trees. We recorded all species of green material present in 63 nests from 2003 to 2005 in suburban Cincinnati in southwestern Ohio, and in 35 nests in Hocking Hills in southeastern Ohio, United States. We identified all trees within 0.08-ha plots at 33 nest sites in southwestern Ohio and 30 in Hocking Hills. Red-shouldered Hawks in southwestern Ohio and Hocking Hills used black cherry (Prunus serotina) branches as a nest lining more frequently than expected, based on Bailey’s 95% confidence intervals. Black cherry was found in _80% of nests but present in only 57–58% of the vegetation plots, and composed only 4–5% of the trees in the forests of the study areas. White pine (Pinus strobus), red pine (P. resinosa), and eastern hemlock (Tsuga canadensis) also were used more than expected in both study areas.

Black cherry is a cyanogenic species and may provide an advantage to nesting Red-shouldered Hawks by functioning as a natural pesticide.

Received 26 February 2008. Accepted 1 July 2008.

Many raptors line their nests with fresh green vegetation consisting primarily of
branches or sprigs of trees (hereafter ‘‘greenery’’) (Preston and Beane 1993, England et al. 1997, Buehler 2000, Ferguson-Lees and Christie 2001). The purpose of the lining has
not been definitively shown. However, it has been suggested that vegetation brought to the nest might serve a signaling function, indicating the occupancy status of the nest to con-specifics and others (Newton 1979), or a nest sanitation function, covering prey remains and waste (Newton 1979).

Red-shouldered Hawks (Buteo lineatus) line their nests with branches of fresh vegetation
(Dykstra et al. 2008). They begin bringing greenery during the nest-building phase (early Feb in southern Ohio; Dykstra et al. 2008) and continue to add fresh vegetation throughout the incubation and nestling phases. Red-shouldered Hawks in southern Ohio,
United States, bring only coniferous greenery from February through mid-April, but after
leaves appear on deciduous trees, they carry both deciduous and coniferous branches to the nests (C. R. Dykstra and J. L. Hays, unpubl. data). The objective of our study was to examine if Red-shouldered Hawks selectively used particular species of green vegetation to line their nests.

METHODS
Study Areas.—We studied Red-shouldered Hawks nesting in two regions of southern
Ohio. The southwest Ohio study area (SWOH) in Hamilton, Clermont, and Warren
counties in the suburbs of Cincinnati, is composed of residences surrounded by lawns and
non-native plantings, interspersed with small areas of natural forest dominated by second-growth mixed mesophytic, oak-hickory (Quercus spp., Carya spp.) and beech-maple
(Fagus grandifolia, Acer saccharum) associations. The Hocking Hills study area (HH) in
southeastern Ohio is composed of portions of Wayne National Forest, Hocking State Forest, Zaleski State Forest, and associated private lands in Athens, Hocking, Vinton, and Perry counties. The predominant forest type is oak-hickory with plantations of white pine (Pinus strobus) and red pine (P. resinosa).

Green Vegetation Used in Nests.—Red-shouldered Hawk nest locations and breeding
areas were previously known to us (Dykstra et al. 2000, 2004). We climbed to all accessible nests containing nestlings between 4 May and 13 June, 2003–2005 to document greenery and to band nestlings. We identified all branches or sprigs of fresh green vegetation in the nests to species or species-group, and recorded the presence/absence of each species. We identified only fresh greenery; it is likely this vegetation was collected by hawks after deciduous leaves had emerged in mid-April.

Tree Species Available in the Study Areas.
We recorded tree species and diameter at breast height (dbh) of trees _8 cm dbh to provide a sample for trees available near Red-shouldered Hawk nest sites in the SWOH and HH study areas in 1997–1998 (Dykstra et al. 2000). We centered a 0.04-ha circular plot (James and Shugart 1970) on each nest tree (n _ 33 in SWOH, n _ 30 in HH) and located a paired random plot at a distance of 75–200 m in a random direction from the nest. All trees within the plots were identified and measured (Dykstra et al. 2000). We combined data from each nest plot with that from its paired random plot to create a combination vegetation plot of 0.08 ha. Nests for which we identified trees in circular plots in 1997–1998 were not the same as those where we identified green vegetation in nests in 2003–2005; however, plots and nests were well distributed throughout the same study areas. This study design necessitated a pooled statistical analysis.

Statistical Analyses.—We recorded nest lining vegetation at some breeding areas in 2 or
3 years during 2003–2005. We randomly selected 1 year of data for inclusion in the data
set for these breeding areas to avoid pseudoreplication associated with individual pairs of
birds or territories, leaving 63 independent nests in SWOH and 35 in HH. We limited our
analyses to the 10 species of greenery most commonly found in nests. We used Bailey’s
95% CI (following Boal et al. 2005) constructed following a _2 goodness-of-fit test
(with Systat 8.0). The CIs for the proportion of nests using a particular species for nest lining were compared to the proportion of vegetation plots containing at least one tree of
that species (availability). If the proportion of plots containing the species was below or
above the 95% CI, we considered the nesting Red-shouldered Hawks had used that species as a lining more or less than expected, respectively.

We also recorded the number of trees of each species in the combination plots
and reported the sum as a percentage of total trees in all plots combined.

RESULTS
Red-shouldered Hawks in both study areas used black cherry (Prunus serotina) branches
as a nest lining more frequently than expected (Table 1). Black cherry was found in _80%
of nests but present in only 57–58% of the 0.08-ha vegetation plots (Table 1), and composed only 4–5% of the trees in the forests of the study areas (i.e., in the vegetation plots). The 95% CI assessment indicated white pine, red pine, and eastern hemlock (Tsuga canadensis) were used as nest-lining material more than expected based on availability in both SWOH and HH (Table 1). Red cedar (Juniperus virginiana) was used more than expected in SWOH. The remaining species were used in proportion to their availability or less often than expected (Table 1). The average number of species of greenery was 4.3 _ 0.2 (SE) per nest in SWOH and 3.5 _ 0.2 per nest in HH. However, the amount of greenery in nests varied widely, from a few small sprigs in the center of the nest cup to many large branches that covered the entire nest (C. R. Dykstra and J. L. Hays, unpubl. data). The average number of trees per 0.08-ha plot was 32.7 _ 2.5 in SWOH and 33.4 _ 2.5 in HH. Totals of 1,079 and 1,001 trees were identified in circular plots in SWOH and HH, respectively.

DISCUSSION
Use of Coniferous Species as Nest Lining.— Red-shouldered Hawks in both study areas apparently used red pine, white pine, and eastern hemlock more than expected based on availability. Both pines are non-native in the study areas and are nonrandomly distributed (i.e., planted primarily in plantations or in residential areas); it is possible that our vegetation plots may not have adequately sampled the distribution of pines in the habitat. Hemlock is native in the Hocking Hills region, growing primarily in north-facing ravines and along streams. It is not native to southwestern Ohio although it is planted in some residential areas. Red cedar, used more than expected in SWOH but not in HH, is native to both study areas. It is much more common in SWOH than in HH, probably because the species’ nature as a scrubby, early-colonizer makes it more suited to the developed habitats of SWOH than to the heavily forested HH. Red-shouldered Hawks also carry significant amounts of these conifers to their nests before deciduous leaf-out (C. R. Dykstra and J. L. Hays, unpubl. data).

Use of Deciduous Species as Nest Lining.
Red-shouldered Hawks in both study areas used black cherry more than expected based
on availability: more than 80% of nests we studied contained this species as a nest lining.
Black cherry, a medium-sized tree native to both study areas, is present in small numbers
in most forest types throughout the region. Black cherry is a cyanogenic species, releasing volatile hydrogen cyanide (HCN) from its leaves when they wilt or become damaged by herbivory (Conn 1979). The cyanogenesis reaction, in addition to HCN, also releases other volatile compounds such as acetone, 2-butanone and benzaldehyde, the last of which has been shown to repel ants (Formicidae) (Peterson et al. 1987). We suggest the black cherry used by Red-shouldered Hawks may provide an advantage to the nesting birds by functioning as a bactericide, insecticide, or insect repellent. Clark (1991) suggests that greenery some passerines add to their nests may release volatile compounds having insecticidal properties. The addition of yarrow (Achillea millefolium) to Tree Swallow (Tachycineta bicolor) nest boxes reduced flea abundance (Shutler and Campbell 2007), and removal of greenery from nests of European Starlings (Sturnus vulgaris) resulted in an increase in mite populations (Clark 1991). It would be interesting to examine the relationship between microbe and insect abundance, and presence of black cherry in hawk nests, to learn if black cherry has a positive effect on reproductive success. It would also be interesting to learn if Red-shouldered Hawks in other regions selectively line their nests with black cherry or any other species.

ACKNOWLEDGMENTS
We are grateful to Ann Wegman and Sandra Stone for assistance with field work. We thank the many landowners in southwestern Ohio and Hocking Hills who allowed access to private property. D. E. Andersen kindly reviewed an earlier version of this manuscript.
This research was supported in part by RAPTOR, Inc., Martin and Julie Wiltz, Brad and Marsha Lindner, and the Ohio Biological Survey.

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