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Use of the invasive pale swallowwort by birds and small mammalsChristine Hanrahan
Pale swallowwort (Cynanchum rossicum), also known as Dog-strangling Vine (DSV), is a highly invasive species of natural areas. It can rapidly out-compete existing vegetation, thereby reducing overall biodiversity of a site. It can survive in sun and shade, and in damp or dry conditions. Once established, it is extremely hard to eradicate. At the Fletcher Wildlife Garden, the plant is widespread in both wooded and open sites. Various methods of control have been tried at FWG over the last few years including pulling, digging, cutting, mowing, and smothering. None have so far proven effective but we need to look at some of these methods over the long term. We are still researching other methods. Invasive alien plants have been in this country for decades and in some cases, for centuries, although the incursion of new species has occurred more rapidly in recent times. As they displace native vegetation and change the composition of natural ecosystems, the impact on the fauna is significant. There is no doubt that invasive species present a considerable environmental and economic problem wherever they occur. However, sometimes native species of wildlife have adapted to the invasive alien plants and occasionally appear to have benefited (for example, the adaptation of the Henry's Elfin butterfly to Glossy Buckthorn, Rhamnus frangula, has meant an increase in abundance of this species, Layberry, et al.1998). Many invasive species, such as Purple Loosestrife, Lythrum salicaria, are known to attract various insects, and I seem to remember reading that biologists have found 40 species of native wildlife using this plant for various purposes. Locally, beaver were reported eating Purple Loosestrife near the Champlain Bridge (Darbyshire and Consaul 1999). In light of this, and given the abundance of DSV at the Fletcher garden, I wondered if it too was being used by birds or other wildlife for nesting material or some other purpose. BirdsIn 2003 I noticed a yellow warbler gathering DSV seed fluff (coma) leftover from the previous year and incorporating it into its nest. The following year I found DSV fluff in the nest of a house sparrow. Knowing that many birds will gather material close to their nest site and utilize a variety of items from natural to man-made, it seemed logical they would use at least some parts of this abundant plant when building nests. Between 2002 and 2005 I collected the dislodged nests of various species at FWG and put them in the Interpretive Centre. In early 2006 I began examining the material in these nests. With the expert help of botanist Stephen Darbyshire, much of the material in these nests was identified. What was found was very interesting! Nests of seven species from the FWG were analyzed. Generally only one nest per species was collected, but 4 nests for yellow warbler were examined for a total of 11 nests. Ten of the 11 nests contained at least some DSV, and some were made only from this plant. Both the stem fibre and the fluff were used. The DSV stems provide long, flexible fibres, strands of which are readily stripped off the winter-softened stems. The seed pods retain a significant amount of fluff over the winter, providing good lining material at a time when early nesters especially, may be hard pressed to find other similar stuff from plants such as thistles and milkweeds. The Baltimore oriole nest was made entirely of DSV stem fibre and lined primarily with DSV fluff (see photo above). The nest was exceptionally strong and even after 4 years, shows no signs of disintegrating. Only one nest of warbling vireo was obtained and it too was constructed largely of DSV stem fibre, although interestingly, no DSV fluff was used as lining.
Red-eyed vireo, house sparrow, song sparrow and red-winged blackbird nests all contained varying amounts of DSV stem fibre and/or fluff. Only the American goldfinch nest did not contain any DSV. However, goldfinches are a common nesting bird at the garden with 4 or 5 or more pairs nesting every year. Only one nest was examined and it may be that an analysis of more goldfinch nests will show use of DSV. MammalsTwo large winter nests of meadow voles were found in the Old Field area of the FWG, although distorted and difficult to determine structure. Both were made of approximately 90% DSV stem fibre, fluff and seed pods, and 10% grass. At the bottom of each nest was an exceptionally thick pile of DSV fluff which could have been lining, or a seed stash, although very few seeds were found. Whether the voles were indeed stockpiling DSV seeds (while still attached to the fluff) or whether they were simply using the silky fluff for a snug lining is hard to say. But wouldn't it be interesting if DSV seeds formed part of their diet? I would be interested in hearing of any other uses by wildlife of Pale Swallowwort. Please contact me at vanessa@magma.ca. References
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Every yellow warbler nest examined used DSV stem fibre and seed fluff, and one was made almost exclusively from this material. All were lined with, amongst other matter, large quantities of DSV fluff. In one nest, the DSV fluff was packed tightly around other fibres on the inside of the nest as well as tamped down at the bottom to create a thick lining.