Birders Need Information on why invasives are a problem
The following message was sent from the Pittsburgh Parks Conservancy and we feel that members would find this information interesting. Louisa Thompson writes....
I will let others address the issue of invasiveness per se, i.e. displacement of native plant species by Paulownia, Norway maple, etc. Here's some information your birder colleagues may find more compelling.
Someone has already mentioned the nutrient value of native fruits vs. some of the exotics. Here's a little more on that: Many native fruits, such as spicebush and dogwoood, are produced in fall and are high in fat. By timing and because of the high caloric value of fat (9 calories per gram, vs. 4 for sugar), they are ideally suited to the nutrient needs of migratory birds. Some of these fruits persist into winter, and again the high fat content helps to keeps resident birds warm when the temperature drops. Some of the invasive fruit-bearing shrubs and vines may also produce high-fat fruit in fall, but oriental bittersweet produces a seed capsule that looks like a fruit and really just wastes the bird's time. Paulownia produces large seed capsules and I think the seeds are winged and spread by wind, not by birds. Norway Maple seeds are wind-dispersed.
Those invasives which produce fruit in summer (e.g., bush honeysuckles) probably compete with native blueberries, elderberries, and brambles, which also fruit in summer. Relatively few indigenous bird species are summer fruit-eaters. Mockingbirds, which have expanded northward along with multiflora rose, are year-round fruit-eaters, and feed fruit to their young during the summer, as do catbirds. Chipmunks and squirrels, and probably other rodents which can reach them, also eat summer fruits.
But most birds feed their young insects (high in fat and calcium) and small meaty creatures such as earthworms (high in protein). So bush honeysuckle favors mockingbirds and catbirds over chickadees and titmice, for example. (I am guilty of mis-reporting the timing of bush honeysuckle fruit in the past - I thought it fruited in autumn and would tempt migratory birds to feed on a less nutritious food source.)
A much larger problem for birds, rarely mentioned, is that most birds - something like 98% in North America - feed on insects some or all of the time. As I said above, insects are particularly important for feeding nestlings, and the calcium in their exoskeletons is also essential for egg-laying. Insects in general (not just butterflies) need specific larval hosts. Invasive plants usually are invasive precisely because they lack natural controls here - natural controls usually being leaf-eating insect larvae. So, Paulownias, Norway Maples, and so on indirectly reduce the amount of food available to birds because fewer insects are found on them. What you can tell your birder acquaintances, then, is that invasive trees and shrubs favor the few birds that are fruit-eaters over the much larger number that are insectivores - and this is true even of those invasives that do not themselves produce fruit.
I agree with Kathy that it is not necessary nor advisable to pull vines out of trees - actually, at any time of year, not just nesting season - unless the vines are girdling the branches. Usually, Japanese honeysuckle can be unwrapped from the trunk as high as you can reach, and that will be good enough. The other vines don't twine so tightly and can be left in place. Pulling them down can cause a lot of damage to the tree. More generally, complete physical removal of invasives often causes greater disturbance than figuring out how to kill the plant and leave it on site. For example, you can often dig up the roots, get them out of contact with the soil, and leave the plant there.
Lynn Wagner raised the problem of old information (recommending various invasives for wildlife habitat plantings) still being circulated, especially by government agencies. We had another discussion about this recently, in regard to Georgia Cooperative Extension. I reiterate what I and others said then: make your views known, politely, to the agency.
The people in charge may not realize that their staff are handing out brochures, etc., which recommend invasives. Or, they may need public support to justify the expenditure of printing a revised version when there is still a good stock of the old publication. If so, a letter from you can be very helpful.