Illinois Natural History Survey - University of Illinois

Effects of Exotic Plants on Bird Nesting Success

Invasive species have been implicated as a major or contributing cause of declines for nearly half the endangered and threatened native species in the U.S. In Illinois, the number of exotic or invasive plants is unknown, but several (garlic mustard, kudzu, autumn olive) seriously threaten Illinois habitats.

Wetlands in Illinois, already reduced by 85%, are invaded due to disturbance from development, altered hydrology, or runoff. Purple loosestrife (Lythrum salicaria), a Eurasian plant that invaded North American wetlands in the 1800s, now dominates many Illinois wetlands. Due to the ineffective management of loosestrife through other methods (e.g., herbicides, burning, flooding), purple loosestrife has been the target of biological control agents (natural enemies such as insects and pathogens) imported from Europe.


Illinois has one of the nation's most active biological control projects against purple loosestrife, with nearly 2 million agents released and apparent success seen at several sites. However, the national loosestrife biological control program has been criticized as unnecessary. Critics claim purple loosestrife poses little threat to native species, especially birds, because nesting females have been observed in loosestrife-infested wetlands.

Data regarding the effects of invasive plants on birds are limited. Numerous factors could result in lower (or even higher) reproductive success, but we believe two stand out: limited prey abundance and elevated predation rates. Exotic, invasive plants thrive in new areas not because they are intrinsically superior to their native counterparts, but because their natural enemies are absent. Natural enemies, usually insects, control plants in their native distributions, but most are never introduced with their host plant. Thus, large stands of purple loosestrife may contain fewer insect prey, which could result in increased starvation by nestlings relying on insect prey as food. This notion is supported by the finding that about 120 species of insects are found feeding on purple loosestrife in Europe, whereas only 12 were found in a study in southern Manitoba.

Nest failure is largely the result of nest predation, and predation rates are greatly influenced by vegetation characteristics of nest sites. The encroachment of an invasive species will, to some extent, modify vegetation characteristics and perhaps render nests more susceptible to predators. For example, a 1999 study by Schmidt and Whelan showed that birds nesting in exotic plants experienced higher predation rates and lower reproductive success than nests built in comparable native shrubs. These elevated predation rates were attributed to structural differences of exotic plants. Structure of purple loosestrife differs drastically from native wetland plants.

In cooperation with the Chicago Department of Environment, Illinois Natural History Survey scientists initiated a two-year study in the Lake Calumet region of northeastern Illinois to examine the effects of purple loosestrife on wetland birds. Preliminary data of approximately 90 nests found during the 2000 breeding season suggest

1. Red-winged Blackbirds nesting in purple loosestrife were less likely to succeed (31% nest success) than birds nesting in cattail (37% nest success);

2. Redwings avoid nesting in purple loosestrife until early to mid-June, whereas nests in cattail were initiated in early May;

3. Starvation was equally common in loosestrife and cattail habitats;

4. Marsh Wrens were never found nesting in loosestrife, only cattail.


Also, in 2001, one of the wetland sites was burned. Although large stands of loosestrife did not burn, all cattails burned, leaving no plant structure for nesting. At that particular site, neither Red-winged Blackbirds nor Common Grackles nested in loosestrife until after June and Marsh Wrens were entirely absent. This anecdote seemingly further supports our assumption that loosestrife is not suitable habitat for nesting birds at least until later in the breeding season.

It appears that elevated predation rates in loosestrife are not related to predation patterns commonly reported in other nest predation studies, e.g., edge, plot, or seasonal effects. Our results may indicate findings similar to those of Schmidt and Whelan in that vegetation structure might facilitate the movement of predators in purple loosestrife. We hope current data collection will provide more insight to these findings.

J. Dylan Maddox, NRES-UIUC, and Robert N. Wiedenmann, Center for Economic Entomology

Charlie Warwick, editor

Illinois Natural History Survey

1816 South Oak Street, MC 652
Champaign, IL 61820

Terms of use. Email the Web Administrator with questions or comments.

© 2019 University of Illinois Board of Trustees. All rights reserved.
For permissions information, contact the Illinois Natural History Survey.

Staff Intranet