Kutztown University’s Dr. Todd Underwood Shares Research on Brood Parasitism

Posted on October 31, 2019 in Visitors

brown-headed cowbird
Brown-headed cowbird photo by David Hebert.

Have you ever noticed an egg out of place in a songbird’s nest? It could be parasitized, meaning a bird of another species has snuck its egg in the nest. Birds that never build their own nests are called obligate brood parasites. This behavior gives these birds an advantage, allowing them to lay more eggs without tending a nest of their own.

Dr. Todd Underwood, professor of Biology at Kutztown University, is taking a sabbatical at Hawk Mountain’s Acopian Center for Conservation Learning. He has studied the interactions of brood parasites and their hosts birds for over twenty years. Dr. Underwood is currently preparing several papers for scientific journals, along with a manuscript detailing his collaborative work on raptor morphology with Hawk Mountain’s Dr. Laurie Goodrich and Kutztown University alum Chelsea Johnson.

Dr. Underwood’s research focuses on several key questions, including why many hosts accept parasitism, what limits there are on brown-headed cowbird egg ejection, how costly ejection methods are, and more recently, how geography affects egg ejection behavior.

For most of his research, Dr. Underwood studied at the University of Manitoba field station in Delta Marsh, located at the southern tip of Lake Manitoba. It is one of the largest freshwater marshes in North America, spanning over 60,000 acres. Between the marsh and the lake rests a hundred-meter-wide forest with one of the highest breeding densities of songbirds in the world.

In Delta Marsh, Dr. Underwood sought to further understand the evolutionary constraints affecting egg ejection among hosts of brood parasites. A host may employ several methods to avoid raising a cowbird chick, which include attacking cowbirds to prevent parasitism. Other tactics include egg burial and nest desertion, meaning that hosts will either build new nests on top of parasitized ones to avoid incubation of the parasitic egg or move away entirely and build a nest somewhere else. Despite these efforts, some birds may still find their nests parasitized.

Directly removing cowbird eggs is also useful strategies for hosts. Hosts may peck into the eggs to carry it from the nest, a tactic called puncture ejection, but this a high-risk approach. Parasitic eggs have thick shells, so, according to Dr. Underwood, “if they miss or glance off that cowbird egg, they can often hit their own eggs and cause damage.” Nests with punctured eggs also have higher predation rates due to scent cues that attract predators. Grasp ejection allows a bird to circumvent these side effects by simply grabbing the whole egg by the bill and throwing it out of the nest.

cowbird egg
A brown-headed cowbird egg in an eastern phoebe nest. Photo by Galawebdesign.

There are two hypotheses explaining why many host birds don’t eject parasitic eggs and instead raise cowbird chicks alongside their own: the evolutionary lag hypothesis and evolutionary equilibrium hypothesis. The evolutionary lag hypothesis claims that ejection is an adaptive response to parasitism, and hosts who accept cowbird eggs haven’t had time to evolve that behavior. The evolutionary equilibrium hypothesis claims that accepting and raising the parasite is an adaptive strategy, because ejection is too costly, and some hosts are simply unable to eject a cowbird egg due to its large size and durable shell.

Dr. Underwood sought to determine whether the warbling vireo, a small songbird, was capable of grasp ejection. By constructing plaster cowbird eggs of the same size, shape, and weight of actual eggs, and placing them in warbling vireo nests, the team was able to recreate a parasitic scenario. Researchers recorded the hosts’ reactions to the mock eggs and determined that most vireos were capable of grasp ejecting these plastered eggs, and the ones that weren’t had peck marks, indicating an attempt at puncture ejection.

There are two varieties of the warbling vireo, eastern and western, which are divided by the Rocky Mountains. The western variety has never been documented ejecting parasitic eggs, so the team developed a grasp index, which determines if a host has the necessary minimum bill length needed for ejection, about 16.0 mm. The western warbling vireo has a bill length of 15.9 mm, suggesting that they are just too small, unlike the eastern variety.

If a host bird is larger than a western warbling vireo and accepts cowbird eggs, it supports the evolutionary lag hypothesis. These birds can remove parasitic eggs from their nests, but they haven’t had time to evolve this behavior. Conversely, the evolutionary equilibrium hypothesis explains why birds smaller than the western warbling vireo would accept a host egg. Bill size limits egg ejection and ensures a scenario for cowbird young to grow successfully.

Dr. Underwood also tackled geographic variation in rejection behavior by studying the red-winged blackbird, which is rarely parasitized but often accepts cowbird eggs when it does happen. He monitored all stages of nest building in red-winged blackbirds around Lake Ontelaunee between Reading and Hamburg, Pennsylvania. During the nest building stage, red-winged blackbirds will practice nest burial, but this is not believed to be a response to parasitism. After the building stage, egg rejection drops from 60% to around 15%.

This study concluded that there is a lower density of cowbirds in the east, and they show a preference for smaller birds, parasitizing more than 7% of songbird nests, meaning that cowbirds rarely parasitize redwings, averaging at 2% parasitism. Since red-wings accept parasitic eggs, this supports the evolutionary lag theory, since they can remove cowbird eggs but don’t.  

Small birds may be negatively impacted by brood parasitism, and are important to cowbird success. Evolutionary constraints help and hinder different birds at all stages of brood parasitism, and it’s important that these behaviors are studied in more species so that we can better understand this aspect of our local ecosystem.

Although you might be tempted to intervene by removing a cowbird’s egg from a parasitized nest, brown-headed cowbirds are native species that are protected by federal law, and it would be illegal to remove the egg. Hosts are also likely desert nests that have experienced a reduction in the number of eggs present. The best efforts to protect songbird populations go toward preserving large tracts of contiguous habitat, such as at Hawk Mountain Sanctuary, where cowbirds are less common and host species have higher reproductive success.