Neotropical Songbirds
DB: This is Earth and Sky, on the subject of songbirds.
JB: Scientists say that populations of Bell’s Vireos, Yellow-breasted Chats, Wood Thrushes, Prairie Warblers and several other species of songbirds are declining. One reason for the decline may be that fewer baby birds are making it out of the nest.
DB: For some animals, a songbird’s nest is a meal in a bowl. Snakes, raccoons and other birds eat eggs and nestlings. Cowbirds also cause problems. A mother cowbird lays her egg in a songbird’s nest, leaving the songbird parent to raise the cowbird chick at the expense of the songbird’s own offspring.
JB: Although nest robbing is nothing new, it seems to be on the rise. A study by the U.S. Forest Service suggests that it’s partly because of changes in the songbird’s forest breeding grounds. For example, in the Midwest, increasing numbers of human settlements are fragmenting the forests – breaking up areas of deep forest and creating more forest edges.
DB: A songbird’s nest built at the edge of the forest is more likely to get robbed than a nest deep in the forest. That’s because nests at the forest edge are more exposed to predators and cowbirds. Also many nest predators are birds, mammals and snakes that do well in human-altered habitats.
JB: But this pattern varies across the country, depending on the local situation. That’s our show – made possible in part by the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.
Our thanks to the following individuals and institutions who assisted in the preparation of this script:
Dr. Frank R. Thompson
Project Leader
North Central Research Station
USDA Forest Service
Columbia, MO
The following books, articles and web sites were used in preparing this script:
For more information about the Breeding Bird Survey, visit their web site here.
Author’s Notes:
Notes from an interview with
Dr. Frank Thompson,
US Forest Service North Central Research Station
Can you describe the model for songbird predation and forest fragmentation?
Before doing that, it’s important to understand why we need this model. We know that some species are declining in numbers and there are various levels of conservation concern for these species. However, we don’t know what is causing most of these declines.
The model identifies factors that occur at many different spatial scales. Migratory birds undergo an annual cycle of activity in many countries. This model has been chosen to focus on breeding grounds.
On the breeding grounds, factors that influence nesting success occur at several scales. The smallest scale occurs at the bird’s nest, whether breeding at that nest was successful or not. Two important factors can cause breeding to fail: nest predation and brood parasitism. These factors are influenced by such things like the location of the nest, time of the season, differences between species [birds and predators], and nest site.
The next scale addresses what the patch of area around the nest is like. What types of habitat affects the predator community, or what type of habitat determines the abundance of cowbirds?
The next biggest scale is the landscape scale, ranging from thousands to tens of thousands of acres. Broad patterns in land use in the surrounding landscape such as forest, cropland, and pastures can affect nesting success. These patterns affect kinds of, and numbers of predators that are found in the landscape.
This is a conceptual model, an organization of hypothesis that can be used to make predictions and then tested. An important aspect of the model is it addresses interactions among scales as well.
What does the model predict?
For example, edge effects and landscape effects affect nest predation and cowbird parasitism. A higher rate of nest predation and cowbird parasitism is found in nests close to forest edges because there are more predators and cowbirds in these areas. Furthermore, if the habitat is a highly fragmented forest landscape, there is a higher incidence of nesting failures. These scales may interact, and landscapes with low levels of forest fragmentation may have less of an “edge effect” than landscapes with high levels of forest fragmentation.
How well has the model done in predicting nesting success?
A field study was done across three states. Sites close to the edge and far from the edge of the forest, in landscapes with high and low levels of fragmentation were surveyed for abundance of cowbirds and levels of nest predation.
The results of the surveys indicated that a higher incidence of predation was found in highly fragmented landscapes. Individual effects at different land scales were interacting to produce a combined effect. In other words, at a smaller scale, nesting at forest edges exposed a bird to more predation. But at a larger landscape scale, that rate of predation along the edges was higher in landscapes with highly-fragmented forest cover. These results have important implications for conservation efforts.
Can you describe a generalized model of nesting success or failure over the entire country?
The model changes over different parts of the country – what are the continental scale patterns that affect nest predation?
Let’s start with the simpler case. Cowbirds are brood parasites. There is a continental pattern to their activity. Parasitism rates are always higher in the Midwest and Central Plains, compared to the East and West Coasts. This geographical distinction overrides local effects. This is due to the historic distribution of cowbirds, their range expansion, and the habitat in those parts of the country. Nesting songbirds in the central part of the country are more likely to be parasitized, compared to other parts of the country.
The patterns for nest predation are more complex. There are many kinds of nest predators, dozens of different species over different ranges. These patterns need to be sorted out at a regional level. There are numerous regional studies underway to identify important predators, and the types of nests that are predated.
In the Midwest, nest predators tend to be generalists – birds, mammals, snakes. These animals do well in human-altered habitats. Forest fragmentation increases the negative effect of these predators on nesting birds.
But this pattern can vary across the country, depending on the local situation. In some western landscapes, the opposite is true, where the primary nest predators in the forest are squirrels.
This model is a conceptual model that can be applied to other parts of the country to organize different hypothesis and information, addressing variations at different scales.
How can this model be applied to practical land management decisions?
There is now enough evidence, enough useful information, to verify several ideas. For instance, specifically in the Midwest, we’re confident that for forest breeding birds, forest fragmentation in the landscape is a very important issue. For example, landscapes with low amounts of forest and high forest fragmentation may have high nest predation and strong edge effects. Conservation efforts in these landscapes might best be directed at reducing levels of fragmentation through reforestation or eliminating cowbird feeding habitats. In extensively-forested landscapes with low levels of fragmentation, conservation might address management to provide the appropriate range of habitats for desired species.
How do you collect information about nesting activity?
The field techniques for this are time and labor intensive. We search habitats to find nests using cues like the behavior of the birds near nests. Once nests are found, they are monitored. One way to do this is to visit the nest every 2 days, and check on it – is there a cowbird egg in it, has a predator eaten the eggs or nestlings?
The second method is to monitor the nest with a video camera. Video cameras take time lapse photos that record the predators at the nest. This is very important. There is a big difference in the types of predators that visit nests. In old fields, snakes are the primary nest predators. But this is not true in forests, where there are a more diverse array of predators, like jays, hawks, mammals, owls. Predators respond to different types of habitat.
How do you know songbirds are declining?
Songbird populations vary up and down over time, are variable in different environments where the birds do well in some landscapes and not in others. This variation in bird numbers is expected at a certain level.
But there is evidence of a long term consistent decline in some bird species populations. The best source of this data in North America is the Breeding Bird Survey, conducted by the U.S. Fish and Wildlife Service. Thousands of roadside routes are covered in this survey. The data has been collected at the Patuxent Research Center since 1966 and is the best data we have to indicate trends in bird populations. [For more information about the Breeding Bird Survey, visit their website at http://www.mp1-pwrc.usgs.gov/bbs/bbs.html ]
What are the other factors contributing to songbird declines?
Habitat loss. Better understanding of songbird habitats is key to understanding the viability of bird species – how habitat is used by different species, how reproduction success and survivial of birds depends on habitat. Species demographics is an important area, understanding the survival rate of juveniles and adults across different habitats and determining the normal variability in these bird populations.
In the Midwest, there are not a lot of federally-listed endangered or threatened birds. Some birds that are considered species of concern are found in shrublands, such as the Bell’s Vireo, Yellow-breasted Chat, Blue-winged Warbler, and Prairie Warbler. In the Midwest and Eastern U.S., the Wood Thrush is declining, according to data from the Breeding Bird Survey. The Prairie Warbler, a Midwestern and Southeastern, species is also in decline.
Additional Teacher Resources
Smithsonian Institution, National Zoo Migratory Bird Center: Travel Alert for Migratory Birds: Stopover Sites in Decline?
A lengthy article explains the importance of ?stopover sites’ for neotropical migratory birds. It covers the location of stopover sites, the difference in sites, the reason these areas and the bird populations themselves are declining and what is being done to counter the decline. The site also contains links at the end of the article to migratory bird fact sheets.
Wildlands CPR: “Where have all the Songbirds Gone? Roads, Fragmentation, and the Decline of Neotropical Songbirds”: http://www.wildlandscpr.org/databases/biblionotes/biblio8.3.html
This informative article touches on why neotropical songbirds are declining, on nest predation and on solutions to the decline. There is also a lengthy list of resources at the end of the article.
