Threats to Birds

Deer browsing alters sound propagation in temperate deciduous forests, Plos, Feb 13, 2019The efficacy of animal signals is strongly influenced by the structure of the habitat in which they are propagating. In recent years, the habitat structure of temperate forests has been increasingly subject to modifications from foraging by white-tailed deer which alters vegetation structure and thus the foraging, roosting, and breeding habitats of many species. However, despite a large body of literature on the effects of vegetation structure on sound propagation, we do not yet know what impact deer browsing may have on acoustic communication. We found that sound fidelity, but not amplitude, differed between habitats, with deer-browsed habitats having greater sound fidelity than deer-excluded habitats. Difference in sound propagation characteristics between the two habitats could alter the efficacy of acoustic communication through plasticity, cultural evolution or local adaptation, in turn influencing vocally-mediated behaviors. Reduced signal degradation suggests vocalizations may retain more information, improving the transfer of information to both intended and unintended receivers. Overall, our results suggest that deer browsing impacts sound propagation in temperate deciduous forest, although much work remains to be done on the potential impacts on communication.

DEER POPULATION AFFECTS BIRD POPULATION, Fairfield County, Deer Management Alliance, n.d.The increasing populations of various deer species are edging out bird species in North America, with the white-tailed deer being the species plaguing our region. The decline in forest birds has long been blamed on factors such as disease, loss of habitat, and an increase in the number of animals that prey on bird nests. These factors continue to impact bird populations; however a recent study has focused on the effects of overabundance of deer on birds.
As reported in the October 2005 issue of Biological Journal, the white-tailed deer is overabundant in 73 percent of its range in North America and other deer species are overabundant in 41 percent of their ranges. Because these animals are devouring the forests’ native shrubs and saplings, birds that use the understory for either nesting or foraging for berries, insects, and worms are also being impacted.
The Biological Journal article focused on a study conducted on the Haida Gwaii archipelago in British Columbia that examined the relationship of deer and forest birds on six islands. The study found that birds that relied on the understory of a forest for food or nesting were the most adversely affected. Islands with highest deer density had no fox sparrows, which nest in the understory and no rufus hummingbirds, which forage in the understory. These species were commonly found on the islands with no deer.

    Improving Bird Habitat Through Forestry, Feb 17, 2017

    The City of Plainfield to Launch Deer Management Operation, TAP into Plainfield, Dec 28, 2017According to the Izaac Walton League of America which is an American environmental organization that promotes natural resource protection and outdoor recreation and is dedicated to conserving outdoor America for future generations; when wildlife is already at carrying capacity, “more” can be a disaster. Among the countless wild creatures hurt by overabundant deer are the deer themselves. Across vast expanses of their range, whitetails are sickly and scrawny.
    Birds suffer as well. The U.S. Forest Service found that when deer exceed 20 per square mile, cerulean warblers, pewees, indigo buntings, least flycatchers, and yellow-billed cuckoos can no longer survive. At 38 deer per square mile, phoebes and even robins disappear. Ground nesters, including wild turkeys, ruffed grouse, wood-cock, ovenbirds, and whippoorwills, can nest successfully in ferns. But as adults, these birds need thick cover, so they take a massive hit from predators when deer denude the understory.

    Cue the Jaws Theme: Deer Eat Birds, Birdwatcher Digest, 2017Preconceptions aside, it turns out that the behavior Buckley witnessed on video might not be entirely out of character. Although carnivorous whitetails have flown under the radar of most of us, scientists have been catching them in the act, not often but occasionally, for years.

    Study: High deer populations mean fewer songbirds, The Bulletin, April 20, 2017 A team used lasers to build 3-D maps of English woods, and discovered that places with high deer populations had less than one-third the shrubbery under 2 meters tall that deer-free forests enjoyed. “It is clear from our research that if we want to encourage more woodland birds, then we need to take action to restore the woodland structures they require,” Eichhorn said in a news release. “But in many areas it will need a drastic reduction in deer to have any real impact.Deer eat the bushes and saplings birds need for cover, forage and nesting material. And when concentrated in urban areas, safe from natural predators, they can become an ecological pest.

    Success and Predation of Bird Nests in Grasslands at Valley Forge National Historical Park [Abstract], Northeast Naturalist, March 2016Estimated nest-success probability for Eastern Meadowlark at Valley Forge was 0.25 (0.04–0.65, n = 7) and similar to estimates from the Midwest, but slightly lower than other studies in the Northeast. Nest success for Field Sparrow; 0.77 [0.31–0.98, n = 8] and Red-winged Blackbird; 0.48 [0.18–0.80, n = 10] was higher than estimates from other studies. The local predator community identified at Valley Forge was less diverse than documented in other studies, with only 4 species depredating 8 of 25 monitored nests. The primary predator was Odocoileus virginianus (White-tailed Deer; 38% of nest predation events) followed by Vulpes vulpes (Red Fox; 25%); Procyon lotor (Raccoon; 13%), and a probable Mustela sp. (weasel; 13%).

    Woodland and Forest Birds: The Role of Sustainable Management, Audobon New York, July 20, 2015

    White-tailed Deer and Their Influence on Forest Vegetation, Long Island Natural History Conference, Tom Rawinski of the U.S. Forest Service, April 2015
    Deer Are Major Songbird Nestling Predators, Top Birding Tours, March 8, 2015Deer are apparently much more than opportunists. In 2000, scientists with the U.S. Geological Survey (USGS) used mini video cameras to document systematic predation of songbird chicks in North Dakota. As was the case in earlier predation episodes, the deer were filmed eating nestlings at night.
    Researchers Document Deer Eating Birds, Field and Stream, March 6, 2015Researchers keep on finding evidence that deer like a protein snack of bird from time to time. Deer eating birds opportunistically seems to be a wider-spread phenomenon than we previously thought. Scientists in North Dakota watching song birds via “nest cams” in a recent study found whitetails raiding more nests than either foxes or weasels.
    Deer Have Been Eating Birds For Years… They Will Eat Us Next, Huffington Post, March 5, 2015For those of you thinking we can kill our way out of this predicament, please put your hunting devices away. It’s too late. Our collective fate is sealed. There is a certain calm that washes over you when you accept your doom. Don’t fight it, let it bring you peace.
    Field Cameras Catch Deer Eating Birds—Wait, Why Do Deer Eat Birds?, io9, March 4, 2015These supposed herbivores placidly ate living nestlings right out of the nest. And if you’re thinking that it must be a mistake, that the deer were chewing their way through some vegetation and happened to get a mouthful of bird, think again. Up in Canada, a group of ornithologists were studying adult birds. In order to examine them closely, the researchers used “mist-nets.” These nets, usually draped between trees, are designed to trap birds or bats gently so they could be collected, studied, and released. When a herd of deer came by, they deer walked up to the struggling birds and ate them alive, right out of the nets.
    Abundant Deer Hurt Bird Populations, AboutNews, Jan 23, 2015Recent studies provide some good insight about the relationships between deer and songbirds:In one study in Quebec where researchers used a manipulative approach, areas where white-tailed deer density was experimentally reduced saw important increases in bird abundance and diversity (Cardinal et al., 2012).
    Similar results were found in a series of small islands off British Columbia where deer (mule deer in this case) occur at various densities. On the islands with no deer, abundance and diversity of birds was highest (Martin et al., 2011).
    Another study examined all the existing research done on deer and birds across North America, and then analyzed Breeding Bird Survey data, a very large citizen-science dataset. The authors report strong evidence for a continent-wide, negative effect of deer on the abundance of ground nesting or ground foraging birds (Chollet & Martin, 2013).

    Long-term consequences of invasive deer on songbird communities: Going from bad to worse?, Biol Invasions, Sept 2014By 2009, severe deer impacts extended to islands that were initially less affected by deer. The severity of impacts also increased even on islands that had been dramatically affected by 1989. Declines in bird abundance occurred before declines in bird diversity.These results support the need for actions by wildlife managers to curtail deer impacts as soon as these become evident, especially in reserves and protected areas that lack hunters and other deer predators.

    Exclusion of deer affects responses of birds to woodland regeneration in winter and summer, Ibis, 2014 Using an exclosure experiment in managed woodland in eastern England, we examined species and migrant responses to vegetation growth and its modification by deer herbivory, contrasting winter and the breeding season over 4 years. Several migrant species responded positively to deer exclusion and none responded negatively. The shrub-layer foraging guild was recorded less frequently in older and browsed vegetation. Exclusion of deer also increased the occurrence of ground-foraging species in both winter and spring, although these species showed no strong response to vegetation age. The canopy-foraging migrants were unaffected by deer exclusion or vegetation age in either season. There was seasonal variation in the responses of some individual resident species, including a significantly lower occurrence of Eurasian Wren and European Robin in browsed vegetation in winter, but no effect of browsing on those species in spring. Bird assemblage compositions also revealed seasonal differences in response to gradients of vegetation structure generated by canopy-closure and exclusion of deer. Positive impacts of deer exclusion in winter are probably linked to reduced thermal cover and predator protection afforded by browsed vegetation, whereas species that responded positively in spring were also dependent on a dense understorey for nesting. The effects on birds of vegetation development and its modification by herbivores extend beyond breeding assemblages, with different mechanisms implicated and different species affected in winter.

    Prior information reduces uncertainty about the consequences of deer overabundance on forest birds, Biological Conservation, Sept 2013For several bird species examined here, the inclusion of informative priors strengthened the conclusion that their populations were negatively affected by changes in vegetation structure caused by deer browsing. Our findings suggest that deer browsing in these island archipelagos must be managed if the risk of local extinctions among native flora and fauna is to be avoided.

    The Meat-eating Habits of Deer, Outdoorhub, Dec 9, 2013“If they come across a nest, where the food doesn’t move or run away, they’ll take advantage of it,” Pietz said. Other small animals are game too, as long as the deer can actually eat them.
    Birds, Birders Benefit from Fewer Deer?, D+DH, May 22, 2012Cristol does not advocate killing more deer in the column, stopping short of what probably would earn him the howling screams of anti-hunters and animal “rights” activists. Instead, he posits that ceasing or modifying existing management for deer would help restore forests and habitat for birds. Instead of fencing and eliminating management for one species – in this case, deer – a better solution for multiple species including birds would be to encourage continued wise habitat and wildlife management and, where applicable based on biology, for states to increase the daily bag limits for whitetail deer during hunting seasons.

    Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer, Biological Conservation, Sept 2011At avian guild level, significantly more ground and understorey foraging birds were captured where deer were excluded, and negative responses to browsing were more marked for pooled migrants than pooled residents. At the species level, especially pronounced negative effects were evident for dunnock (Prunella modularis) and garden warbler (Sylvia borin); approximately five times more dunnocks were captured in deer exclosures than in browsed vegetation. We also detected negative responses to browsing by nightingale (Luscinia megarhynchos) and long-tailed tit (Aegithalos caudatus). No significant positive responses to browsing were detected. For some species the use of young re-growth increased post breeding relative to the breeding period, including a marked shift by pooled residents that involved a disproportionate number of juveniles. Previous studies in North America have shown that, through vegetation modification, ungulate activity can alter woodland bird assemblages.

    Effects of Deer Herbivory on Birds, Wisconsin Bird Conservation Initiative, Sept. 2010

    White-tailed Deer (Odocoileus virginianus) Predation on Grassland Songbird Nestlings, USGS Northern Prairie Wildlife Research Center, Feb 2013

    Abundant Deer Hurt Bird Populations, Environmental Issues, 2013

    Wisconsin Bird Conservatory Initiative

    Specific Threats to Connecticut’s Birds, Connecticut Audubon Society, 2007

    White-tailed DeerAlter Diversity of Songbirds And Their Habitat in Northwestern Pennsylvania, Pennsylvania Birds, 1996

    US songbirds decline as deer populations rise, Invasive Species Council, Sept 21, 2012


    Managing the Abundance and Diversity of Breeding Bird Populations through Manipulation of Deer Populations, Conservation Biology, 2001Deer densities in forests of eastern North America are thought to have significant effects on the abundance and diversity of forest birds through the role deer play in structuring forest understories. We tested the ability of deer to affect forest bird populations by monitoring the density and diversity of vegetation and birds for 9 years at eight 4-ha sites in northern Virginia, four of which were fenced to exclude deer. Both the density and diversity of understory woody plants increased following deer exclosure. The numerical response of the shrubs to deer exclosure was significantly predicted by the soil quality (ratio of organic carbon to nitrogen) at the sites. Bird populations as a whole increased following exclosure of deer, particularly for ground and intermediate canopy species.


    Deer Behind Britain’s Great Bird Decline?, National Geographic, October 28, 2010

    A perfect storm: two ecosystem engineers interact to degrade deciduous forests of New Jersey, Biol Invasions, 2008We look at how two ecosystem engineers, the white-tailed deer and the invasive plant Japanese stilt grass, interact to completely alter the structure and composition of the subcanopy within northern deciduous forests. This interaction has wide-ranging repercussions on forest food webs which we explore through a case study of breeding woodland birds in the state of New Jersey. We show that the guilds of birds that rely on the subcanopy have experienced greater declines from 1980 to 2005 than birds that specialize on the intact upper canopy of impacted forests.

    Deer Culling – A Critical Tool for Conserving Local Habitat Diversity: A Green Paper by the Bird Conservation Network, Bird Conservation Network, Nov 2007This paper outlines concerns that habitat within forest preserves and other natural areas in the region are being adversely affected by widespread overpopulation of white-tailed deer, and recommends ongoing proactive efforts to reduce this deer overpopulation

    Canada Warbler, Bird Conservation Western Great Lakes Basin, 2006 Densities of breeding Canada Warblers dropped significantly as deer density increased from 8/sq mile to 35/sq mile (DeGraaf et el. 1991)

    Deer Decreasing Forest Bird Population, Scientific American, Oct 21, 2015Large populations of deer are edging out forest birds in North America, report scientists in this month’s issue of the journal Biological Conservation. The study is the first to evaluate the impact deer grazing can have on nest quality and food resources in areas unaffected by human activities such as forestry or hunting. It also offers general rules for predicting the influence these animals could have on bird ecosystems in the future.

    Managing the Abundance and Diversity of Breeding Bird Populations through Manipulation of Deer Populations, Conservation Biology, 2000
    Deer densities in forests of eastern North America are thought to have significant effects on theabundance and diversity of forest birds through the role deer play in structuring forest understories. We tested the ability of deer to affect forest bird populations by monitoring the density and diversity of vegetation and birds for 9 years at eight 4-ha sites in northern Virginia, four of which were fenced to exclude deer. Both the density and diversity of understory woody plants increased following deer exclosure. The numerical response of the shrubs to deer exclosure was significantly predicted by the soil quality (ratio of organic carbon to nitrogen) at the sites. Bird populations as a whole increased following exclosure of deer, particularly for ground and intermediate canopy species. The diversity of birds did not increase significantly following exclosure of deer, however, primarily because of replacement of species as understory vegetation proceeded through successional processes. Changes in understory vegetation accounted for most of the variability seen in the abundance and diversity of bird populations. Populations of deer in protected areas are capable of causing significant shifts in the composition and abundance of bird communities. These shifts can be reversed by increasing the density and diversity of understory vegetation, which can be brought about by re- ducing deer density.


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