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Birds in Forest Ecosystems

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In book: Handbook of Forest Ecology, Chapter: Birds in Forest Ecosystems, Publisher: Routeledge Press, Editors: Richard Corlett, Kelvin Peh, Yves Bergeron, pp.281-296
Authors and Editors
Abstract
Birds are ubiquitous and highly interactive members of forest communities. As insect predators, birds influence tree growth by reducing the effect of folivorous arthropods. Coffee plantations, for example, benefit from insectivorous birds and have increased productivity as a result of bird control of insect pests. As frugivores that can move large distances, birds are the most important seed dispersers in tropical forests. Many crows and jays play critical roles as nut dispersers in temperate forests. Large vertebrate predators, such as hawks, may affect seedling establishment by preying on scatter-hoarding mammals or affecting their behavior. Pollination by birds is an important element in influencing the genetic structure of tree populations. Many of these ecosystem functions vary by latitude and by season. In return, forests provide food, nesting sites, and, in some cases, thermal refugia for birds. Forest structure, particularly in tropical sites, is closely tied to avian species richness on local and regional scales. Major threats to forest birds include deforestation, forest fragmentation, and urbanization. Invasive predators on nests and adults are also an important threat to island birds and climate-related changes.
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20
BIRDS IN
FOREST ECOSYSTEMS
Jerey A. Stratford and Çağan H. Şekercioğlu
Avian diversity in forests
Birds have been associated with forests as long as there have been birds (Sereno and Chenggang
1992). Since their origin, birds have diversied to occupy a remarkable array of habitats and
foraging strategies, unparalleled by any other terrestrial vertebrate (Naish 2014). Over the eons,
birds have formed intimate relationships between their habitats, their prey, and formed tight
symbiotic relationships, such as ower-pollinator symbiosis. Because the majority of birds are
conspicuous and relatively easy to study, they are among the best studied animals in forested
ecosystems (
Şekercioğlu 2006b; Şekercioğlu, et al. in press).
Despite their relative ease of being detected, new birds are still being discovered, particularly
in the rainforests of the Neotropics and Southeast Asia (Jenkins, et al. 2013; Lohman, et al.
2010). Currently, there are between approximately 10,300 (www.birdlife.org/datazone/info/
taxonomy; www.birds.cornell.edu/clementschecklist) to 10,546 (http://www.worldbirdnames.
org/) accepted extant species of birds. The exact number of avian species is unknown since we
still debate species concepts, new species are still being discovered, and the loss of species
through extinctions is happening in real time (Newton 2003; Sodhi, et al. 2011). The importance
of forests to birds cannot be overstated: forests are home to about 75 percent of avian species
and comprise the primary habitat of the majority of bird species (
Şekercioğlu, et al. 2004). The
highest diversity of birds (>5,000 species) occurs in lowland tropical and subtropical forests near
the Equator in the Americas and Africa and 25°N in Southeast Asia and declines towards the
poles (Birdlife International 2014; Newton 2003).
Lowland tropical forests have the greatest number of species and, among tropical forest sites,
the Neotropics have the greatest number of species. At least 30 bird families are endemic to the
Neotropics (not including Pluvianellidae or Thinocoridae). The understory of Neotropical
forests is dominated by approximately 1,100 suboscine (suborder Tyranni) species that include
the endemic antbirds (Formicariidae), treecreepers (Dendrocolaptidae), ovenbirds (Furnariidae)
and the incredibly colorful manakins (Pipridae) and contingas (Cotingidae). Non-passerines
that are endemic to the Neotropics include the tinamous (Tinamidae), motmots (Momotidae),
toucans (Ramphastidae) and others. Oscines (suborder Passeri), such as the tanagers (Thraupini)
dominate the Neotropical forest canopy. Only a few suboscines are found in the canopy or in
bright treefall gaps and only 52 suboscine species are found outside the Neotropics (Corlett and
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Jerey A. Stratford and Çağan H. Şekercioğlu
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Primack 2011). In the Afrotropical forests, the dominant groups include the cuckoos
(Cuculidae), shrikes (Laniidae) and others. More than 20 families are endemic to the Afrotropics,
most of which are found in forests. Tropical forests of Southeast Asia have at least ten endemic
families including the ioras (Aegithinidae), leafbirds (Chloropseidae) and fairy bluebirds
(Irenidae). Of the roughly 30 families endemic to Australia, New Guinea and surrounding
tropical islands, approximately 10 are found only in tropical and subtropical forests. Some of
these endemic families include the large and unusual cassowaries (Casuariidae), beautiful birds-
of-paradise (Paradisaeidae), terrestrial logrunners (Orthonychidae) and others. There are fewer
endemic species in the Australian, Afrotropical and Southeast Asian forests, but many shared
families between these biogeographic areas. Madagascar, despite its relatively small size and
proximity to Africa, has ve endemic families.
The highest species richness is in lowland rainforests and decreases with increasing elevation
(Able and Noon 1976; Rahbek 1997; Terborgh 1977), latitude, and decreasing productivity
such as seasonal tropical and boreal forests (Rahbek and Graves 2001). For example, the
Palaearctic Region is the largest biogeographic region and includes temperate forest, boreal
forest and tundra but has just over 900 species, many of which are shared with the Indomalaysian
and Afrotropical Regions (Newton 2003).
Forests aect birds
Forests provide shelter and sustenance
Forests provide the essential resources necessary for the completion of life cycles, including
food for adults and nestlings and nesting sites. Birds occur on various trophic levels in forests;
from primary consumers to vertebrate predators, as well as omnivores and scavengers. Because
birds are endotherms, their caloric requirements are higher than equivalently-sized ectotherms,
and hence their demands for food are higher and are likely to be more sensitive to changes in
forest resources.
As primary consumers, birds get nutrients from nectar, fruits, seeds and vegetative tissues
(roots, shoots and leaves). Birds that consume the vegetative parts of plants may supplement
their diet with other sources of protein such as insects (Karasov 1990; López-Calleja and
Bozinovic 1999). A strictly folivorous diet is rare among birds; only 3 percent of avian species
are strictly herbivorous (
Şekercioğlu, et al. 2004) and these tend to be large (>1 kg) non-forest
birds (López-Calleja and Bozinovic 1999) with most species in Asia (Kissling, et al. 2012).
Though forests have an abundance of leaves, secondary plant compounds and indigestible bers
(e.g., cellulose) make a diet of mature leaves an unusable food source for most birds (López-
Calleja and Bozinovic 1999).
Granivory (seed-eating) and frugivory (fruit-eating) are much more common among
herbivorous birds. Granivorous birds get most of their calories from the starches in seeds and
there are just over 1,000 avian species that are primarily granivores (
Şekercioğlu, et al. 2004).
Though not numerically the predominant guild, granivores can make up the greatest proportion
of avian biomass the Amazon (Terborgh, et al. 1990).
In subtropical and deciduous forests, acorns and beech nuts are an important source of lipids
and starches for a number of gallinaceous birds (fowl), corvids, woodpeckers and titmice.
Acorns for example, make up a signicant portion of the diet in turkeys in all seasons (Steen,
et al. 2002). Nuts are also a key food source during the winter months for birds that cache seeds.
In coniferous forests of the Northern Hemisphere, the seed crop inuences several species of
birds that forage on the seeds of rs and spruces (Petty, et al. 1995).
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Birds in forest ecosystems
Nearly 600 species of birds are nectivorous (Şekercioğlu, et al. 2004) and are concentrated in
the tropics (Brown and Hopkins 1995). In the Americas, hummingbirds (Trochilidae) and
owerpiercers (genus Diglossa) are the primary nectivores. Throughout Africa and Southeast Asia,
sunbirds (Nectariniidae) are the primary nectivores, and honeyeaters (Meliphagidae) are the main
nectivores in Australia. All of these groups include species that forage in forested habitats. Many
other species many also consume nectar including orioles, Phrygilus nches, bulbuls and white-
eyes. Flowers are often brightly colored to attract pollinators and oer nectar as a reward.
Nectar is an energy-rich food source although the quantities are small to promote movements
between owers (McCallum, et al. 2013). The composition of nectar is variable, but typically
the owers of avian pollinated plants contain sucrose and amino acids (Baker, et al. 1998).
Other sugars are often found in nectar and provide energy for pollinators, which are often
among the most energy-demanding taxa in forests. Hummingbirds, for instance, because of
their small size and hovering behavior, have the highest mass-specic metabolic rates for any
animal and require energy-rich foods.
Frugivory, in some form, is relatively common in forests birds and consists of birds consuming
a eshy pulp associated with a seed or seeds (Howe and Smallwood 1982). Nearly one in seven
bird species are frugivores (
Şekercioğlu, et al. 2004) and twenty-three families of birds include
fruit in at least half of their diet. For many frugivores, the proportion of fruit in the diet varies
seasonally as fruit abundance changes (Jordano 2000). Another 16 families are mostly frugivores
(Jordano 2000; Kissling, et al. 2012; Wenny, et al. in press), however only a few species are
exclusively frugivorous (Izhaki and Safriel 1989; Jordano 2000; Wenny, et al. in press). Fruits
are such a key resource for birds that the diversity of fruiting plants may play a role in determining
avian diversity (Kissling, et al. 2007). In tropical and subtropical forests, gs (Ficus spp.) are
particularly important and eaten by over 1,200 species of birds in 92 families, including birds
that are typically carnivorous.
Fruit availability increases with proximity to the Equator and with increasing moisture.
Consequently, tropical rainforests tend to have the highest biomass of fruit available and lower
seasonal variation in abundance compared to temperate forests (Jordano 2000). On local
scales, fruit availability is variable in both space and time, with a trend for fruits to be spatially
and temporally aggregated (Jordano 2000). For example, fruit availability in temperate and
tropical forests is greater in gaps, such as treefalls (Blake and Hoppes 1986; Levey 1988;
Willson, et al. 1982).
As a food source, fruits are highly variable in quality as a consequence of several traits
including nutrients, secondary plant metabolites, total size, relative seed size and water content
(Jordano 2000). These traits, however, are partially constrained by phylogeny and show
considerable overlap within families and genera (Jordano 1995). Generally, fruit content falls
into three categories based on sugar, ber and lipid content with fruits tending to be either lipid
or sugar rich. Fruits also tend to be low in nitrogen and proteins, which probably explains why
there are few birds that are exclusively frugivorous (Snow 1981). In the temperate zone, fruits
provide a source of lipids and other resources that allow birds to put on fats required for
successful migration (Bairlein 2003; Stiles 1980). Though fruits might be a high source of
energy, the lack of protein and the presence of secondary plant metabolites, such as tannins,
may reduce the nutrient value of fruits (Cipollini and Levey 1997). In the tropics, birds may
move on smaller scales to track fruit availability (Blake and Loiselle 1991; Blake and Loiselle
1992). There are also tropical birds that are nomadic and search large areas for adequate fruit
crops for reproduction (Stouer and Bierregaard 1993).
Insectivory is a commonly used term to describe a diet based on insects but this is probably
too narrow a term for many birds that should be called invertivores, since their diet includes
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284
other invertebrates, such as spiders and gastropods (Poulin, et al. 1994). Insectivores are typically
divided into aerial insectivores and terrestrial/arboreal insectivores. This division is based on
foraging strategy with the former, such as swifts and swallows, catching insects while staying in
ight for long periods of time. There are far fewer aerial insectivores (228 species) than terrestrial
arboreal insectivores (4,900 species), which is the largest guild of birds (Kissling, et al. 2012).
Insects and other invertebrates provide proteins and nitrogen in bird diets, which are particularly
important for growing birds. Insectivores have diverse strategies for nding insects. Terrestrial
insectivores, for instance, forage in the leaf litter but there are various methods of foraging in
litter including leaf tossing, searching the surface or searching under living leaves while walking
across the surface of leaf litter (Stratford and Stouer 2013). A number of tropical species are
obligate ant-followers and only forage for invertebrates escaping from army ant swarms (Willis
and Oniki 1978).
In temperate forests, lepidopteran larvae are important food sources that are fed to nestlings.
Their importance is underscored by the fact that the timing of reproduction in many birds is
correlated with the highest abundance of lepidopteran larvae in forests (Martin 1987). North
American Coccyzus cuckoos may track gypsy moth caterpillar (Lymantria dispar) outbreaks
(Barber, et al. 2008). Other birds that forage for invertebrates on the ground, the terrestrial
insectivores, are inuenced by insects in the leaf litter, where are, in turn, inuenced by
microclimate (Johnston and Holberton 2009).
Forests are also home to many carnivorous birds, including hawks, falcons and owls. There
are also several carnivorous birds of other orders found in forests including wood rails, ground
cuckoos and hornbills. Carnivores, like insectivores, eat animals, but carnivory typically refers
to species that eat vertebrates by killing them. There are some 300 species of carnivorous birds
(Kissling, et al. 2012). Other prey items found in forests include small mammals, lizards, snakes
and amphibians. Thirty-six species of avian scavengers (
Şekercioğlu, et al. 2004) exclusively
consume dead animals and other organic material. New World vultures (Cathartidae), in
particular, forage frequently in forests and play an integral role in nutrient cycling therein
(Houston 1985;
Şekercioğlu 2006a).
Most food items, by themselves, do not provide a complete diet so that frugivores, nectivores
or insectivores will often supplement their diet with alternative food items. These are often
taken opportunistically, such as abundant fruits during fall migration or termite emergences.
Though many birds take multiple types of food items, there are only 500 omnivores proper
(sensu Kissling, et al. 2012) and even fewer species in forests (Kissling, et al. 2012). Part of the
reason for the lack of omnivores is the need for specialized anatomy or physiology to capture
or procure food items and digest them (Gill 2007). Omnivores pay the cost of increased
handling time or decreased digestive eciency but have the benet of higher encounter rates
of potential food items. Omnivores may also be less susceptible to the eects of forest
fragmentation (Blake 1983; Henle, et al. 2004; Willson, et al. 1994).
Forests provide nesting sites
Forests also provide nesting sites for birds. Nests can be found in all forest strata: from the
ground, in the shrubs and in the treetops. Forest structure, such as canopy openness, inuences
reproductive success of forest birds. For many forest species, nesting success generally increases
with increasing canopy closure (Bakermans, et al. 2012).
Cavity nesting birds use hollowed out parts of trees or other tall plants as nesting sites.
Primary cavity-nesters are species that excavate the cavities and typically include woodpeckers.
Secondary cavity-nesters use the cavities created by primary cavity-nesters or use naturally-
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Birds in forest ecosystems
occurring cavities and include a taxonomically diverse assemblage. Secondary cavity nesters
include ducks (e.g., mergansers), falcons (e.g., kestrels), parrots, several owls, swifts, swallows,
trogons, several ycatchers, tits, wrens, bluebirds and many others. The availability of dead
trees and branches (snags) in forests can limit populations of both primary and secondary cavity
nesting birds (Kroll, et al. 2012; Newton 1994).
Forests provide wintering sites
Forests are important habitats for migrating birds in all the major yways (Kirby, et al. 2008)
where birds forage on insects and fruit and replenish fat reserves used to cross seas or non-
forested habitat (Moore and Kerlinger 1987). In southern North America, riparian forests are
important stopover habitats (Buler, et al. 2007; Hutto 1998) and forests are selected even when
less representative than other habitats in the landscape, such as areas in the Midwest, US
(Grundel and Pavlovic 2007). Many migrant birds then join tropical forest species for several
months (Wunderle and Waide 1993), primarily in Neotropical and Indomalaysian forests, and
become part of tropical food webs (Bauer and Hoye 2014). Habitat quality on the tropical
wintering grounds aects reproductive success in the temperate breeding grounds and inuences
populations of many long-distance migrants (Norris, et al. 2004).
Forests provide thermal refugia
Forests can provide microclimates that are refugia to physiologically challenging temperatures.
For example, small passerines will move into trees and forested habitats during the winter to be
in an environment that is sheltered from winds. The implication is that birds do not expend as
much energy maintaining body temperature (Wolf and Walsberg 1996; Wolf, et al. 1996).
Temperate birds will move towards the ground to minimize exposure to wind (Dolby and
Grubb 1999). On the other extreme, forests can be cool refugia places when temperatures are
high enough to cause thermal stress (Seavy 2006). During the nesting season, canopy cover may
aect the temperature of chicks during development and inuence reproductive success, at
least for cavity nesting birds (Dawson, et al. 2005).
Forest structure aects avian communities
The scale of measuring diversity has a key importance in elucidating drivers of diversity. Across
the globe, evolutionary history, plate tectonics and other historical factors inuence patterns of
diversity. Rainfall and temperature inuence biomes and create plant types such as the dierent
forests, grasslands and deserts. Within these dierent biomes, increased structural complexity of
vegetation is associated with increased avian species richness (MacArthur, et al. 1966; MacArthur
and MacArthur 1961; MacArthur, et al. 1962; Orians and Wittenberger 1991). One measure of
forest structure is foliage height diversity and is dened by the variation in the layers of a forest.
Increasing foliage height diversity is associated with increasing avian diversity, particularly
insectivores (MacArthur and MacArthur 1961; MacArthur, et al. 1962). Increasing foliage
height diversity is associated with increasing foraging sites and increased niches available to
exploit (MacArthur et al. 1966). The diversity of the tropics might be increased by increasing
specialization of forest birds. For example, there are a number of birds that specialize on dead
leaves that gather on branches (Rosenberg 1997) or foraging with raiding army ants (Johnson,
et al. 2013; Willis and Oniki 1978).
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Birds aect forests
Just as forests provide several resources for birds, birds inuence forests. This interaction is both
direct (e.g., seed dispersal) and indirect (e.g., eating phytophagous insects). Birds are now
widely recognized as having several positive interactions with forests, providing several
ecological services (
Şekercioğlu 2006a,b; Şekercioğlu, et al. in press; Wenny, et al. 2011).
Dispersal agents
Seed dispersal is a key aspect of plant ecology. Seeds that disperse away from the parent plant
are less likely to suer mortality from herbivores and pathogens (Janzen, et al. 1976). Moreover,
seeds landing farther away from the parent plant will be less likely to compete and reproduce
with relatives. Because of their abundance, breadth of taxonomic interactions and mobility,
birds are important as seed dispersers in forested ecosystems (Jordano 2000), particularly the wet
tropics where nearly 90 percent of tree species produce fruits (Howe and Smallwood 1982).
Birds disperse seeds for a taxonomically diverse range of plants including angiosperms and
gymnosperms (Jordano 2000; Stephens and Fry 2005).
Birds disperse seeds by eating fruits (endozoochory) then vomiting or defecating seeds or by
having seeds stick to them (epizoochory). At least during migration, endozoochory far
outnumbers epizoochory (Costa, et al. 2014). Even though epizoochory by birds might be
relatively rare, these long-distance events might be incredibly important from the perspective
of the plants (and, incidentally, small invertebrates and parasites (Darwin 1859)). The same is
true for endozoochorous seeds, which may need to be consumed to germinate (Jordano 2000).
Many seeds are deposited where germination is favored, such as elds (Carlo, et al. 2013).
Moreover, endozoochory by birds can remove fungi and bacteria that can damage seeds and
add a small amount of nutrients in feces (Fricke, et al. 2013).
There are however, few examples of close associations between birds and fruits. That is,
many species of birds eat many species of fruiting plants but there are few cases of specialized
relationships between birds and their fruits (Blüthgen, et al. 2007; Herrera 2002). A general
relationship has the advantages of resiliency. Birds would be able to use alternative fruits should
the abundance of any plant decrease. Likewise, fruiting plants do not rely on any particular
species for dispersal.
The lack of specialization between frugivores and plants also means that invasive plants can
nd a dispersal mechanism in novel areas (Gosper and Vivian-Smith 2006; Merow, et al. 2011).
Birds may, in some situations, even prefer fruits of invasive species to native species and play a
key role in their expansion (Greenberg and Walter 2010).
Much less is known about avian dispersal of nuts compared to avian dispersal of fruit but
recent work on jay-acorn dynamics has shown the importance of European, Blue and Scrub
Jays in oak dispersal, particularly long-distance dispersal (Gómez 2003). Though birds that
consume nuts are primarily seed predators, they are also key dispersal agents when seeds are not
recovered after caching them in soil (Steele, et al. 2010). Jays remove seeds from individual oaks
by the thousands and cache sites can be several kilometers away and often in sites where
seedling establishment is high, such as elds (Steele, et al. 2002). Thus jays are incredibly
important in forest succession. Their propensity for long-distance dispersal of acorns may
explain the rapid northern advancement of oaks after glaciers retreated in North America and
Europe (Johnson and Webb III 1989).
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Birds in forest ecosystems
Insectivory
Birds that consume phytophagous insects can potentially reduce the eects of leaf damage and
increase plant growth. These tritrophic interactions have been shown to exist experimentally
in temperate (Barber and Marquis 2009; Marquis and Whelan 1994; Mols and Visser 2002) and
tropical forests (Van Bael, et al. 2008). Birds can reduce damage to commercially important
trees including cocoa (Van Bael, et al. 2007) and coee plantations (Johnson, et al. 2010),
preventing up to US$310/ha/yr in damage to coee crops (Johnson, et al. 2010).
reats
Directly or indirectly, humans have been causing avian extinctions in the wake of our species
settling the globe. Human-caused extinctions are best documented starting in the sixteenth
century, though forensic genetics can demonstrate human-induced extinctions before this time
(Pimm, et al. 2006; Allentoft, et al. 2014). Most extinctions have occurred on islands (Szabo, et
al. 2012) though future extinctions are predicted to occur in biodiversity hotspots where they
are often associated with high rates of deforestation (Orme, et al. 2005). Deforestation rates are
increasing in many tropical forest biodiversity hotspots as a result of increasing production and
global trade of agricultural crops such as oil palm, soy beans and corn, as human food (Phalan,
et al. 2011), livestock feed (Anonymous 2014), biofuel (Danielsen, et al. 2009), and industrial
use (Fitzherbert, et al. 2008).
Deforestation, forest fragmentation and degradation
Deforestation can occur by removing a single area of habitat and leaving the remaining habitat
intact. Forest fragmentation results from a combination of forest removal and the creation of
forest patches amid a non-forest matrix. Strictly speaking, deforestation can occur without the
creation of forest fragments. More typically though, deforestation is accompanied by forest
fragmentation and both have negative consequences for avian populations. Forest degradation
can occur as a result of selective logging, which results in bird species losses at medium-to-high
intensities (Burivalova, et al. 2014). Moreover, logging and deforestation opens areas to hunting
and development. Consequently, habitat loss, high land-use intensity, forest degradation and
fragmentation are major drivers of avian species endangerment (Birdlife International 2014;
Fahrig 2003; Kerr and Deguise 2004; Newbold, et al. 2014; Sodhi, et al. 2011).
Typically, the number of forest interior species is reduced in forest fragments (Banks-Leite,
et al. 2012; Leck, et al. 1988; Whitcomb, et al. 1981) consistent with species-area relationships
(MacArthur and Wilson 1967). Globally, the most fragmentation-sensitive groups are tropical
insectivores and large frugivores (Bregman, et al. 2014). Less sensitive are small-bodied canopy
frugivores and pollinators (Stouer and Bierregaard Jr 1995). Though the species richness of
these groups is not strongly aected by forest fragmentation – other aspects, such as behavior,
are aected by fragmentation (Hadley and Betts 2009). Temperate species appear less sensitive
to forest fragmentation and the declines are less predictable (Bregman, et al. 2014). Causes of
sensitivity are likely to vary geographically (Stratford and Robinson 2005b). In North America,
for example, forest fragmentation increases exposure to brood parasites (Robinson, et al. 1995)
and nest predation (Rodewald 2002). In Neotropical forests, birds appear to be more sensitive
to light conditions (Patten and Smith-Patten 2012), changes in vegetation structure (Stratford
and Stouer 2013) and many are unwilling or unable to cross open habitats that separate forest
from forest fragments (Ibarra-Macias, et al. 2011; Moore, et al. 2008; Powell, et al. 2013).
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288
The loss of species as a result of forest fragmentation is likely to aect ecosystem services that
birds provide (Saunders, et al. 1991;
Şekercioğlu 2006a,b). Fruit removal rates tend to be lower
in forest fragments (Rey and Alcántara 2014), which implies reduced seed dispersal. Other
processes may be less aected or balanced by other eects. For example, population genetics of
Heliconia plants appear to not be strongly aected by fragmentation because their avian
pollinators and frugivores are able to move long distances between fragments and continuous
forests (Suarez-Montes, et al. 2011). Remnant native trees are important for sustaining some
tropical forest bird species and their ecological services in agricultural landscapes (
Şekercioğlu,
et al. 2007; Douglas, et al. 2014). Restoration eorts in these landscapes can result in the return
of some forest bird species (Catterall, et al. 2012).
Over-exploitation
Forest birds, especially large, edible species like guans, curassows, pigeons and pheasants, or
those that are victims of the pet trade, such as parrots and nches, are greatly over-exploited
and many of them are threatened with extinctions as a result (Birdlife International 2014; Sodhi
et al. 2011). In some forests, especially in western Africa and Southeast Asia, this has resulted in
the elimination of large birds and mammals, leading to the phenomenon of the “empty forest”
(Redford 1992) where the forest is devoid of many large vertebrates and their ecosystem
services (Kurten 2013;
Şekercioğlu 2010). Globally, over 400 bird species are threatened with
extinction due to over-exploitation (Birdlife International 2014). These comprise a third of all
threatened bird species, the majority of which are found in forests (IUCN 2014). The cage bird
trade is increasingly understood to be far more widespread for southeast Asian bird species than
previously thought and is pervasive even in many protected areas in the region (Kai, et al. 2014;
pers. observ.; Bert Harris, pers. comm.)
Invasive species and emergent infectious diseases
Invasive trees may compete with native trees and alter food resources and nesting sites. Invasive
plants are associated with lower species richness (Ortega, et al. 2014) and higher nest predation
rates (Borgmann and Rodewald 2004; Borgmann and Rodewald 2005). Invasive species can
aect birds not only directly but indirectly by aecting forest structure. Numerous invasive
pests have altered the North American landscape. For example, chestnut blight, Dutch elm
disease and the wooly adelgid have aected the amount of American chestnut, American elm
and eastern hemlock, respectively, and gypsy moths are currently reducing the proportion of
oaks. The loss of American chestnut may have contributed to the extinction of the Passenger
Pigeon (Ectopistes migratorius) (Bucher 1992).
In general, invasive bird species have failed to colonize most tropical and subtropical forests,
but there are some major exceptions on islands, including Hong Kong, Singapore and many
oceanic islands, such as Hawaii and New Zealand, where invasive birds have fundamentally
altered bird communities (Sol, et al. 2015). Other invasive vertebrates have profoundly aected
avian populations. The extent to which cats have aected avian populations has only recently
been appreciated (Loss, et al. 2013). The eect of invasive vertebrates on islands is particularly
damaging (Blackburn, et al. 2004; Towns, et al. 2006). The brown tree snake (Boiga irregularis),
for instance, has resulted in the extinction and population declines of at least 25 native species
on Guam, where it was introduced (Wiles, et al. 2003).
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Birds in forest ecosystems
Emergent infectious diseases may also have negative impacts on forest birds, such as West
Nile virus in North America (Kilpatrick, et al. 2007; LaDeau, et al. 2007), mycoplasma
conjunctivitis (Fischer, et al. 1997)
Urbanization
Urbanization is the conversion of forested habitats to human-created impervious surfaces.
Urbanization is a particularly pernicious form of land-use change since urban areas are rarely
converted back to a natural state. Another feature of urbanization is the frequent increase in
pollution (Pickett, et al. 2001). Associated with increasing urbanization is a loss of native
species, particularly insectivores (Chace and Walsh 2006). However, granivores and omnivores
increase in abundance and species richness, though these increases do not oset the loss of
other species (Marzlu 2001; Zhou, et al. 2012). In eastern North America, long-distance
migrants are highly sensitive to urbanization and most species are not found in landscapes that
are more than 20 percent urbanized (Stratford and Robinson 2005a). The eects of urbanization
can be mitigated by preserving urban forests, which can be important stopover sites for
migrants (Kohut, et al. 2009).
Climate change
As with all ecosystems and taxa, climate change is rapidly approaching habitat loss in the
magnitude of the threat it poses for forest birds (Harris, et al. 2011;
Şekercioğlu, et al. 2012;
Wormworth and
Şekercioğlu 2011). Climate change particularly threatens bird species endemic
to tropical montane forests, as most of these species are endemic to narrow elevational ranges
and have specic microhabitat and microclimate requirements. Species without access to higher
elevations, coastal forest birds and restricted-range species are also highly vulnerable (
Şekercioğlu,
et al. 2012). Some forest birds are especially susceptible to increased rainfall seasonality and to
extreme weather events, such as heat waves, cold spells and tropical cyclones. Protected forest
areas will be more important than ever, but they need to be designed with climate change in
mind. Networks of protected forests need to incorporate extensive topographical diversity,
cover wide elevational ranges, have high connectivity and integrate human-dominated
landscapes into conservation schemes (
Şekercioğlu, et al. 2012).
e future of birds in forests
The threats listed above to forest birds are not mutually exclusive. Forest birds can simultaneously
face increased habitat loss, invasion from introduced species, and novel diseases. Managing
forest birds, therefore, requires detailed knowledge of interacting threats as well as understanding
of long-term trends in avian abundance. Two long-standing tools that have given ornithologists
great insight are citizen science programs, such as the Breeding Bird Survey (Link and Sauer
1998) and the Christmas Bird Count (Butcher, et al. 1990). Other programs such as Monitoring
Avian Productivity and Survivorship (MAPS: www.birdpop.org/maps.htm) are providing
greater detail on age structure and reproductive success in dierent habitats. Involving the
public in forest bird research and conservation should not be optional. The advent of eBird
(Sullivan, et al. 2009) has demonstrated the power of citizen science in generating data on bird
distributions and movements. As is the case in conservation and research in general, increased
public involvement increases the sense of public ownership, augments public participation in
conservation policy and funding, and improves the chances of success bird conservation
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Jerey A. Stratford and Çağan H. Şekercioğlu
290
programs (Şekercioğlu 2012). Comprising the majority of the world’s bird species threatened
with extinction, forest birds are in urgent need of much greater levels of public and political
support if we are to have any chance of reducing the ever growing numbers of threatened and
extinct bird species.
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    ... From an applied point of view, this might help to understand the complementary role of bats and birds in providing pest control services to forests and surrounding landscapes (Maas et al. 2015; Whelan et al. 2015). We tested four predictions: (1) bat and bird species richness in European forests decrease with increasing latitude, from Mediterranean to boreal forests (Hurlbert 2004); (2) bat and bird richness and functional evenness increase with the amount of broadleaved tree species because they are more likely to provide more nesting and feeding resources than evergreen trees (Russ and Montgomery 2002; Lindenmayer et al. 2015; Charbonnier et al. 2016); (3) forest understorey structure (e.g., vertical stratification) have opposite effects on the two predatory vertebrate taxa, being negative for bats which use acoustic signals to locate their prey, and positive for birds which are mostly foliage gleaners, thus ultimately resulting in reduced intraguild competition (Müller et al. 2013; Maas et al. 2015; Stratford and Sekercioglu 2015) and (4) the strength of forest habitat effects on bat and bird diversity increases with latitude as local environmental filters become more dominant (Ricklefs 2004). ...
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