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
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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.
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 (
taxonomy; 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
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
M20_HANDBOOK OF FOREST ECOLOGY_CH20.indd 283 15/06/2015 10:39
Jerey A. Stratford and Çağan H. Şekercioğlu
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).
M20_HANDBOOK OF FOREST ECOLOGY_CH20.indd 285 15/06/2015 10:39
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Although twice as many species were supported in low-lying forest than in upland forest, ecological overlaps among species in each habitat were usually similar. Behavioral similarity among species was not related to dietary overlap. Size of prey taken, however, was correlated with bill size, except that the largest species, Xiphorhynchus guttatus, ate surprisingly small prey. Diet composition of all species differed significantly from prey availability in dead leaves, with orthopterans selected by all species and small roaches and spiders often avoided. Censuses of 92 mixed-species flocks revealed no negative and only two positive associations between species, suggesting that birds join flocks independent of other species present. Co-occurring Myrmotherula leucophthalma and M. ornata in the same flocks were not aggressive and converged in foraging height and substrate use. In contrast, co-occurring Automolus foliage-gleaners tended to diverge in foraging height and exhibited overt aggression. Niche segregation among dead-leaf foragers therefore represents a balance between benefits and constraints imposed by feeding in a mixedspecies flock; that is, increased vigilence and group defense of territories versus feeding close to potential competitors. Dead-leaf specialization evolved independently in several bird families but shows strong phylogenetic constraints among genera. Phylogenetic study of Myrmotherula antwrens revealed that all specialist species were related and that they have been evolving separately from other antwrens perhaps for as long as nine million years. Foraging specialization is therefore a primitive trait within this group (and probably others), appearing before the radiation of modern species. Study of present-day ecology may not elucidate factors leading to the evolution of such specialization, especially without concurrent phylogenetic analyses. /// Una de las razones sugeridas para la gran diversidad de aves en los bosques tropicales es el aumento en la especialización sobre recursos que están ausentes en hábitats templados. Este estudio investiga en detalle una de tales especializaciones, esto es, el forrajeo para artrópodos en hojarasca suspendida en bosques tropicales de tierras bajas. Hasta 16 especies de aves en dos sitios en el suroeste de Amazonia consituyen un tipo de forrajeadores especializados en la hojarasca, los cuales forman aproximadamente el 11% de las especies de pájaros insectívoros de la región. La mayoría de los especialistas en hojarasca son pizpitas (Furnariidae) o pájaros comehormigas (Formicariidae), que son miembros característicos de bandadas mixtas de forrajeo en el sotobosque o el dosel. Estos especialistas, comparados con otros insectívoros, tendieron a colocarse en posturas más acrobáticas y a manipular el sustrato de forrajeo con las patas o el pico. Estas especies se segregaban por hábitat, hasta cierto punto, incluyendo varios congéneres que les reemplazaban. Esta asociación alimentaria alcanzó su mayor diversidad en la faja a través del suroeste de Amazonia y a lo largo de la base de los Andes, donde las bambúas y otros microhábitats de áreas perturbadas le añaden heterogeneidad al bosque. Hojas muertas individuales, como recurso para los pájaros, eran abundantes en todos los tipos de bosque y sostuvieron una mayor densidad de presas (número por hoja) que el follaje vivo adyacente. La densidad de presas fue mayor en hojas más grandes, especialmente en las hojas arrugadas grandes de Cecropia. La fauna artrópoda de la hojarasca suspendida fue similar a través de las estaciones del año, hábitats, y sitios, siendo dominada por arañas, cucarachas, otros ortópteros y escarabajos pequeños. Esto contrasta grandemente con los artrópodos disponibles en el follaje vivo. Miembros de las distintas asociaciones alimentarias difirieron significativamente uno del otro en altura de forrajeo, tamaño o tipo de hojas forrajeadas, composición de la dieta, o el tamaño de la presa, aunque los solapamientos ecológicos entre parejas de especies fueron usualmente altos (≥0.900). Aunque se encontró el doble de especies en terrenos bajos comparados con bosques en terrenos altos, el solapamiento ecológico entre las especies en cada hábitat fue usualmente similar. La similaridad en el comportamiento entre especies no estuvo relacionada con el solapamiento dietético. El tamaño de las presas cazadas, sin embargo, estuvo correlacionado con el tamaño del pico, excepto que la especie más grande Xiphorhynchus guttatus, consumió presas sorpresivamente pequeñas. La composición dietética de todas las especies difirió significativamente de la disponibilidad de presas en hojas muertas, siendo los ortópteros seleccionados por todas las especies, mientras que cucarachas y arañas pequeñas fueron evitadas. Censos de 92 bandadas de especies mixtas no revelaron asociaciones negativas y solamente dos asociaciones positivas entre especies, lo que sugiere que las aves se asocian con bandadas independientemente de qué otras especies estén presentes. Individuos de Myrmotherula leucophthalma y M. ornata que se encontraban en las mismas bandadas no eran agresivos y convergían en la altura y substrato utilizado. En contraste, Automolus tendió a divergir en la altura de forrajeo y exhibió agresión abiertamente. La segregación de nichos entre forrajeros de hojas muertas, por lo tanto, representa un balance entre beneficios y costos impuestos por alimentarse en bandadas de especies mixtas; esto es, aumento en la vigilancia y la defensa grupal de territorios versus la alimentación cerca de competidores potenciales. La especialización en hojas muertas evolucionó independientemente en varias Familias de aves, pero muestra fuertes restricciones filogenéticas entre Géneros. El estudio filogenético de Myrmotherula reveló que todas las especies especialistas estaban relacionadas y han estado evolucionando separadamente de otros del mismo Género, quizás por tanto como nueve millones de años. La especialización en el forrajeo, por lo tanto, es un rasgo primitivo dentro de este grupo (y probablemente otros), que aparece antes de la radiación evolutiva de especies modernas. El estudio de la ecología actual puede no elucidar factores que conlleven a la evolución de tal especialización, especialmente sin los análisis filogenéticos correspondientes.