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 oer 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-specic 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 (Stouer 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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