Topic 4.3: Birds

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Topic Introduction

In this topic, we look at the avifaunas of tropical forests. There are approximately 10,500 known species of birds, and over 21,000 recognized subspecies. It is difficult to say how many of these depend on tropical forests as while many birds are resident in tropical forests, there are also migratory species that depend on certain tropical forests for a part of the year. For example, many Neotropical forests are critical to overwintering migrants from North America, and serious declines in the populations of many these species be linked to the loss of Neotropical forests.

In addition, some birds that are resident in the Tropics are only dependent on tropical forests for part of their life cycles. For example many noddies (a species of tern (Anous spp.; Laridae) spend most of their lives at sea, and are entirely reliant on the sea for food. However, they need coastal forests for breeding, nesting in a variety of coastal tree species, including the so-called ‘bird-catching trees’ (several different species have been given this name, mostly within the Pisonia (Nyctaginaceae) genus). These trees have sticky seeds that can entangle both young and adult birds so severely that they are unable to fly. The birds die, and their decomposing bodies provide nutrients for the trees.

Considerable stratification occurs within tropical avifaunas. Some species are restricted to the forest floor, such as the cassowaries (Casuarius spp.; Casuariidae) and tinamous (Tinamidae). Others occur very close to the ground, but also use shrubs and the lower strata within the forest. Examples include many of the species that follow army ants (such as the antbirds (Thamnophilidae) and ant-tanagers (Habia spp.; Cardinalidae). On the other hand, there are many species that rarely descend from the forest canopy, many of which are frugivores. Finally, some groups, such as the swifts (Apodidae), feed almost exclusively in the air above forests.

Birds are the best known group of organisms in tropical forests. Nevertheless, new species are still being discovered and others are being reclassified as new genetic information becomes available. Classification has been particularly challenging, as genetic and molecular studies have revealed that many birds that were assumed to be closely related, based on their appearance, have actually very little relationship. This is often reflected in the common names, which tend to indicate where and when a bird was first described. Many of these changes have occurred quite recently, and will no doubt continue to be made.

In determining how to present the incredible diversity of bird life found in tropical forests, we have departed from a strictly systematic approach and grouped families broadly by their feeding strategies. This in itself created a number of issues, as many species vary in their diets, either seasonally or opportunistically. For example, birds that normally eat seeds (granivores) may feed their young insects, as the young need more protein to develop quickly. Others may change their diets depending on season. Many migrants, for example, feed on insects during their summer breeding season, but include more fruit and seeds in their diet when over-wintering in the tropics.

To take into account these factors, we took each family and looked at their dominant diet, allocating them to a particularly group based on this. The only exception was the pigeons and doves family (Columbidae), where there is a clear distinction between seed-eating species and fruit-eating species (frugivores). Obviously, for individual species, there will be exceptions.

We also had to simplify the diet-based categorization. There is a huge range of potential dietary specializations, and we have grouped these into particularly groups. For example, under carnivores, we have grouped the birds that eat vertebrates, although they might more correctly be put in different categories, including avivores (a diet of birds) and piscivores (a diet of fish), or those practicing anurophagy (a diet of frogs), ophiophagy (a diet of snakes) and so on. Similarly, we have grouped all the birds that eat arthropods as insectivores, even though not all arthropods are insects. We have placed all families that eat more than one major class of food under the general grouping of omnivores, even though strictly speaking an omnivore eats everything (very few do).

Within each of these feeding behaviours, families are described systematically.

By the end of this topic, you should be able to:

  • Describe the range of birds that you might encounter in a tropical forest.
  • Identify the avian groups found in particular regions.
  • Assess the role that different groups of birds play in tropical forests.
  • Determine which birds are likely to be seed predators, and which are likely to be seed dispersers.
  • Identify which avian groups are important for the pollination of plants.
  • Assess the habitat needs of different groups of birds.
  • Determine which groups of birds might be benefited or adversely affected by forest management decisions.

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