9

less humid Pacific slope. Likewise, for a bird described as occurring from mid-March to September, an

occurrence in early March or early October would not be exceptional, whereas an occurrence in January

would be highly unusual. World range is summarized in parentheses at end of the Status section.

Abundance terms are by necessity subjective, as well as relative and averaged out over several years;

migrants, especially transients, may be common one week, rare the next, depending on local weather

conditions. Abundance tends to vary across a species’ range (often a species is less numerous at the edge of

its range), or by habitat and elevation. Apparent abundance can also be affected by time of year (species

are often most conspicuous when singing in spring, and thus seem commoner at that season than when

quiet, retiring, and molting in late summer and fall), and also by observer familiarity (some species are

rarely seen but often heard, and will appear rare or even absent to somebody who doesn’t recognize the

vocalizations). Habitat can also influence real or perceived abundance: species of open habitats are more

conspicuous, and often more numerous, than species of forested habitats, where greater species diversity

is balanced by fewer individuals of any given species. And simply by virtue of lifestyle, common hawks are

numerically less numerous than common tanagers.

That said, most species are (relatively) fairly common in appropriate habitat and season within their

mapped range. Many species, however, especially those of forests, are now absent from much of their

mapped range courtesy of habitat modification by humans; and even species that should be common

in open and modified habitats are oddly scarce in large areas of the deforested Caribbean lowlands,

presumably thanks to pesticides and other anthropogenic factors.

Figure 1. Costa Rica, showing population centres and places mentioned in the text.

Lake Nicaragua

PANAMA

NICARAGUA

Caribbean Sea

Pacific Ocean

11°N

8°N

83°W

86°W

Towns, cities

National parks, reserves

Volcanoes, peaks

∆

∆∆

∆

∆

∆

∆

∆

∆

∆

∆

∆

Santa Rosa

San José

Palo Verde

San Vito

Carara

Cartago

Corcovado

Turrialba

Limón

Caño Negro

La Selva

Braulio Carrillo

Buenos Aires

Liberia

Monteverde

Puntarenas

Tortuguero