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Common: encountered (seen or heard) on ostensibly all days in range and habitat/season, often in relatively

large numbers. For example, Black Vulture, Great Kiskadee, Blue-gray Tanager.

Fairly common: encountered (seen or heard) on most or all days in range and habitat/season, but in smaller

numbers than common species. For example, Gray Hawk, Northern Ochre-bellied Flycatcher, White-

shouldered Tanager.

Uncommon: not usually encountered (seen or heard) on most days in range and habitat/season, and then

only in small numbers. For example, Hook-billed Kite, Sepia-capped Flycatcher, White-winged Tanager.

Also used for species encountered on most days in range and habitat/season, but only in very small numbers

because of limited or specialized habitat; for example, Fasciated Tiger Heron, Green-fronted Lancebill.

Scarce: species that appear rare, perhaps more due to their behavior or to observer coverage than to actual

rarity, such as Masked Duck, Audubon’s Shearwater; or to population reduction through habitat change

and hunting, as for Great Curassow in much of its range.

Rare: species that occur in low density, missed far more often than encountered (seen or heard) on a day

in the field and perhaps only encountered a few times a year. For example, Tiny Hawk, Great Jacamar.

Very rare: not usually encountered every year, and should be documented carefully. For example, Gray-

bellied Hawk, Brown-banded Martin.

Irregular: Abundance varies appreciably between years, not necessarily annual in occurrence. For example,

Green-winged Teal, Tree Swallow, Myrtle Warbler.

Range Maps

In a country as topographically complex as Costa Rica, where lowland valleys cut far into high mountains,

elevations can change greatly across a few kilometers, and it is impossible to show truly accurate bird

distribution on a tiny range map; moreover, for reasons not readily discernible to humans, a species may

be fairly common in one valley and absent from an adjacent but seemingly equally suitable valley. Hence,

the range maps are by necessity simplified and often extrapolated, but a quick glance at them should give

a good idea of whether you are in the right place and season, and the common patterns of distribution

should soon become second nature (Figure 3). The maps are based on peer-reviewed literature, specimens,

our own field experience, and eBird (but see below).

Maps should be used in conjunction with the Status sections of the species accounts. For example,

a quick visual check of, say, p. 172 shows immediately that Yellow-billed and Black-billed Cuckoos

are widespread transient migrants (usually absent from the highest mountains); Mangrove Cuckoo is a

widespread nonbreeding migrant at low and mid-elevations (check the account for seasonality); Groove-

billed Ani occurs widely as a resident breeding species; and Smooth-billed Ani occurs as a resident

breeding species only on the south Pacific slope.

Because seasonal occurrence can be complex at a local level, and is poorly known in many cases, we

map only the primary seasonal status, with notes in the text about local differences; for example, Yellow-

billed Cuckoo is mapped as a transient migrant and Mangrove Cuckoo as a nonbreeding migrant, but the

text notes that Yellow-billed can occur rarely in winter on the Pacific slope and that Mangrove may breed

locally along the Pacific coast.

Most resident tropical species do not wander much, but migrants might on occasion be found well

outside the mapped ranges. Distribution can also be dynamic, as for many species that favor open and

disturbed habitats and whose ranges are expanding north and upslope to higher elevations. Maps are not

included for very rare migrants and also do not usually show areas of very rare occurrence, where a species

is not to be expected; this information may be covered in the text. Thus, the lack of a range map usually

indicates a species unlikely to be encountered.

A note on eBird (see www.ebird.org). Based on the infinite number of monkeys theorem, this popular

resource has great potential and was extremely helpful in creating the range maps. However, for numerous

taxa in Costa Rica and elsewhere in Middle America we often found at least 10–20% of documented (by

photo or sound recording) eBird reports to be misidentified (in some cases more than 50% misidentified;

exceptionally 100% when only one or two reports existed). This is not a commentary on the gallant eBird

reviewers, who have volunteered for this thankless, Sisyphean task, but simply an inevitable consequence

of the sheer volume of unfiltered input far exceeding the foreseeable capacity for proofing. Hence, eBird

can give misleading impressions of distributional and seasonal occurrence patterns, and should be used

with caution (Figure 4). When our maps or range statements do not agree with eBird it is usually because

we discount unverified or inaccurate reports rather than having overlooked them. Undoubtedly this will

lead to errors of omission for some species, and we encourage users to document records outside the

mapped ranges.