mates can differ by several orders of magnitude de-
pending on the scale of measurement and the time
that the census occurs relative to the annual cycle of
the bird.
6.5.2. Demography
Understanding the demographic parameters of a popu-
lation requires quantitative knowledge of many life-
history parameters, many of which are difficult and
time consuming to measure in wild birds. Brown Jays
can begin breeding when they are 1-2 years old but
rarely do so (Lawton 1983). Older members of a flock
usually perform most of the breeding activities
(Lawton and Lawton 1985). Flocks made up entirely
of 1—2-year-olds occur, but an activity such as nest-
building takes three times as long as for more mature
flocks (Lawton 1983). In contrast, House Wrens begin
breeding at the age of one year; yearling females are
just as capable at raising young as older females in
terms of clutch size, number of nesting attempts, egg-
hatching success, and nestling growth (Young 1993b).
In Long-tailed Manakins, as in many lek-mating sys-
tems, the sexes differ greatly in their age at first repro-
duction. Females can breed when they are 1-2 years
old, but males rarely copulate before they are at least
8 years old (McDonald 1993b).
Fertility for birds is measured as the average num-
ber of eggs, chicks, or fledglings produced per female
per year. The fertility term in matrix population mod-
els based on postfledging censuses is the number of
offspring produced per female per stage (usually one
year for models of bird populations) times the prob-
ability that these offspring survive to the next stage
(usually the first year of age). In Monteverde, fertility
has been estimated or calculated directly for three
species. Female House Wrens at Monteverde produce
on average 4.2 three-day-old nestlings per breeding
season (Young 1996). Of these, 3.7 survive to fledge,
2.5 survive the first two weeks post-fledging, and 0.4
survive to the following breeding season (calculated
from Young 1996). Female Long-tailed Manakins pro-
duce an average of 1.5 offspring each year that sur-
vive to the following year (McDonald 1993b). Brown
Jay flocks produce an average of 0.7-3.8 fledglings per
year (Williams et al. 1994). Other studies have based
calculations on the productivity of nests; these do not
translate into annual fertility without quantitative
information on first-year survival and how many nest-
ing attempts a female, pair, or flock makes per breed-
ing season.
Survivorship has been examined in Long-tailed
Manakins and House Wrens in Monteverde. In male
manakins, the average annual survival rate for 1-3-
year-olds was 0.68, whereas survivorship in olderbirds was slightly better (0.78). The survival rate for
females was 0.75 (McDonald 1993b). Average survi-
vorship for one-year-old or older House Wrens (as-
suming age-independent survivorship) was 0.46 for
males and 0.47 for females (Young 1996). Direct ob-
servations of breeding birds indicate that tropical
birds may live longer than related temperate birds
(Karr et al. 1990b). No temperate species is directly
related to manakins; however, survivorship of tem-
perate House Wrens is 17—49% lower than that of
tropical House Wrens (Young 1996).6.5.3. Sources of Mortality
Predators are probably important sources of mortal-
ity for Monteverde's birds, but direct observation of
predation events is rare. In a 2-year study of redstart
participation in mixed-species flocks, Shopland (1985;
Shopland, "The Cost of Social Foraging," pp. 206-207;
Fig. 6.7) never witnessed a predation attempt and
concluded that redstarts do not join mixed-species
flocks to gain protection from predators. Eight at-
tacks (half of which were successful) by either Barred
Forest-Falcons or Collared Forest-Falcons on Common
Bush-Tanagers were witnessed over a period of 15
months (Valburg 1992c; see Valburg, "Why Join Mixed-
Species Flocks?," pp. 205-206). However, an observer's
presence can frighten away predators, causing under-
estimation of their importance (Powell 1985). Also,
predation can be a rare but important selection factor
(Lima 1987). Anecdotal evidence accumulated over
many years in Monteverde supports the importance of
predation. Hawks and margays attack quetzals (Wheel-
wright 1983). An incubating House Wren was killed by
a Mouse Opossum. Barred Forest-Falcons attack fledg-
ling and adult House Wrens (Young 1993b) and are fre-
quently seen clutching recently captured birds.
Predation on bird nests is much easier to quantify.
Predators leave obvious traces in the form of dishev-
eled nests, egg shells, or blood in nests. Nest preda-
tion rates have been calculated for 11 species of birds
in Monteverde (Table 6.6). Predation is responsible
for variable but substantial losses in bird nests in
Monteverde, as is typical for the tropics (Ricklefs
1969). Although cavity nests are considered safer from
predators than open nests (Lack 1954), this pattern has
exceptions in Monteverde. House Wrens and Emer-
ald Toucanets build cavity nests and lose relatively
few nests to predators, but quetzals, which also nest
in cavities, have high rates of nest predation (Table
6.6). Some species can defend their nests against
predators. Adult quetzals successfully defend nests
against Emerald Toucanets and squirrels (Wheel-
wright 1983). Brown Jay mobs defend their nests from
Broad-winged Hawks, Common Black-Hawks, Varie-196 Birds