Is Predation Responsible for Small Clutch Sizes of
Neotropical Birds?
It is a straightforward fact that forest species of
Neotropical birds lay fewer eggs in their nests than do
their counterparts in the temperate zone. A clutch size
for an American Robin (Turdus migratorius) is typically
four eggs and often five. Its Neotropical equivalent, the
Clay- colored Thrush (T. grayi; plate 9- 29)— which is the
national bird of Costa Rica— has a typical clutch size of
two or three eggs, though it occasionally will lay four
eggs. A study by Alexander Skutch of 115 nests showed
an average clutch size of 2.68 eggs.
One reason tropical birds may have lower clutch
sizes could be food- related. Tropical bird species nest
throughout the year, though various species have
various peak times. In contrast to the tropics, in the
temperate zone there is a major insect flush in the
spring as leaves open, which makes protein- rich food
abundantly available to nesting birds. Not only that,
days are longer, allowing more time for parents to
provision food for their growing nestlings. Perhaps this
scenario alone allows temperate- zone birds to increase
clutch size compared with tropical species. But it is likely
not the most compelling factor.Nest predation and thus nest failure are very high in the
tropics. Birds that make open- cup nests are vulnerable
to predators ranging from monkeys to ants (plate 9- 30).Bridget Stutchbury and Eugene Morton cite that nest
failure is in the general range of 80– 90%. That’s right—
something like eight to nine out of 10 nesting attempts are
destined to fail. (The failure rate is significantly lower in the
temperate zone.) A study in Panama cited by Stutchbury
and Morton that had been performed by Morton found
that 58% of Clay- colored Thrush nests were destroyed in
dry season, compared with 85% in rainy season. Food is
more abundant in rainy season but so is predation risk.
Morton also found that thrushes fledge (leave the nest)
earlier and when they are smaller in body mass than their
temperate equivalents. It appears there is strong selection
pressure for the birds to get out of the nest.
Given the high rate of nest failure in tropical forest,
why aren’t the birds extinct? The answer may be in
the adage “if at first you don’t succeed, try, try again.”
Really. Temperate passerines have, on average, much
reduced life spans compared with tropical species. A
small tropical bird like a manakin may live for 15 or more
years. Such a life span is unheard of with a temperate
species like a chickadee. The lifetime reproductive
success of a manakin, with its small annual clutches and
its high rate of nest failure, may nonetheless equal or
even exceed that of a shorter- lived chickadee.The breeding biology of Neotropical forest birds does
indeed suggest that predation acts very strongly as
a selection pressure and that it may contribute to
influencing diversity patterns.Plate 9- 30. This open- cup nest of a Palm Tanager (Thraupis
palmarum) contains two nestlings, a typical clutch size for
tropical forest passerine birds. The rate of nest failure from
predation is very high. Photo by John Kricher.Plate 9- 29. The Clay- colored Thrush suffers very high rates of
nest failure due to various forms of predation. Photo by John
Kricher.152 chapter 9 why are there so many species?