cavities. Sites that were unsuitable because they were easily accessible to preda-
tors were permanently blocked ( Jones et al. 1991). The use of nest boxes has
greatly increased the numbers of breeding kestrels on Mauritius by providing
nest sites in areas previously lacking them. In one subpopulation exposed to a
shortage of natural cavities in the 2002–03 season, 27 (63%) out of the 43
known pairs used nest-boxes.
Nest cavity modification has been a major feature of the Echo Parakeet
restoration work. Cavities were modified according to the characters of the most
secure and successful sites, with changes to every occupied cavity, and others
where parakeets were seen prospecting. In the 2002–03 season, there were
21 breeding pairs, 17 in modified tree cavities and 4 in nest-boxes.
Any nest-sites in rotten trees that were in danger of falling were destroyed,
while others were reinforced and repaired. In sites that flooded in wet weather,
drainage holes were inserted, or weather guards were placed around the entrance
to keep out driving rain. Shallow cavities are deepened to at least 70 cm. An
entrance door was built into the side of every cavity, so that field-workers could
gain access to eggs and young. The substrate was changed in all cavities before the
breeding season, and again every week or two during the nesting period, in order
to maintain hygiene and to prevent the build up of nest parasites.
In some cavities, the size of the entrance hole was reduced to exclude White-
tailed Tropic Birds Phaethon lepturus, (which have plenty of other sites), in order
to increase the numbers available to parakeets. At some cavities a network of
branches was placed near the hole, to prevent long-winged, non-perching tropic
birds from gaining access to the cavity, but to provide perches for adult and newly
fledged parakeets. In all nest trees, predator guards in the form of smooth plastic
sheeting wrapped around the trunk were fixed and any interlocking branches
from neighboring trees were pruned off to discourage monkeys and rats. Between
1987 and 2002, 45 modified cavities were used between one and seven times.
Nest-site enhancement has been an important component in the restoration
of the Puerto Rican Parrot Amazona vittata. For this species nest-sites were found
to be in short supply and accessible to predatory Pearly-eyed Thrashers Margarops
fuscatus, a recent colonist of Puerto Rican forests (Snyder 1978; Wiley 1985;
Snyder et al. 1987). Some cavities were used for 20 years or more, as good sites
that were both secure and of suitable size were scarce. The modification of nest
cavities was also a component of the California Condor Gymnogyps californianus
restoration project, where cliff cavity floors were leveled and rock baffles built for
protection (Snyder and Snyder 2000).
The Echo Parakeets on Mauritius would readily accept modified cavities, but
for many years refused to use nest-boxes, until at last a design was found that was
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