Snakes & Ladders: Tree-trimming Reduces Predation of Rare Parrots

Black-billed Amazon: Black-billed Amazon chick sitting on a Jamaican Boa that had previously consumed two nest mates in May 1999. Cockpit Country, Jamaica (Photo: Rudolph Diesel).

Nest predation by snakes - a particular concern for several endangered parrot species - may be reduced by removing vines from the nest tree, and isolating its canopy from neighbouring trees.

In a review of the results of two field studies (Vines and canopy contact: a route for snake predation on parrot nests, Susan E. Koenig, Joseph M. Wunderle, J R and Ernesto C. Enkerlin-Hoeflich, Bird Conservation International (2007) 17:1-15), the authors examine the causes of nest failure of Black-billed Amazon Amazona agilis in Jamaica, and habitat use by Puerto Rican Boas Epicrates inornatus in Puerto Rico.

Puerto Rican Boa is closely related and ecologically similar to Jamaican (or yellow) Boa Epicrates subflavus, the major predator of nestling Black-billed Amazon.

Of 63 nesting attempts by Black-billed Amazons in the study, 35 (56%) failed before fledging at least one chick. Predation of nestlings accounted for over one-third of total nest failures.

Two structural features of nest-trees were associated with nestling predation: nests with vines on the trunk were more likely to fail, and as canopy connectivity increased to neighbouring trees, so too did the probability of predation. Nest-trees in edge habitat were noted particularly for their abundance of vines. Of the nests that failed, 50% experienced predation in regenerating edge, compared with 21% in isolated trees in pasture, and none in the interior forest.

The authors say the appropriate isolation distance will depend upon the gap-bridging abilities of the target predators. In Puerto Rico a gap of 1.5 metres is likely to be adequate to prohibit canopy access of boas to nest-trees, as this species has not been observed crossing gaps greater than 0.5 metres.

To reduce the risk of nest abandonment by sensitive species, "snake-proofing"' activities should be done in the non-breeding season for species that show high site fidelity to a traditional nest-site, or at night for species that do not roost in the nest at night. If trimming is expected to cause too much disturbance to a nest-site, neighbouring trees can also be snake-proofed to isolate the nest stand.