As Dave indicates, the winter diet of the Prothonotary Warble may certainly
include berries.
Lisa Petit in her Prothonotary Warbler life history account in NA Birds online
indicates:
"Primarily insectivorous throughout annual cycle (butterflies and moths
[Lepidoptera], flies [Diptera], beetles [Coleoptera], and spiders [Araneae]),
but also takes mollusks (Mollusca) and isopods (Isopoda). In nonbreeding season
can be more opportunistic, feeding on some seeds, fruit, and even nectar in
addition to insects and spiders. Breeding-season diet includes mayflies
(Ephemeroptera)."
A study by Morton (1980) of stomach contents in Panama wintering grounds
revealed that 10 % of the volume was fruit.
Morton, E. S. 1980. Adaptations to seasonal changes by migrant land birds in
the Panama Canal Zone. Pages 437-453 in Migrant birds in the Neotropics:
Ecology, behavior, distribution, and conservation. (Keast, A. and E. S. Morton,
Eds.) Smithson. Inst. Press, Washington, D.C
Buckthorn is highly invasive and is often spread by birds. The berries are
generally considered a starvation food source, however, since they are toxic
and cause diarrhea. This toxicity apparently diminishes after the berries
ripen. Buckthorn has an allelopathic chemical in its roots that suppresses the
growth of surrounding plants, much like black walnuts do.
Doug Kibbe
Littleton
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