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Three organisms are now working together to make some birds sick enough to vomit.  Common Milkweeds are beginning to bloom.  This increases visits to the plant by Monarch Butterflies.  The insect may lay an egg on a leaf when it investigates the flowers.  A larva will hatch and begin feeding on the plant's leaves, consuming tissues and some milky latex. 

Milkweeds produce a toxin that has no impact on the Monarch larva or the mature butterfly, but when some birds, including Blue Jays, eat a Monarch Butterfly, they get sick and seem to recall the experience leaving other Monarchs alone...