|Goldfinch- from Wikimedia|
Sciencenews.org reports a study from the University of Western Ontario where goldfinches were housed with Canadian thistles. When they were exposed to summery heat and saw the thistles bloom, they got the "breed now" message. Their testosterone levels were twice that of other finches which were shown thistle plants without blooms. They were separated from the plants by transparent shower curtains, so it is unlikely that taste or smell was the stimulus.
Thistle seed is prized by goldfinches both as baby food and their own feeding. It is interesting that the finches don't eat the thistle flower. Apparently they have made the association between the blossom and what is coming in a few weeks.
The American Goldfinch (Spinus tristis) is also called the Wild Canary. It migrates South in the winter but is found in most of the upper US throughout the year. It molts twice a year, the springtime molt producing the vivid yellow colors in the male that both we and female finches treasure.
Many birds are threatened by human's conversion of forests and fields to urban settings, but the finches have thrived. Not only do they love our feeders, but also our disturbed areas such as roadsides and untended fields which produce their favorite foods such as thistle, dandelion, ragweed, mullein and teasel.