By the way, these days, the ume(梅) are blooming beautifully in my garden, and, some mejiro visit it almost everyday. I tried to take a picture several times, but I cannot have them which satisfy me. I’ve stopped asking too much and am writing the post now (^^;).
We have an idiom “ume(梅) ni uguisu(鶯)”, which we use as a positive meaning of “hand and glove”. But I’ve never seen uguisu(鶯) at the peak bloom period for ume blossoms. Though I know a card called “ume(梅) ni uguisu(鶯)” exists in Hanafuda, the bird on the card looks more like a mejiro than a uguisu. Actually, the uguisu mainly eats insects, so they maybe have no interest in ume blossoms, which bloom in early spring and don’t have a lot of insects, I guess.
You can see the image of uguisu on Wikipedia and can get its subdued color. As its song is beautiful, our ancestors might have expected them have like mejiro’s color. Then, the misunderstanding expression was maybe created. The same way as 仏法僧.
Now, I show you my mejiro photos. Sorry, the captions still remain in Japanese because I pasted its source from my Japanese blog.