“Next, the two things which you couldn’t be expected to puzzle over. The breeding box which you did see but did not understand, and the undercurb of the step which even I didn’t see but felt with my hand. That breeding box had the usual little doorway or round opening for the nesting bird to enter by, but to my surprise the doorway had a door and the door was closed. Now, that’s going too far in pet-love sentimentality and although very cruel people are often very kind to animals, that kind of soapy gesture to birdmother comfort seemed to me strange—until I noticed, on the under-side of the next box, a small wheel. When I felt under the jamb of the step, I found two more such wheels—flanged wheels, and running along from one to the other, a black thread. Then when I knew what to look for, I could see the same black thread running up to the wheel fixed in the bird-box. I couldn’t doubt my deduction any longer. That little door could be opened if someone raised his foot slightly and trod on the black thread that ran under the step curb.

“Now, one doesn’t have to be a bird fancier to know that birds don’t want to breed in boxes where you shut them up with a trap-door. What then could this box be for? You do, however, have to be something of a bird specialist to know about hawking and hummingbirds. The main technique of the former is the hood. When the bird is hooded it will stay quietly for long times on its perch. Cut off light and it seems to have its nervous reactions all arrested. Could that box be a hood not merely for the head of a bird but for an entire bird? Now we must switch back, as swoopingly as a hummingbird, to Miss Hess. You remember the description?”

It was my turn to be ready. I reached round to the paper rack and picked out the sheet that had started the whole adventure. I read out, “The late Miss Hess, whose huge fortune has gone to a very quiet recluse nephew whose one interest is birds, was herself a most colorful person and wonderfully young for her years.” I added, “There’s a colored photo of the colorful lady. She’s wearing a vivid green dress. Perhaps that’s to show she thought herself still in her salad days?”

“A good suggestion,” replied Mr. Mycroft generously, “but I think we can drive our deductions even nearer home. Of course, I needed first-hand information for that. But I had my suspicions before I called.”

“Called where?”