The walk through aviary has a smart feeder that reads each bird's leg ring and weighs it on the perch, so we know who is eating and how much they weigh without catching anybody. It logs a ring every night at about eleven that belongs to a bird recorded dead twenty years ago, and the perch weighs it at forty one grams, which is what she weighed.
I am the bird section's senior keeper, and the feeder logs are how I know my birds are well.
How the feeder works.
Every bird in the aviary wears a ring with a chip in it. The feeder's perch is a balance and there is a reader in it. A bird lands, the reader picks up the ring, the balance takes its weight, and the visit is logged with the time. The system flags a bird that has not visited or one whose weight is falling.
The rings are unique and the numbers are on a national register.
At about eleven every night the feeder logs a ring that is not on any bird in the aviary.
The number is in our records. It was on a small finch that came to us in 1999 and died in 2004, and the ring was removed at post mortem and is in a drawer in my office, in an envelope, with her record card. I have it in front of me.
The perch weighs the visit at forty one grams, which is her weight on the last card entry.
What has been checked.
The reader. Replaced, and the software reset, and the ring in my drawer read on the bench to confirm the number, which it is.
The ring in the drawer, at eleven at night, is in the drawer. I have sat with it. The feeder, forty metres away in a locked aviary, logs it on the perch.
The perch. A balance reads what lands on it. Forty one grams landed on it. Not a reader glitch: a mass, at the time of the read, for the length of a feed, and the feed's own dispenser, which meters seed by weight, logs about half a gram taken.
The birds. All present, all ringed, none of them forty one grams, none of them visiting at eleven, because they roost at dusk, and the aviary's night camera shows the feeder perch empty at the moment of the read.
What forty one grams is.
I want to be plain that a small finch is a small thing. Forty one grams is a large one, which she was. It lands, the perch dips, the reader reads, seed is taken, and it leaves, and the whole thing takes twenty seconds, which is a finch at a feeder.
The seed is gone in the morning by the half gram, from a hopper that meters it by weight, and the perch has, on three mornings, had a dropping on it of the size a finch leaves, which I have had looked at, and which is a finch's.
What is done about it.
The feeder log has a filter on it now that excludes her ring from the health flags, because for two years it flagged a bird that had not visited in daylight, and the flag was a dead bird's name in red on my screen every morning.
I have not deleted the ring from the register, because the register is a record and you do not delete a record, and because I have come to feel that she is entitled to be on it.
What I do.
I keep the section.
The one thing I have changed is that I fill the feeder last thing, at ten, rather than first thing in the morning, so there is seed in it at eleven.
That started after the autumn I forgot, and the hopper was empty at eleven, and the log shows her ring on the perch for eleven minutes rather than twenty seconds, forty one grams, waiting, and the night camera, which had never shown anything on that perch at that hour, showed the perch dipping, empty, every few seconds, the way a perch dips when a bird on it shifts its feet.