The birds are already telling you
Birds run an alarm system you have been hearing your whole life. Most of what the calls carry has been measured — including what they cannot tell you.
From people describing what they had noticed in their own gardens, then checked against the published work on alarm calls — which has measured almost all of it, and reversed one part.
There is a sound outside your window most of the day, and you have almost certainly filed it as weather. It is not background. It is a running commentary, made by animals whose survival depends on knowing what has just walked into the garden, and a fair amount of what it carries has now been measured rather than guessed at.
The signal is a change in the background sound of a place, and it tells you something is there without telling you what.
Both halves of that matter, and the second half is the one that keeps people from getting into trouble with it.
What the calls actually carry
Black-capped chickadees make the call they are named for, and the number of dee notes on the end of it is not decoration. Researchers presented chickadees with fifteen species of live predator and counted. The number of dee notes went up for the smaller raptors, which is the reverse of what anybody guesses, and is exactly right, because a small agile hawk in tight woodland is far more dangerous to a chickadee than a big slow one. Play the calls back to other chickadees and they mob harder for the calls that named the smaller bird. The call is not “there is a hawk”. It is closer to “there is a hawk, and here is how much of a problem it is”.
Other species listen in. Red-breasted nuthatches, which are not chickadees and have their own affairs, respond to those variations appropriately — a hunting network running on somebody else’s language.
And then there is the part worth stopping over. A later study looked at what nuthatches do when they have only heard the chickadee and have not seen the thing themselves. They still pass the warning on. But they encode less in it: their own call is more general, more of a something than a specific something. The birds distinguish between what they saw and what they were told, and it shows up in the sound.
One more, and it corrects the version people usually repeat. Birds do not go silent for a hawk. What they do is switch to a different kind of call — a thin, steady, high note with a soft beginning and end, around seven kilohertz, which is built to be difficult to place. It fades fast over distance, and hawks hear high frequencies worse than the small birds do, so it carries to the neighbours and not to the predator. For something on the ground — a cat, a snake, a raccoon — they do the opposite: loud, abrupt, broad, easy to locate, because that call is an invitation. It is meant to bring everybody over to make the intruder’s afternoon unpleasant.
So a garden that has gone eerie and thin is telling you to look up. A garden making an unholy racket in one spot is telling you where.
What it cannot tell you, which is most of it
Most of what people wanted to know about their own gardens went unanswered, and the list is worth having straight, because the gap is the reason the advice is what it is.
Hawk or cat: not separable from the sound alone. Silence meaning weather or silence meaning predator: the same. Which species in your particular region means what, and how to tell one alarm call from another by ear: asked, never resolved. There are people who can do this, and what they say about how they got there is that it took a long time and a great deal of listening, which is an honest answer rather than a discouraging one.
There is also a folklore layer running through all of this — birds going quiet before rain, staying loud through a long one, dragonflies flying low, ants moving uphill before a flood, pine cones opening and closing with the damp. None of it is backed up here and this page is not going to pretend otherwise. Somebody made the sharper objection anyway: city birds on a reliable human food supply do not seem to mind rain at all, which is what you would expect if the behaviour is about food rather than weather.
And a great deal of ordinary garden noise is not an alarm in the first place. Birds hold territory and argue about it loudly. If your patch is mostly pigeons, or mostly a few species living off what people drop, there is less signal in it, and this page has less to offer you than it does someone with a hedge.
What to do when they go off
Go and look, from a distance, and look before you walk — that part is above and it is the whole safety of this.
What people describe finding, when they follow it: cats, hawks, a nest being raided, a snake. Snakes are worth two specifics. First, a nest being mobbed with no obvious animal in it may still be a snake, since they eat eggs routinely — the assumption that a bird alarm means a snake you can see is incomplete. Second, and this is the one that matters: most snakes are harmless and are doing you a favour with the rodents, and somebody made the point plainly: do not kill one unless you know it is dangerous, and if in doubt leave it be. Wildlife and humane organisations say the same and add the practical half — back off, give it room, and it will usually take the chance to leave; if it genuinely has to move, a garden hose will do it from a distance. Trying to kill it is how people get bitten, and some species are protected.
If you keep chickens, you already have a version of this and may not have named it. People who keep them describe a rooster spotting a hawk before any human does and driving the hens under cover — which makes that noise a smoke alarm you did not install.
And there is a smaller thing, harder to be practical about, that came up more than once. Some birds know which one you are. People described jays changing their call when one particular member of the household came out and not when anybody else did, and squirrels approaching one person and ignoring the rest. The crows, as above, will remember you for years and tell the others.
There is nothing to act on in that. It is worth knowing anyway: you have been a recognised individual out there for as long as you have lived in that house, whether or not you were paying attention, and the noise you have been hearing as weather has had you in it the whole time.
Common questions
The birds outside sometimes go completely quiet. What does that mean?
Something changed, and that is genuinely as far as this goes. A hawk overhead will do it. So will a cat on the fence, or a fox, or a person nobody recognises. People also read it as weather, and the honest position is that this page cannot separate those for you — telling the two apart in the moment is one of the specific things left unworked-out here, and it was asked more than once. Treat it as a prompt to look up rather than as a fact you have learned.
There is a nest right where I need to work. Will the parents abandon it if I go near?
Probably not, and the popular reason for that is wrong. The usual explanation given is that birds cannot smell you. Cornell's ornithologists say most birds smell perfectly well — small songbirds simply recognise their young by sound, by sight and by where the nest is, and no species is known to react to human scent as such. What does cause a bird to give up on a nest is disturbance and the sense of being watched by something dangerous. So the practical answer is not touch freely, it is be quick, be boring, and go back to your own business. Expect to be shouted at meanwhile, and possibly hit — one person described small birds throwing themselves at people who got too close.
A crow seems to have taken against me. Is that a real thing?
It is one of the better-documented things here. Researchers at a United States university wore rubber masks while catching and banding wild crows, then walked the same ground later in the same masks. The crows scolded that face and ignored every other one, and were still doing it years afterwards — the published figure is at least 2.7 years from a single encounter. It also spread: crows that had not been there joined in, over more than a kilometre, and on one measured walk most of the crows encountered scolded a mask worn by a researcher who had trapped only a handful of birds years before. The practical version, from someone who had to intervene against crows, is the same trick from the other side: change how you look while you do it.
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