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This year in October I saw a lot of robins, male and female, finches and chickadees, and geese flying south.

Surprisingly I didn't see many juveniles from previous broods. My guess is that juveniles are harder to see.

And the robins were flying above me, looking for worms below me, and most ended up in tall shrubs. The finches stayed in place longer than the robins.

So why this big congregation of birds when it is still going to be relatively warm for most of the fall(which it ended up being)?

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    I'm voting to close this question as off-topic because is more suitable for Biology.Stackexchange.com. – Benedikt Bauer Dec 12 '15 at 22:32
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    See meta.outdoors.stackexchange.com/questions/631/… "We have an ongoing problem of getting our question rate up. I feel that the key to this is to get a broader range of questions on the site (bird watching being a good example)." for argument against closing this question. @Sue – ab2 Dec 13 '15 at 3:06
  • I'm voting to close this question as off-topic because it is about arbitrary behaviour observed outdoors that does not specifically affect how to enjoy the outdoors. – gerrit Apr 27 '17 at 13:45
  • I voted for closing this question, because as is - after almost 1.5 years - the question is too arbitrary (I think) and no one was able to turn it into a more specific (e.g. birdwatching) one – knitti May 5 '17 at 12:33
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As far as I could extract it, the general question contained in your text is something like:

This year in October I saw a lot of robins, male and female, finches and chickadees, and geese flying south. [...] So why this big congregation of birds when it is still going to be relatively warm for most of the fall(which it ended up being)?

The simple answer is, that birds are not looking into the weather forecast thinking things like "Well, looks like it's gonna stay quite pleasant here for another two weeks, so let's take things easy and stay a bit longer here in this neat little forest." Instead, the timing of each species on when to migrate has been calibrated over generations of birds. It is the time slot which on average over the years produces the least mortality on the journey, i.e. the least damage to the whole population, which is of course not necessarily the least hazardous option for the single individual.

Also, you have to consider, that the fact that the weather is still fine at where the birds are right now, that might not be true for the rest of their journey. For example, a migrating bird with its summer habit somewhere in central or northern Europe will have to cross the Alps and/or the Mediterranean Sea on their way to Africa. While the weather might still be fine for some time in their summer habit, the chance of finding the Alps covered in snow and hitting a fall storm over the Mediterranean increases if they wait for too long.

Concerning the question, how they know, Wikipedia provides some insight into that topic. According to that, the primary trigger for when it is time to go is the change of day length. So basically, if day length reaches a certain point, they go, even if look out of the window and are more in the T-shirt than in the winter coat mood.

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