It's been pretty cold here were I live (18 degrees at night, and in the 40's during the day) and haven't seen much critter activity locally.  There is a group of probably 1,000+ Ladybugs in the cracks of tree trunk bark, in bushes, and on the forest floor, along one section of the road.  I've seen them in this same spot every fall, and into winter, for the 25+ years I've lived here. I'm pretty sure they survive the cold temperatures like the moss does, by creating an "anti-freeze" in their body fluids.  I also know that they come up from lower elevations to spend their winters here where it's colder! 

Why do they migrate up to this colder environment?

I couldn't find an answer to this question!  

I did find out that their migration is triggered by food scarcity (mainly aphids), shortening of daylight hours, and cooling temperatures. 

After they complete their migration and temperatures cool even further, they enter diapause, a suspended state (sort of like hibernation in mammals), to survive the winter.

Why do they congregate in such large numbers?

Our local species of ladybug, Hippodamia convergens, congregate to mate 

and share warmth. 

They don't congregate for the "safety in numbers" reason, as not many critters 

eat ladybugs. They taste bad!  

This nest held two baby robins this past summer, and now, since the rains, it's sprouting!  The robins have migrated to warmer, lower, elevations for winter.  

Will the robins reuse this nest next Spring?

Most birds do not return to old nests for 3 main reasons. 

1) The nest has deteriorated over the winter.  

2) The nest has odors left from birds, which could attract predators. 

3)The nest has ectoparasites that could harm baby birds.

However, some birds do reuse old nests. 

Usually birds that build large nests (eagles, herons, etc.) reuse their old nests, but refurbish them with new sticks and nest materials.  

Some small birds also refurbish old nests, such as Phoebes and Robins! 

Robins sometimes make new nests, sometimes refurbish an old nest, and sometimes build a new nest on top of an old nest!