A swarm in May
is worth a load of hay;
a swarm in June is worth a silver spoon;
a swarm in July isn’t worth a fly.

This quoted verse is about a swarm of bees.  A post on my other blog observed the capture of a swarm.

Will-o’ -the-wisp has a number of meanings.  The reason it belongs in a post about nature is that it is the phosphorescent light over marshy ground due to combustion of methane (Oxford Concise Dictionary).  It is more often used to refer to a person who may or may not turn up or is unpredictable.

Like a dog’s hind leg means crooked. I have often heard it used about an unsuccessful attempt at hairdressing. A parting like a dog’s hind leg is not usually intentional!

No flies on you!
This expression is a compliment, meaning someone is alert and intelligent. Flies trouble cattle, which whisk them away with their tails.

Dog someone’s footsteps means follow them. It may apply to repeated disasters.

Swanning around is an older expression for being noticeable but ineffective. Swans are very beautiful. Do they work for a living?

Packed to the gills means full up. The gills through which fish breathe are not far from their mouths.

Chewing the cud means talking something over without reaching a decision or action. Ruminants, such as cattle, regurgitate their food and chew it again as cud.

Autumn leaves may be lots of different shades. It is also the title of a jazz standard.

Darling buds of May is also a song.  (Oops, please see the comments below!)

Horses for courses is an expression about employing people with appropriate skills and talents. The course horses run races on may be flat or with jumps – a steeplechase for example. The size and shape of the horse affects its suitability for a particular course.

The mountain labours and brings forth a mouse. This is a line from Aesop’s Fables. A mountain giving birth to a mouse seems out of proportion. I had a copy of Aesop’s Fables on my Kindle app. However, I now have an updated app without that particular book, which is much longer than I previously realised. Do children still learn the most popular stories from it?

Kill the fatted calf
is an expression used in the Bible in Jesus parable about the prodigal son. It is an idiom meaning celebrate lavishly. The parable may be found in Luke 15:11-32

I wrote a related post here.