In the shade of the banks. Here, every spring,
I would fill jampotfuls of the jellied
Specks to range on window-sills at home,
On shelves at school, and wait and watch until
The fattening dots burst into nimble-
Swimming tadpoles. Miss Walls would tell us how
The daddy frog was called a bullfrog
And how he croaked and how the mammy frog
Laid hundreds of little eggs and this was
Frogspawn. You could tell the weather by frogs too,
For they were yellow in the sun and brown
In rain.
Then one hot day when fields were rank
With cowdung in the grass the angry frogs
Invaded the flax-dam; I ducked through hedges
To a coarse croaking that I had not heard
The voice changes to the older Heaney.  There is a sharp contrast between his perspective as a child, fascinated and curious about the natural world, and his later attitude. 
 The second stanza signifies passing time, now probably summer.  The ‘hot day’ seems oppressive and the ‘rank’ fields' create an unpleasant, restless atmosphere and mood.
Death of A Naturalist