Colourful Migrant Birds! & Odd Egg?

Siskin - male

Chaffinch - Brambling

Brambling - Male
It's nice to have some colour in the garden.

Siskin - Greenfinch - Brambling

(European) Goldfinch

Dunnock -
 New to my Twitching Trees - Bird on a Stick Project.

Twite - with Fair Isle ID colour rings

Songthrush

Chaffinch - Brambling - Greenfinch - Siskin - feeding together under my feeder.

Dunnock

Triplets! Lambs are popping out all over the Isle.

(European) Robin

What type of egg is this?
I found this small egg on the edge of my pond?
small, 1 inch wide and completely round? Shell was pliable but brittle.
Very strange? any ideas? no turtles, snakes or reptiles live here?

My best guess is a reject chicken egg laid in the pond edge?
the yoke looks weird?
 I have seen small eggs, when a young chicken starts laying but not round and this small.
Could it be another bird? Duck? Coot? Snipe? Waterail? I really don't know?
What do you think?