Snails in the rooftop garden!

My facebook recently brought up one of their 'photo reminders' where they remind you of what you posted in the past. Here is a recent one that popped up, when I went snail hunting!

My little rooftop terrace garden is four floors up. I am pretty organic up here, all my vegetable scraps and coffee grounds go into a compost bin.

Considering I'm off the ground and only bring up soil in commercial bags, I'm amazed that I have a plethora of living creatures up here like worms, bugs, snails and slugs. 

Periodically, i.e. when they start eating too much of my produce, I gather them all up and relocate them to a more suitable place at ground level in our public park or in woodland on the way to our allotment. If I don't clear them regularly every year, I end up with a situation like this when I skipped a year:

I'm not sure where they're coming from but I've eliminated a few sources:

1. Ground level: I live in a concrete jungle. They're not climbing up the side of the building.

2. Neighbours: My neighbours don't have plants so they're not hitching a ride on a pot or in soil.

My current thinking is to blame the birds! As I have a roof terrace garden, I still have additional roof space which is above my living space. We regularly get birds up here: crows, sea gulls, magpies and occasionally sparrows.

The bring a mix of gifts to my rooftop. They like to perch on the TV ariel and over time the pots below this area started to grow a variety of plants that were not from me such as rapeseed and general weeds. I have since moved these pots.

In addition to their organic matter the crows LOVE a good takeaway and we have plenty fast food shops nearby that cater to drunk folks on a Saturday night who leave their empty boxes in inappropriate places. The clever crows will bring up the leftovers, in particular, the fried chicken remnants. Every time that we have a good storm I am guaranteed to find chicken bones or various dead bird bits on my rooftop.

I much prefer the friendly magpie. He seems to have stolen a ping pong ball from the local outdoor community tables (looked like an egg?).

Imagine the conversation I had with my neighbor who also has a terrace, we're separate by frosted glass but have 5 inches of clearance so things can blow underneath. 'Excuse me kind neighbour, are you missing a ping pong ball?'

In the meantime I found some slugs in the lettuce that I picked this week. They're back! Time to go hunting again...

If anyone has any other ideas of where they're coming from, feel free to comment.  :)