So why are birds better at growing sunflowers than I am?

I sowed dozens of sunflower seeds this year and not a single one came up.

Granted, I didn't really expect great success as:
a) our soil is pure sand;
b) muntjac deer find sunflower leaves really juicy and tasty;
c) the seeds were mostly from ancient half-used packets (I have a vast stash of all kinds of old seeds and keep intending to strew them randomly round the garden to see what happens).

However, the garden birds have done their best to help. I've counted at least a dozen modest sunflowers dotted round the garden. They're mostly teensy and spindly...





...though some stronger ones have made a home amongst other flowers in pots.




I'm now wondering whether next year I should simply replace the seed on the bird tables at sowing time with some of Mr Fothergill's finest giant varieties, on the basis that the bird gardeners couldn't do worse than I did!