Let's start by getting the maths right...

24 (Hours) * 60 (Minutes) * 2 (every 30 Seconds) * 2 (Bytes) = 5760 Bytes/Day

If you pick a 64kB Micropower 18F PIC and be frugal with your code and your Hardware usage, you could record ten days worth of Data with an on-board 3v Lithium Battery... unless of course your nest happens to be adjacent to a convenient tree-top Power outlet.

I can envisage some problems with the Thermal Dynamic properties of your 'egg' material affecting the temperature readings. If you make it thermally conductive (ideal), the bird will detect it as alien and throw it out of the nest. If you make it look, feel and weigh akin it's own eggs, it won't have great thermal properties (and it'll still chuck it out of the nest when it doesn't hatch!).

Good Luck!

BTW... I recall seeing 'plastic eggs' being used in the Food Industry for Data-Logging fridge temperatures more than ten years ago.