Then again there are those species that almost always need a decent view to allow any degree of certainty. Some birds for notoious 'species pairs' - Griffon vs Ruppell's Vulture, Lesser Short-toed Lark vs Short-toed Lark, Thekla vs Crested Lark and Iberian Chiffchaff vs Chiffchaff. Arguably it's the last pair that causes the greatest headache, but distinguishing Thekla Lark from Crested is a close second.
So with some trepidation I'm going to attempt to have a systematic look at the Thekla/Crested problem. I've tried to put the criteria in a heirarchy and weighted points according to my own experience. Naturally, others will give a different weight to some features, but I hope this makes a good starting point.
For me the this bird is 'obviously' a Thekla on bill shape alone. the lower mandible is clearly strongly convex whilst the upper manidible curves only towards the tip. A Crested would have a longer 'smother' curve to the upper mandible and a flatter lower mandible. The stronger eyestripe in front of the eye, whitish ground colour and half collar, more 'contrasty' face pattern also point towards Thekla.
Yet the crest looks a bit 'pointy' although it doesn't seem to rise sharply from behind the eye, but emerges smothly from the crown. The chest streaking may look a fine compared to many Theklas, but the flanks are very well marked.
What you can't see in this photo is where the photo was taken. I had this bird at c550m in the Alcornocales. It was in the car park of a small venta, but the nearby 'fields' were strewn with rocks. Other larks I've watched here persistently perch on bushes and sound OK for Thekla.
Not the best of photos, but it's the only one I've got!
The crest looks long and more 'abrupt'. The chest can't be seen, but it was finely streaked and the ground colour was somewhat sandy. The ear-coverts & face pattern, as far as they can be seen, look a bit bland and plain.
However, the key thing here is typical Crested Lark bill; long bill with a nicely evenly curved upper mandible.
It was seen on good quality farmland near Espera in an area I wouldn't expect to find Thekla. Note it was perched on a fence post not bush!
I also posted my ID table on "Bird Forum" (see http://www.birdforum.net/showthread.php?t=195118) where several people joined in the discussion and posted a series of instructive photos. Well worth a look!