We have at least a couple of dastardly duos of our own here in Portugal. Let's take, for instance, Crested and Thekla Larks. We are regularly surprised how readily birders visiting here, often with no previous experience of either species, confidently identify them. Maybe we're missing something!
So what are the differences between the two?
Let's start with the excellent Collins Bird Guide (Mullarney, et al). This tells us that close views are required to separate them. Fine, that shouldn't be too difficult. But it then goes on to say that Thekla is marginally the smaller of the two and then mentions a number of other features that really aren't much help unless you've got one of each species side by side or until you've got your eye in and see them regularly.
It tells us amongst things that the plumage of Thekla is greyer, that its bill is 'usually not so pointed' and 'often slightly darker' and that 'the mantle and back on average (are) somewhat more distinctly streaked'. There's not much to argue about there but equally none of that is going to clinch that sought-after tick for the first-time visitor. When it says 'the uppertail coverts (are) more reddish-brown' we start to wonder whether we shouldn't just tick both of them and move on!
In our view, no more helpful is the difference in the shape of the crests of the two species that is suggested by the illustrations - this seems to vary with individuals and can be affected by the wind, by moult and probably age. The illustrations do show that the streaking on the breast of Thekla is always distinct but the text concedes that some Cresteds are similar.
So what about songs? Again, although there are a number of comparative descriptions in the field guide, the statement at the end of them that Crested Lark is 'often difficult to separate from song of Thekla' doesn't send us out into the field full of confidence! It does, though, confirm our experience.
And then there's habitat. Although there is some overlap, as pointed out by Mullarney et al, Thekla is generally a bird of arid, barren, rocky areas, while Crested is the much more likely bird in cultivated areas or near human habitation. You might well see a Crested in Thekla habitat but a Thekla in Crested territory is much less likely.
As far as we are concerned, almost as useful as any of these things is one of the first bits of advice we were given when originally faced with this problem: if it's perched in a bush, a tree or on a pile of rocks, it's very probably a Thekla; if it's on the ground, the likelihood is that it's a Crested.
Finally, what for us is easily the most useful feature: bill shape and length. Simply, Theklas have shorter, stubbier, more triangular bills with the lower mandible often slightly convex, while Cresteds have longer, more tapering bills with a generally straighter lower mandible. Usually! As the man said, 'close views required'.
Let us know via a comment what you make of these birds, all photographed in Portugal: