The Thekla Lark (Galerida theklae) is a species of lark that breeds on the Iberian Peninsula, in northern Africa, and sub-Saharan Africa from Senegal to Somalia. 
The Thekla Lark looks very similar to the Crested Lark (Galerida cristata) that also occurs in the Extremadura, and birders usually find them very difficult to tell apart.
The main differences between the two are the following:
1. Thekla has larger, thicker chest stripes on a whiter background; Crested has finer, less distinct stripes running along the flanks. 
2. The eyebrow stripe is shorter on Thekla (until just after the eye; on Crested it is 1cm after the eye).
3. The bill of the Thekla is somewhat shorter and stubbier than that of the Crested (due to the biotope of harder (rock) soils of the Thekla, this seems to have a good evolutionary explanation). 
4. The lower mandible of the Crested is quite straight, while the lower mandible of the Thekla is slightly curved.
5. Thekla usually has a less pointed and less high crest than Crested.
6. Thekla is often found in rocky areas with scattered trees and more at altitude, while the Crested is not on altitude/rocks but on the steppe. 
7. Thekla often sits at the top of a bush or tree; Crested sits more on the ground.

1st photo: Villareal de San Carlos, Monfragüe National Park, Extremadura, Spain, 4 May 2012
2nd & 3rd photo: Garganta del Fraile, Cáceres, Extremadura, Spain, 12 June 2024
4th photo: Monfragüe National Park, Extremadura, Spain, 12 June 2024