The following text is available now on the Spanish Herpetological Society website:
• A. Acuña Rodríquez, F. Martinez-Freiría, G. Velo-Antón, 2013 – Ingesta de tres especies de herpetos (Bufo calamita, Anguis fragilis, Vipera seoanei) por un ejemplar de águila culebrera (Circaetus gallicus) abatido en un parque eólico [Three species are found in a Short-toed Eagle killed by wind farm] // Boletín de la Asociación Herpetológica Española, vol. 24(1): Pre-publication, ms685. – 
(Es) /map/.
This is one more article from the Ardeola website:
• IRIBARRER, J.J. and RODRÍGUEZ-ARBELOA, A.., 1973 – Observaciones en un nido del Águila Culebrera (Circaetus gallicus), Navarra 1972 [Observations on a nest of Short-toed Eagle, Navarra 1972] // Ardeola 19(1), 101-106. – (Es) /map/.
The text was published in 1973 including some black-and-white images. The placed PDF file is not of the best quality, but it’s fairly readable.
Here you can acquaint yourself with the work on the wintering of Short-toed Eagle and Booted Eagle in Spain:
• José Enrique Martínes y José Antonio Sánchez-Zapata, 1999 – Invernada de Aguililla Calzada (Hieraaetus pennatus) y Culebrera Europea (Circaetus gallicus) en España [Wintering of Booted Eagles and Short-toed Eagles in Spain] // Ardeola 46 (1): 93-96 – (Es) (source – Ardeola website).
…and with the abstract of an article about Greek Short-toed Eagles:
• D.E. Bakaloudis, C.G. Vlachos, G.J. Holloway, 1998 – Habitat Use by Short-Toed Eagles Circaetus gallicus and Their Reptilian Prey During the Breeding Season in Dadia Forest (North-Eastern Greece) // Journal of Applied Ecology, Vol. 35, No. 6: 821-828 (En) /map/ (source – the JSTOR database).