The presented article refers to the following: in spring ’81 the author discovered that a nest found in the previous year in the Pilis Mountains /map/ had fallen down. They built an artificial nest and kept watching the site. Later, they observed the pair occupying the nest and found egg remains under it on April 25, so the egg laying took place a few days earlier. It was the earliest known egg-laying of this species at that time. Since the pair was holding the territory they checked back in June 4 and found the female incubating. Despite the repeated egg-laying the fledgling left the nest early August.
• L. Haraszthy, 1982 – Kígyászölyv (Circaetus gallicus) pótköltése műfészekben [Repeated egg-laying of Short-toed Eagle in an artificial nest] // Madártani Tájékoztató, Oct-Dec.: pp. 266-267. – 
(Hu).
Thanks to Gábor Papp for sharing the text and its brief summary!