19世紀の科学文化は、ロック鳥の神話の起源は、生まれたばかりの子羊を運び去ることがしばしば目撃される、鷲の能力を誇張したものではないかとし、神話の起源に対するいくつかの「科学的」合理的な推論を示した。1863年、ビアンコーニはロック鳥が猛禽類であることを示唆した(Hawkins and Goodman, 2003: 1031)。最近、マダガスカルで発見された巨大な半化石マダガスカル冠鷲は、かつてオオキツネザルやコビトカバといった大型動物が存在していたこの島の鳥類捕食者の頂点に君臨していた<ref>Goodman, 1994</ref>。
この神話のもう一つの起源は、16世紀までに絶滅したマダガスカルの鳥、アエピオルニス象鳥の卵であり、この鳥は高さ3メートルで飛べない巨大な鳥であったとされる点である<ref>The Eighth Continent , https://archive.org/details/eighthcontinent00pete , registration , Tyson, Peter , 2000 , New York , 138–139</ref>。少なくとも民間の記憶では、1658年にエティエンヌ・ド・フラクールが書いたように、象鳥が目撃されたことが報告されている<ref>ey196608</ref>。1456年に出版されたフラ・マウロの世界地図のキャプションによると、ロック鳥は「象やその他の大きな動物を運び去る」とあり、喜望峰に向かう船乗りがロック鳥の卵を見つけた1420年には、その卵は生きているか半化石化されていると考えられていた<ref>{{Cite book|title=Science and Civilisation in China|last=, Needham|first=, Joseph|publisher=, Cambridge University Press|year=, 1971|, isbn=:9780521070607|pages=501}}, pages501</ref><ref>{{Cite book|title=The Life of Prince Henry of Portugal Surnamed the Navigator, and Its Results, Comprising the Discovery, Within One Century, of Half the World ... from Authentic Contemporary Documents|last=, Major|first=, Richard Henry|publisher=, Biblioteca Nacional de Austria – Asher (Editor)|year=, 1868|pages=311}}, pages311</ref>。
Its egg, live or [[Subfossil|subfossilised]], was known as early as 1420, when sailors to the Cape of Good Hope found eggs of the roc, according to a caption in the 1456 [[Fra Mauro map]] of the world, which says that the roc "carries away an elephant or any other great animal". Between 1830 and 1840 European travelers in Madagascar saw giant eggs and egg shells. English observers were more willing to believe their accounts because they knew of the [[moa]] in New Zealand. In 1851 the [[French Academy of Sciences]] received three eggs. They and later fossils seemingly confirmed to 19th-century Europeans that ''Aepyornis'' was the roc, but the real bird does not resemble an eagle as the roc is said to.{{r|ley196608}}
[[File:Print, Ferdinandes Magalanes Lusitanus, plate 4 from "Americae Retectio", 1580s (CH 18382163).jpg|thumb|290px|''Elephant Carried Away by a Roc'' after design by [[Stradanus]], 1590]]
Another rationalizing theory is that the existence of rocs was postulated from the sight of the African [[ostrich]], which, because of its flightlessness and unusual appearance, was mistaken for the chick of a presumably much larger species. There is, however, a claim that ostriches were known to Europeans in [[Biblical times]] due to a translation of the Old Testament.<ref>{{cite web|url=http://www.biblestudytools.com/job/passage/?q=job+39:13-18|title=Job 39:13-18 - "The wings of the ostrich flap joyfully, though...NIV|website=Bible Study Tools}}</ref>{{vn|date=September 2016}} On the other hand, a medieval Northern European or Indian traveller, if confronted with tales about ostriches, might very well not have recognized them for what they were (compare [[History of elephants in Europe]]).{{Citation needed|date=May 2014}}