All images on that site are drawings from a 1905 field guide, and so they should be public domain. BUT all images were (as they describe on their page) scanned and reworked with a graphics tool in order to change contrast, lightness and hue. I don't know, if this makes the owners of the website new copyright holders.
If the page doesnt say copyrighted ... (sometimes its only on the home page..... ) it should be fair game.. especially if you know the original source, and... if you modify the image a bit yourself... Look at the Redd Foxx portrait which i merged from two crappier photos I found on the web... neither one of these can directly be tied to this one, and theirs were rip- offs anyway... But either way, Ive run across a few images that were questionable in origin, and the solution to removing any tie to the original is to modify the image... Explanation: A company I worked for was engaged in the shady business of scanning other peoples high-end catalogs for use of the images on the web. After consulting a lawyer, who said that it was ok to use the images provided there were seven changes made to them... seven degrees of removal from the original... I don't know if this holds water well enough for here.. (Most places dont worry at all...) but... in the strictest sense, its not hard to alter a photo with a few different operations... 豎眩:sv