IN GENERAL, the longer an ecological study goes on for, the more valuable it becomes. This is usually done, funding agencies permitting, by extending it into the future. Extending it into the past is tricker. It is not, though, impossible—as Jonas Hentati-Sundberg and Olof Olsson of Stockholm University show in a paper published this week in Current Biology.

Dr Hentati-Sundberg and Dr Olsson borrowed an idea that historians of architecture are starting to use to track changes to buildings—looking at tourist photographs taken over the years. That works well for much-snapped edifices in holiday hotspots, but getting equivalent sets of pictures for parts of the natural environment means finding something specific that is equally photogenic. Fortunately, the two researchers think they have done so.

Their something is Stora Karlsö in the Baltic Sea. This island is well known to ornithologists as a nesting site for guillemots and razorbills. Crucially, it has been a destination for daily tours to look at its cliffs since the 1920s—and was, before that, a royal hunting ground. The result is a trove of photographs, albeit one that is scattered around Sweden.

The task Dr Hentati-Sundberg and Dr Olsson set themselves was to find as many of the photos as they could, and subject them to scientific scrutiny. To do that, they searched through national and regional archives, collaborated with photo agencies, and put requests in magazines and internet forums. Working this way, they tracked down more than 200 shots of the same cliff face (see above for a pair of examples), taken in 37 different years between 1918 and 2005, and then every year until 2015.

They concentrated on the breeding season, and on guillemots in particular, since members of this species nest communally and are faithful to the same sites from year to year. They thereby hoped to judge the guillemot population’s health by noting from their photographs how many nests were occupied by birds that were either incubating eggs or brooding hatchlings.

That focus on the breeding season (May 1st to July 10th in the case of guillemots) restricted the set of photos they could use to 113 snaps that were either specifically dated as having been taken between those days, or which appeared to have been from the state of the vegetation shown in them. In each of the 113 images the researchers counted all the birds they could see. They found that there were 182 nesting pairs in 1918, a number that rose slowly, to 304, by 1950. By 1964 there were 508 pairs, but that dropped back to a low of 353 in 1981. After this, the birds started to recover again, and have done so spectacularly. In 1998 there were 571 pairs; in 2015, 1,209.

Dr Hentati-Sundberg and Dr Olsson interpret the initial half-century rise as being a consequence of the end of hunting and egg-collecting on the island. They put the decline after 1964 down to contaminants such as DDT and a growth in the practice of fishing for salmon with drift nets, which snag and drown birds that fly in to grab what looks like an easy meal. And they attribute the latest rebound to the result of curbs on such substances and practices.

How widely deployable the method they have developed will be remains to be seen. What worked on Stora Karlsö would surely work for seabird colonies elsewhere, many of which are the subjects of regular photography. Whether it could be adapted for other habitats is less clear. There may, though, be places like national parks where tourists are so abundant that useful ecological information can indeed be extracted from dusty albums squirrelled away in the world’s attics.