Your seemingly innocuous holiday photos could be more than just a way to brag to friends about where you've been -- they could be helping document the ebb and flow of wildlife populations.
That's the case on the picturesque Swedish island of Stora Karslo, where photos from the past 100 years have been studied -- not for what their subjects are doing, but moreso what's happening in the background.
In the 1880s, the island became a nature preserve and hunting park. Around 40 years later, the owners began organising tours.
From the 1920s, the owners of the island began organising tours after it became a nature preserve and hunting park around 40 years earlier.
The colony in 1918 (Paul Rosenius / Vanersborgs museum collection)
Since then, it's remained home to the common guillemots -- a population of seabirds living on the island -- and is also a popular tourist destination, with 10,000 visitors a year.
That's a lot of photos to look through and it's allowed researchers to look back in time and document the changing numbers of the birds.
And it tells a startling story.
Numbers plummeted in the early 20th century, but have since risen to historically high levels.
"We found that the population is currently increasing at an unprecedented rate of about 5 percent annually. This is interesting in that many common guillemot populations are decreasing worldwide," says study author Jonas Hentati-Sundberg.
Mr Hentati-Sundberg, from Stockholm University in Sweden, says what they were able to determine from the photos shows how powerful "unconventional sources" of evidence can be to helping fill in the gaps of scientific knowledge.
Breeding guillemots from 1960 (Gosta Hakansson / Gotland museum collection)
He and Olof Olsson knew that before the island was given protection, the birds' eggs were collected and the adult birds harvested for food. That ended in the late 19th century, but oil spills and chemical exposures over time also impacted the population as well as the birds getting caught in fishing gear.
The authors spent around five years collecting images from archives, museums and people who had visited the island and they show the modern day colony is more than five times the size than in the early 20th century.
During the 1960s and 1970s there was a drop in their numbers, which coincides with when the use of environmental contaminants, such as DDT and PCB, were extremely high in the Baltic Sea.
Thousands of photos have been taken of the colony including this one from 1975 (Lars-Erik Norback)
"It is reasonable to expect that contaminants had a role in the decline," Mr Hentati-Sundberg says.
"It has not been known previously that seabird populations were affected by the contaminants."
The growth of the colony could be linked to a number of things including a ban on driftnet fishing for salmon in 2008, a drop in environmental contaminants and an increase of the small fish the birds like to eat.
The colony in 2015 (Aron Hejdstrom)
The authors also believe tracking the birds' numbers could unearth trends in the larger marine ecosystem.
The study was published in Current Biology today.