Climate change fundamentally affecting European birds, study shows

A study has found that global warming is changing European birds, not just the increase in temperature.

The researchers found that the garden warblers are having a quarter fewer babies. Chiffchaffs lay their eggs a week earlier. Redstarts are getting larger while passerine family members are getting smaller.

The house sparrow is one of 60 different species collected since the mid-90s in Britain and the Netherlands. They don't know how these birds have changed over time with regard to their egg-laying schedules, number of offspring and morphology.

Scientists weren't sure if the swallow family was getting smaller due to heat stress or if it was because it was harder to find.

The scientists looked at whether warming affected species more than others and whether other factors unrelated to temperature reinforced the effects.

The study, which was published in the Proceedings of the National Academy of Sciences journal last week, found that although more than half of trait changes are linked with rising temperatures, warming is likely the largest factor driving change over the years.

Climate change caused chiffchaffs to lay their eggs six days earlier over the last 50 years, but other unknown environmental factors led to an additional six days, meaning in total they now lay their eggs 12 days earlier than they did half a century ago.

A change in schedule can cause a mismatch between when chick are born and when food is available for them.

According to the study, up to 57% of overall change over the past decades can be linked to temperature warming. 32% of 60 bird species had changes in body conditions due to temperatures, with an average decrease in size of 0.45% for every celsius increase in heat. Changes in egg-laying times and number of offspring were among the changes.

The lead researcher on the study said that a 26% decrease in the average number of offspring over the past half century is really concerning for the long-term fate of this species.

Some species are being affected differently. The redstart is increasing their body condition and offspring number. The researchers think that the variation of how much different species are changing is probably due to non-temperature factors.

The study gives a well-grounded explanation for why different species change at different rates. Shahar Dubiner, an ecologist at Tel Aviv University who was not involved in the study, said that it was not related to temperature sensitivity. Dubiner's research has found dramatic changes in shape and body condition for over half of Israeli bird species, including many who migrate from Europe.

Warming is likely the largest factor in driving trait change, but it is not the only factor. The question is what these other non-temperature factors are, and how they are linked to temperature increase.