Morphological museum data for Israeli birds, including all individuals used for the analyses in Dubiner & Meiri (2022)

Aim: Body size decline has been proposed as a universal response to global warming, but this is often questioned. We describe and characterize recent morphological changes in the avifauna of Israel as a whole, and test several hypotheses regarding their cause.

Location: Israel

Time period: 1950-2020

Major taxa studied: Aves.

Methods: We analyzed the morphology of 7,981 museum specimens. For each of the 106 species we calculated the rate of change in mass, head and body length, wing length, and approximated relative surface area, over time and as a function of temperature anomaly (the difference between temperatures in a given year and the interannual average). We used phylogenetic mixed models (PGLMM) to determine trends and their relation to species’ ecology.

Results: Over the last seventy years there have been consistent changes through time in mass, length, and surface-area-to-volume. Mass declined by 18.3%, length increased by 5.1%, and surface area-to-volume increased by 28.9%. The increases in the ratio of surface area-to-volume through time corresponds to a 12.2% increase per °C of warming. Conversely, wing length changes were few and inconsistent. Most species changed in either mass or length, but seldom in both. The effect of rising temperature on morphology was roughly an order of magnitude stronger than the effect of a comparable geographic difference in habitat temperature. Changes were modulated by migratory habits but not explained by human-commensalism or diet.

Main conclusions: Mass decrease and length increase are widespread, both leading to higher relative surface area. Results conform with predicted responses to global warming, but not with any of our other tested hypotheses. If warming is indeed the driver of these changes, the diverging responses observed between different species may represent different solutions to solve a common problem.

We initially gathered measurements of nearly 11,000 birds from the Steinhardt Museum of Natural History, Tel Aviv University, collected between 1950 and 2020. These represent all birds for which either head and body length, wing length, or body mass data were present. Bird hunting is forbidden in Israel since the mid-1950s, and most specimens in the collection represent birds that were found dead, died in wildlife hospitals, or were confiscated by the authorities after being illegally hunted. Regardless of their source, birds were measured shortly after their death, and these measurements were taken by museum staff.  Because the specimens were collected and measured over several decades not all of them were measured by the same person, but new generations of museum personnel were taught by the previous ones, and used the same methods. We filtered the data to contain only adult birds, collected from the wild, in Israel. We only used species for which we had at least 30 specimens, collected over a period of at least 50 years, and sampled during at least 10 individual years over this period. We scanned the dataset for strong deviations from the general mass-length ratios (which we suspect represent typing errors, misidentifications, specimen in very poor condition etc.). We re-measured specimens with outlier values if possible, and removed measurements we could not verify.