Climate change: spring starting too early for roe deer

While plants are coming into leaf ever earlier due to climate change, the time at which roe deer give birth is changing more slowly. This means that the food supply available when deer are rearing their young is altering. Roe deer populations at lower altitudes are particularly affected, according to the findings of a study by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).
Since 1971, roe deer fawns in Switzerland have been fitted with ear tags. This practice should continue so that changes in the deer population as a result of climate change can be identified in good time. (Photo: Maik Rehnus, WSL)

Wild animals give birth to their offspring at the time when environmental conditions offer the best possible chance of reproducing successfully. Roe deer fawns are therefore born at the start of the growing season, when nursing does can find tender, easy-to-digest grasses and herbs with a high energy and protein content. With vegetation development starting ever earlier as a result of climate change, there is an increasing mismatch on the Swiss Plateau between the time when the best food is available and the period when the deer give birth. This is demonstrated by the results of a study led by Kurt Bollmann, a wildlife biologist at WSL.

Ear-tagged fawns

Since 1971, roe deer fawns in Switzerland have been fitted with ear tags and details of their location recorded. For their research, WSL's Maik Rehnus and Marta Peláez from the Technical University of Madrid compared the birthing dates of 8,983 fawns born between 1971 and 2015 with long-term data on the start of the growing season and the date when the first hay was cut in meadows. This period provides optimal nutritional conditions for nursing does.

On average, the growing season began 0.45 days earlier each year, and the first hay was cut 0.32 days earlier. This means that, over the 45 years spanned by the study, the onset of the vegetation period moved forward by 20 days and the hay cut by 14 days. By contrast, the deer birthing dates only advanced by 0.06 days per year, or three days in total. The rate of change of the birthing dates was therefore seven or five times slower than that of the vegetation, at all altitudes. One reason for the slow adjustment in the birthing date is the fact that the roe deer's reproduction period is controlled by the ratio of day to night, which is not affected by climate change.

Optimal conditions and birthing dates diverging

At lower altitudes, the birthing dates are increasingly falling outside the period when the best food supply is available, while at higher altitudes they remain optimal. According to the researchers, the consequences of this are uncertain. "The relatively small-scale patchwork of different agricultural crops growing at different times means that roe deer can still find enough food even when the conditions in meadows are past their best," says Bollmann. However, it is possible that deer will become rarer on the Swiss Plateau in the future, while populations in hills and mountains may grow, since vegetation development starts later in these areas and therefore better matches the birthing dates.

Whether the discrepancy between deer birthing dates and meadowland trends ultimately affects the population will depend not only on the management of other agricultural crops but also on the weather in winter and during the rearing season. The researchers therefore recommend that fawn monitoring should continue, and should be stepped up at lower altitudes. "This will ensure that changes in the deer population are identified in good time and hunt management practices can be adapted accordingly," says Rehnus, lead author of the study.


Dr. Kurt Bollmann
Eidg. Forschungsanstalt WSL
Zürcherstrasse 111
8903 Birmensdorf