HOW CLIMATE CHANGE CAN INFLUENCE ANIMAL BEHAVIOUR
Over the last few decades, several biological changes have been linked to climate change, such as changes in time of migration and reduction of suitable living places for many species. Indeed, climate change can be considered one of the most serious challenges that humans are facing. One important way to measure climate change is to track the increase in Earth’s average temperature over the years. Temperature changes might cause a shift in the phenology of plants and animals, which means changes in the timing of periodically recurring events, such as egg laying or plant flowering. For example, if temperatures increase, winters will be milder, and spring will start earlier. Earlier springs means a shift in the growth and flowering of plants and can cause a shift in the timing of reproduction for many species. Earlier springs and longer frost-free periods can lead to earlier egg-laying in birds, for instance. There is evidence that earlier egg-laying is associated with milder temperatures, which can be helpful for some species. Other species will avoid changes in climate by seeking new breeding areas or spending winters in new locations.

OUR STUDY SPECIES - THE GRAYLAG GOOSE
Among birds, graylag geese (Anser anser) have been extensively studied and can be considered fairly tolerant of environmental conditions, as they breed from the Mediterranean to the Arctic circle. This means that these geese might be able to successfully cope with climate change. Graylag geese might respond positively to a warmer climate because they forage on grass and seeds, so they could profit from a climate in which vegetation appears earlier in the spring.

Graylag geese live in big flocks for most of the year and pair-partners (male and female) usually stay together for several years, similar to the way humans maintain their relationships as couples. Paired birds spend a lot of time together, including feeding, resting, and of course breeding. They greet each other after short separations, which could signal their pair status to others and might also reconfirm their commitment to each other. In our study, we wanted to find out more about the influence of climate change on graylag goose reproduction.

WHAT DID WE INVESTIGATE?
We collected data over 29 years, from 1990 to 2018. During this time, we monitored a population of graylag geese and collected data about the weather. We were interested in how the changing climate conditions influenced the reproduction of breeding pairs. Particularly, we looked at how the average temperature per year and the amount of snow cover were linked to the start of the flock’s reproductive period. To do so, we looked at the date when the first and last eggs were laid, at the size of individual clutches, and at the number of fledged goslings (the number of young geese that become independent from their parents). The first measurement told us whether increasing temperatures might result in earlier egg laying; the second measurement told us if the geese were laying more eggs, and the last measurement told us whether reproductive success was improving during the gosling period, when parents guide their young to suitable foraging sites and protect them against predators.

To collect this information, we carefully monitored each goose two to three times per week. During the egg-laying period, we checked the laying days and the number of eggs in the nests , and after hatching we observed the goose families until the goslings fledged.

WHERE DID WE CONDUCT THE STUDY?
To conduct such a long-term study, animals must be individually marked—otherwise it would be impossible to distinguish one family, or one goose, from another. Our study was conducted at a Research Centre of the University of Vienna. The centre is found in the Alm Valley in the northern part of the Austrian Alps, which are in central Europe. A flock of graylag geese was introduced there in 1973 by the late Nobel Prize winner Konrad Lorenz. The birds are fully free and can move around as they wish, but they receive food from us so that they do not have to migrate during the winter like other local geese do. All geese are tagged with coloured leg rings, and they are used to the close presence of humans. Data about every individual goose has been collected since 1973, and therefore we know their partners, friends, and relatives, within the flock (Figure 1B). We also know that, over the period of the study, the total number of geese in the flock averaged 150. During the study, we collected a total of 614 laying dates for 300 individuals (148 males, 155 females), with an average of 21 breeding pairs followed per year.

WHAT WE FOUND
Our results confirmed both our expectations and the global climate trend: the average annual temperature in the valley increased by around 2°C across the three decades of the study. This increase is at the lower end of the warming range in the alpine areas. Snow cover was highly variable over the study period and did not show a specific trend. As we expected, the annual average temperature had a significant impact on the reproduction of the graylag geese. Birds started to breed earlier when the winter was warmer, which means that the first eggs were laid earlier in the season (end of February instead of beginning of March). Additionally, we found that the higher the average annual temperatures, the more goslings reached independence. This means increasing average temperatures had a positive effect on the reproductive success of the geese. When we considered the timing of breeding in relation to reproductive success, we found some additional interesting insights. For instance, clutches that were laid earlier in the season contained more eggs, and more goslings fledged from earlier eggs. Thus, an early breeding time improved breeding success.

CONCLUSION
In summary, in the graylag geese that we studied, the increase in local temperatures at the end of winter led to an increase in the number of eggs and offspring. This example shows that some animal populations can take advantage of climate change. But is this always like the case? And how does it happen? Some individuals can adjust the timing of breeding to take advantage of new environmental conditions and increase the number of eggs they lay, for example. In general, birds from temperate zones must deal with a range of environmental conditions as the seasons change, and these birds seem to be adapted to cope with greater environmental variability. Birds from temperate zones must time their reproduction so their offspring survive and have enough to eat when they fledge. In general, the earlier a bird hatches, the more time it has to feed during the season—provided the correct food is available. If a bird hatches late in the season, food might run out and it may not survive. However, the timing is complex because the egg-laying female may benefit from waiting to lay eggs, since that gives her more time to feed before egg laying, which takes a lot of energy. Long-term studies contribute valuable information that can help scientists to clarify the benefits and costs involved in these complex situations.

It is also important to point out that our results might not apply to migrating birds, since our study flock does not migrate. Migrating birds must adjust their arrival time at their breeding grounds according to the local environmental conditions, they do not have a weather forecast in advance. Also, climate change might have different effects on graylag goose populations in the Mediterranean as compared to those in the Alps and the Artic regions, because the temperature increase was less pronounced in the Mediterranean region.

Overall, our long-term study showed how changes in the environment can have long-term consequences on an organism (in our case, a goose), as these changes can affect reproductive success, which can determine the future of a species. This work is important because it sheds light on the complex interactions between climate change and animal behaviour and fosters awareness about potential unbalanced environment. In fact, when one species is increasing, another one is generally decreasing, with a general loss of biodiversity.