In recent decades, the Earth’s climate has changed, manifested mainly by increases in ambient temperatures, changes in rainfall patterns and extreme weather events. These changes are seriously impacting human existence and are being experienced worldwide. But climate change has consequences for plants and animals – they have been extensively described for birds.
Much research examining the impacts of climate change on birds has focused predominantly on extreme weather events. In contrast, small-scale variations in weather conditions that can affect almost every aspect of avian biology have often been overlooked. The earlier arrival of many migratory birds at their breeding grounds and an earlier start to breeding in recent decades have been described many times.
Climate change could be causing changes in the availability of food, leading to alterations in the breeding process of birds, particularly among those that consume insects. This change in breeding phenology could have significant consequences. Higher temperatures cause insects to develop earlier. Since the twentieth century, the Earth’s surface temperature has increased by 0.74°C, and mean temperatures have risen by more than 2°C in some parts of the temperate and arctic climate zones. Most studies have implicitly or explicitly assumed that temperature is the main driver of changes in birds’ breeding phenology and breeding parameters. There are species in which the onset of nesting has sped up at the rate of 5 days in 10 years. Early nesters tend to have larger broods and are more likely to double-brood, which greatly increases the number of chicks they raise.
Precipitation is important
On the other hand, the impact on breeding parameters of weather factors other than temperature, including precipitation, has been analysed much less frequently. Moreover, the nest-building and incubation stages have been less interesting than the brood-rearing stage. Climate change is manifested, among other things, by changes in the intensity and duration of precipitation. This appears to be of great importance for species inhabiting arid and tropical regions with a dry season, as it stimulates plant growth and increases the availability of food for birds during this time, thus improving the condition of females before they lay their eggs. But how does this work in other zones, such as the temperate zone? In this instance, very few data are available, and they are not unequivocal, so it is difficult to draw meaningful conclusions. However, it seems that precipitation may have an even greater impact on reproductive phenology than temperature.
Different species of birds respond to climate change in different ways. In all probability, insectivorous species are quite strongly affected by these changes because of the rate of development of insects in spring when they start breeding. The Red-backed Shrike (Lanius collurio) is one such species because it feeds mainly on insects; the development and activity depend on weather conditions.
Shrikes like sunny weather
Based on a 23-year-long study in eastern Poland (1999-2021), we found a 5-day shift towards delayed breeding. We also studied the clutch and egg sizes but found no directional changes over the 23 years. Detailed analysis showed that the average temperature in May, when the shrikes lay their eggs, positively affected the clutch initiation date. In contrast, the number of rainy days in that month delayed laying. We also evaluated the overall trends in weather conditions in eastern Poland from 1999 to 2021. It turned out that the average May temperature did not change significantly but that the total precipitation and the number of days with rain in May had increased. Thus, the delay in nesting in the shrike population we studied was likely due to the increased rainfall during this period.
Consequences for late-nesting birds
What are the causes of delayed nesting in shrikes? First of all, the energy status of laying females may deteriorate during inclement weather with rainfall because the energy cost of thermoregulation is higher. To this, we have to add the reduced availability of insects, resulting from their delayed development and minimal activity when it is raining. Secondly, birds adjust their breeding to the greatest availability of food for feeding their young. This means that clutch initiation may be delayed as insects develop later in a colder and wetter spring. This can have serious consequences for birds, as females that breed later in a given season produce smaller clutches and have fewer opportunities to re-nest if the first brood is lost. Thirdly, rainfall can also affect gonadal development in birds. This depends on the light intensity; as this is lower during rainfall than on sunny days, the gonads develop more slowly. All these factors may translate into the delayed initiation of nesting by Red-backed Shrikes.
We have shown that local variations in rainfall can affect the breeding phenology of one shrike species, and our results provide a rare example of delayed bird nesting in recent years. We consider the average delay in nesting initiation of 2.2 days/10 years significant. It has been predicted that changes in temperature, wind parameters and rainfall amounts may strongly influence individual behaviour, life history, physiology and morphology, with consequences at both population and species levels. Further study will likely play a key role in shaping our understanding of how birds respond to climate change. It also has wider implications for ecology, evolution and conservation.
Golawski, A., & Golawska, S. (2023). Delayed egg-laying in Red-backed Shrike Lanius collurio in relation to increased rainfall in east-central Poland. International Journal of Biometeorology, 67(4), 717-724. http://doi.org/10.51414/sei2022.045