Long-term nest box research offers insight into trends in nature

Great tits can't complain about a lack of attention. The Netherlands Institute of Ecology (NIOO-KNAW) has hung nearly two thousand nesting boxes throughout the Netherlands for this “model species”. For seven decades researchers have been following the ups and downs of these songbirds. This makes it the longest-running study in the world of individually recognisable animals. The family trees, caterpillar peaks and breeding successes outline the development of climate change, acid rain and adaptation to a changing world.

NIOO has been conducting research since 1955 on birds that breed in nesting boxes, mainly great tits but other species as well. Not every bird behaves the same. Some birds lay many eggs or lay them early, while others lay only a few or do so late. What causes this variation? And what are the consequences? By following the life cycle of many individual tits in the wild and also studying them experimentally, we now understand this much better. This has yielded important, internationally applied knowledge about the effects of climate change on food chains. As a result, we can also predict how populations will continue to change. 

The long-term research builds on even older studies. The first nesting boxes for insect-eating songbirds were hung in 1909 on the Oranje Nassau's Oord estate near Wageningen by mathematics teacher Gerrit Wolda, who was concerned about the lack of natural cavities. The birds' breeding season was monitored here from the spring of 1910 until 1959. In 1920, Wolda joined the then National Plant Protection Organisation (NPPO) as an ornithologist, where the research was subsequently embedded and given a different focus. The aim was to attract birds and use them to control insects in forestry and agriculture. Later, the boxes were retained for bird protection and ecological research. Another next box study launched in 1920, on the Hoenderloo estate, which at the time was part of De Hoge Veluwe National Park. In 1924, Huybert Nicolaas Kluyver continued the NPPO's nest box study. He took the study with him when he became the first director of the Institute for Ecological Research (IOO) in 1954. Almost immediately, the researchers set up four areas with nesting boxes, which ecologists still monitor today: De Hoge Veluwe ( the southern part this time), Vlieland, Oosterhout (near Nijmegen) and the Liesbos (near Breda). The areas were chosen along two axes: oak versus mixed forest and isolated versus part of a larger forest. Over the past 71 years, a total of 30,446 great tit broods have been monitored. During that period, 224,475 eggs were laid and all 140,076 fledglings were ringed. This long-term nest box study is also directly linked to the oldest study of cavity nesters at Oranje Nassau's Oord: the archive, including the original field notebooks, is kept at the NIOO and is being investigated again.

Variation in years and birds

Because all great tits in the populations wear a small ring with a unique number, we can document and study the life course of individually identifiable animals. This allows us to see how “plastic” they are in their behaviour. A bird may have a large clutch one year and lay far fewer eggs the next. This is because circumstances play a role in those “decisions”. During a year with plenty of food, large clutches are advantageous, while during a year with little food, it is better to lay a small clutch. In that case, there must be cues at the time the birds are laying eggs as to whether it will be a year with plenty of food or little food. Without reliable cues, plasticity is not possible.

In addition to this plasticity, birds also differ individually. Some birds always lay more eggs than others; a variation that is partly genetically determined. For tits, this opens the door to genetic adaptation to changing circumstances, known as microevolution. By tracking populations of individually identifiable animals over many years, we can monitor processes such as plasticity and genetic adaptation in wild populations. This enables us, for example, to visualise the effects of climate change on food chains.

Ecological relationship problems

In the case of the great tit, we see that the birds are laying their eggs earlier and earlier in the spring. Where in the 1970s this still happened around the end of April, there are now individuals that start laying their eggs at the end of March. We also see that the great tits that lay eggs late in the season are raising fewer and fewer chicks. Early-laying individuals therefore fare better than their late-laying counterparts: natural selection favours early laying. Another shift is that while in the 1980s many great tits still had two broods per year, this is now hardly ever the case.

We can understand these patterns because we also do long-term research on the food for the chicks, especially caterpillars of the winter moth. We also study the oak trees on which these caterpillars depend. Due to climate change, the approximately 10-day peak in caterpillar biomass occurs about two weeks earlier in the spring, but this shift is much stronger than that of the great tit, which only shifts forward by one week. The phenology of birds and caterpillars are therefore both temperature-sensitive, but not to the same extent. This leads to a “mismatch”, i.e. climate change leads to ecological relationship problems. Because we know the relationship between temperature and the phenology of trees, caterpillars and birds from our long-term research, we can predict the extent of the mismatch between great tits and their food in different climate scenarios. Under the so-called “business as usual” scenario for predicted climate change, the mismatch will become so great that it will have significant consequences for the bird population: far fewer young birds will fledge. If the temperature increase remains below 1.5 degrees according to the Paris Agreement, the mismatch will not become much greater than it is now. Even now, there have been years with a considerable mismatch, resulting in increased mortality among chicks and far fewer fledglings. However, this does not directly lead to smaller populations. This is due to density dependence: if few young birds fledge, each young bird has a slightly greater chance of returning as a breeding bird. In any case, we do not see 95% of the young birds again: the vast majority do not survive their first winter. But that survival rate is also influenced by the amount of winter food: the amount of beech nuts. This has been tracked for fifty years and there are significant fluctuations due to the cycle of mast years, but also due to the current disruption of that cycle. Beech nuts therefore also determine the population size of great tits.

Calcium and acid rain

During the 1980s, certain songbirds, including great tits, were found to lay thin-shelled and shell-less eggs (“wind eggs”) more frequently in forests on poor sandy soils. Many eggs did not hatch. Some birds stopped laying eggs altogether and brooded on empty nests. The NIOO investigated whether a calcium deficiency was causing these problems.

This proved to be the case. During the egg-laying period, tits eat more calcium-rich snail shells to produce strong eggshells; their normal diet consists largely of protein- and energy-rich caterpillars, but these contain very little calcium. However, soil acidification had led to a decline in the number of snails. The cause of this was found to be nitrogen oxides, which cause acid rain and are released during the combustion of fossil fuels. In forests with nutrient-poor soil, such as the Veluwe, the acid rain washed away the calcium before it could be absorbed by plants. There were fewer snails in these forests because they depend on the calcium in leaves and other plant litter to build their shells. 

A follow-up study examined whether feeding chicken eggshells as a calcium supplement to the tits would have an effect. The number of empty nests did indeed decrease and the quality of the eggshells improved. Even today, calcium-deficient eggs are still regularly found in forests on poor sandy soil, with the chicks often failing to hatch or dying before they fledge.

Personality

In order to survive, it is important for animals to be able to adapt quickly to changes in their environment, such as climate change and urbanisation. Individuals within populations have different ways of dealing with change. We call these consistent behavioural differences between individuals personality, both in humans and animals. For example, animals in cities are observed to be more bold and aggressive than those outside cities, and the more cautious animals seem to respond more quickly to change.

Research into animal personalities in great tits has been ongoing since 1990. This research is conducted both in aviaries and in the wild in order to study the causes and consequences of these differences. Approximately half of the underlying sources of personality are genetic, with the environment determining the other half. What is striking is that the success of a particular personality type varies considerably: in some years, the bold birds survive better, while in other years it is better to be more cautious. One reason for this is that bolder birds fare better when densities are lower, while they have a harder time than more cautious types in winters when food is scarce. As a result, natural selection based on personality fluctuates from year to year.

Comparative research between five European great tit populations shows that these shifts in selection occur at every level: from the local population level to the species level. The variation in personality therefore not only affects the animals themselves, but also the stability of the population and the species. The results can also be applied to endangered species, where populations with different personality types are less likely to become locally extinct. This indicates that personality, as a form of biodiversity within populations, is an important factor in reducing the likelihood of populations disappearing or entire species becoming extinct. The great tit serves as a model for many other animals and thus provides valuable knowledge for nature conservation.

Nest box citizen science

In the Netherlands, nest box research receives assistance from many citizen scientists, volunteers who collect breeding data from a single nest box or sometimes from all nest boxes in a given area. Since 2009, all this data has been collated in NESTKAST, a partnership between NIOO, SOVON, Vogelwacht Uden and Vogelwerkgroep Zuidoost-Achterhoek. In 2024, data was received on the start of laying and clutch size from 13,674 nest boxes and 21 bird species.

A hundred years ago, the National Plant Protection Organisation coordinated a similar project. Back then, people across the country also collected data on nest box breeders. In 1925, they met at the very first gathering of nest box researchers. All data from more than 40,000 clutches from 1922-1950 have recently been digitised at the NIOO with financial support from NLBIF, the Dutch branch of the Global Biodiversity Information Facility. What is striking is that some species have increased significantly since then, such as the European pied flycatcher, which only really established itself in the Netherlands in the 1960s (and remained stable from the 1980s onwards). Species such as the redstart and, more recently, the coal tit, on the other hand, have declined sharply. Current techniques also make it possible to collect this type of data on an even larger scale. The SPI-Birds data network, established by NIOO in 2019, collects breeding data from nest box breeders across Europe to make it publicly available for research. Combining this historical and current data gives us a unique insight into nest box inhabitants in a time of climate change and nitrogen problems.