Hans van Balen was an enthusiastic ornithologist and bird ecologist. The grandson of his namesake Johan Hendrik van Balen, who published on birds in the second half of the 18th century, he took up a similar interest at young age. He studied biology in Leiden (1948–1958) in the time that the ethologists Niko Tinbergen and Jan van Iersel were active there. Van Balen started his career by studying the relaying behaviour of Black-tailed Godwits after clutch removal, with the aim of estimating the potential effects of egg collection on Godwit reproduction in the Dutch polder Arkemheen. His supervisor was Herman Klomp (ITBON, Institute for applied nature research), an ecologist involved in meadow bird protection. This work was published in Ardea (1959), discussing not only his own Godwit data but also what was known about other species on relaying after clutch removal.
Being interested in bird migration, he spent a lot of time on the ‘vinkenbaan’ of the Dutch ‘Vogeltrekstation’, a place where he learned to catch and handle birds, as he would do the rest of his life with pleasure. Naturally, he joined the bird migration camps in Putten together with Luuk Tinbergen, Herman Klomp and J.J. Zijlstra, a place where a generation of ornithologists had its roots.
During the start of the 20th century the Plant Protection Service (Plantenziektekundige Dienst in Wageningen) became interested in bird ecology, especially because of the role birds have in regulating insect numbers and preventing outbreaks. Wolda started the scientific work on hole breeding passerines in nest boxes in 1912, and Huib Kluijver, as the successor of Wolda (1951), continued what would become a long tradition of hole nesting bird work. Their work attracted international attention and inspired David Lack to start his own long-term population study on nest box breeding birds in 1947. Both population studies at the Hoge Veluwe and Wytham Woods near Oxford are still continuing today and have become famous, as they have contributed substantially to our knowledge about avian population regulation and the study of adaptation in a changing world.
In 1954, Kluijver became the director of the new Institute for Ecological Research (IOO) of the Dutch Academy of Arts and Sciences, an institute specially devoted to long-term population studies, with emphasis on the population ecology of Great Tits Parus major. In 1958, Kluijver choose Hans van Balen to become his second-in-command, where he took full share in the nest box studies in the different plots that the institute ran. In 1967 Van Balen published an extensive paper on wing length and body mass in different study populations of Great Tits, with the main conclusion that body size and mass in coniferous woods were lower than in deciduous woods. Habitat use had both his and Kluijver's full interest, as illustrated by the paper by Kluijver and Tinbergen (1953) on the buffer effect. Hans van Balen's thesis (defense 2 May 1973, promotor Prof. Dr. D.J. Kuenen, co-referenten Dr. H.N. Kluijver, Prof. Dr. K. Bakker) was titled “The comparative study of the breeding ecology of the Great Tit Parus major in different habitats’ (Ardea 61, 1973) and is a most thorough study of the ecology of Great Tits. The goal of this study was to find out whether the preference of Great Tits for mixed over deciduous woods, that Kluijver & Tinbergen (1953) found, was adaptive. Van Balen convincingly showed that deciduous oak woods were rich in food and had high bird densities, much higher than pine woods, even if nest boxes were not limiting. Caterpillar availability in the breeding season was much higher in oak woods, and the peak food availability was earlier (second half of May in that time) than in pine woods (end of June). In the oak woods, caterpillar peak availability roughly coincided with when Great tit nestlings were seven days old, but this was not the case in pine woods, where the timing of their first brood was not different from the oak woods. Van Balen studied differences in lay date and clutch size between habitats, the diet of the young (Tortrix viridana and Operophthera brumata in the oak stands and, at the peak, Panolis flammea in the pine stands) and their food consumption in relation to brood size and the survival of nestlings. His general conclusion was that Great Tits are adapted to breed in oak woods and populations in the coniferous woods are not able to adapt to the local circumstances, confirming the adaptive nature of the Great Tits' preference for oak wood. Why Great Tits still breed in pine woods may be explained by the buffer hypothesis: as we say now, they may make the best of a bad job. His thesis became very well cited.
In 1967, van Balen took over the leadership of the Great Tit group of the institute until his retirement in 1992, continuing the work of Kluijver together with colleagues Joop Mertens (physiology), Piet Drent (behaviour) and later Joost Tinbergen (behavioural ecology). He was not attracted to this management work, but realised that it was necessary. During Hans’ directorship he managed an appreciable technical staff (among them van Eck (on Vlieland) Westra, Visser, de Goede and Speek) that were indispensable in carrying out all the fieldwork. He firmly secured the high quality of data collection on individually ringed Great and Blue Tits Cyanistes caeruleus in a number of woods such as Hoge Veluwe, Liesbos, Oosterhout and Vlieland. For Hans, the collection of long-term data sets from preferably undisturbed populations of Great Tits was of the greatest importance. This long-term work on population dynamics has regularly been under pressure, as people questioned the ongoing investment into these already well-studied species, both at Oxford and in The Netherlands. However, during the late 1980's the Great Tit population studies were an important illustration of the detrimental effect of acidification on avian reproduction (Graveland et al., Nature 1994), which was only possible because changes could be shown in different habitats. The importance of looking back in time to investigate how populations have been dealing with long-term environmental change has increased since then, and current researchers stand on Hans' shoulders when analysing the effects of global change over the last 50 years (e.g., Visser et al. 2003, 2009). The institute at this moment still collects population data and has done so for the same populations since 1955: 60 years!
Van Balen also felt responsibility for Dutch ornithology. He was on the board of the Huib Kluijver Fonds, secretary of the Nederlands Ornithologische Unie (1971–1976) and he was co-founder of SOVON (1973). He also was editor of Ardea and Limosa, the two journals of the NOU.
Hans himself worked on several aspects of the ecology of Great Tits as can be seen from the papers he wrote. Among them is a paper on time saving mechanisms with Kluijver and Cave (1977), an experimental paper on the role of winter food in determining survival (1980) and a paper on effects of nest box size on reproduction (1984) to compare with reproduction in natural holes (1982). One very important spin-off of the painstaking data collection over the years was the PhD-thesis of Arie van Noordwijk, who, under supervision of Scharloo and van Balen, produced a wealth of papers on estimates of the heritability of a number of traits in Great Tits; one of the classic examples of heritability estimates from the field. Hans was keen to collaborate, as can be seen from the many papers he produced with others. For instance, a paper on density dependent survival with Tinbergen (1985), on seasonal decline in reproductive success with Verhuist (1995), with Perdeck on the tit survival in relation to the beech crop index (2000) and in recent years a number of papers with Marcel Visser, such as the paper on climate change and migration distance (2009). At the moment, the Animal Ecology Group of the now called Netherlands Institute of Ecology (NIOO-KNAW) is flourishing partly because they have these historical data on Great Tit populations, collected by predecessors such as Kluijver and van Balen, which allow them to quantify changes in Great Tit populations over time in a changing world.
Personally, Hans gave me the unique opportunity to do experimental work in ‘his’ long-term population studies that he managed with so much care. To show how I appreciated his drive to advance, in particular Great Tit ecology, and ornithology in general, I have visited him a number of times over the last few years. He was not in a very good shape, talked very softly, his binoculars within reach. His rather ‘dry’ sense of humour was still recognisable. Yet his eyes twinkled whenever the bird were the subject. He loved it when I told him about new developments in the tit work. We discussed the effect of Marten predation that has recently increased in our study area, and I asked his advice on an experiment manipulating nest box size. That will not be possible anymore!
