Ruffs and black-headed gulls differ in quite a number of respects. Female ruffs (reeves) care alone for their offspring, build nests, incubate eggs and lead the chicks. In gulls male and female care together. Yet, both species are not very distant in their evolutionary descent. At a closer inspection, the very narrowly related species to the ruff, the sandpipers, display a wide variety of patters of care for offspring. There are a number of monogamous species in which male and female care together. There are also several species whose members do not form lasting pair bonds, as for instance, in the ruff. In some species male and female, each take care for a separate nest. They have a short lasting bond during the time that the eggs are laid for one of the nests. After completion of the first nest both try to find a new mate. The female cares for her second nest, the male returns to the first. I became impressed by these phenomena, and started to make a study of the evolution of parental care in birds.
Besides, I noticed that there were large differences in the numbers of male and female ruffs that stayed in the Netherlands. In the period I observed them, there were sometimes very large numbers of females visiting the meeting sites. Apparently more females were mating than the number of males at these sites. However, the number of breeding females after the mating season was much lower than the number of males that had been present. Therefore, I supposed that fertilized females moved further north, to the breeding areas in Scandinavia and northern Russia. That could have advantages, as the breeding season is rather short in those areas. This idea induced me to explore some breeding localities in Lapland. That idea was interesting, but, according to current knowledge, not a general pattern. Instead, ruffs were going to leave The Netherlands. It started with breeding females, was followed some years later by the males of the display grounds, and just recently also by most migrating females and males. Their way of life could no longer be combined with changing agricultural practice.
Parental care in birds continued to fascinate me. Especially the question what kind of parental care pattern in the earliest birds could give rise to the present diversity. According to my most probable scenario the females of the archetype of birds were mainly producing eggs, whereas the males were taking care for these eggs, and perhaps in some cases, also for the young. That pattern also appears from research on fossil nests from early birds and closely related dinosaurier species. It is also apparent in the megapodes from eastern Indonesia, and in the ratites, that dispersed from about 85 million years ago onwards from the southern continent Gondwana, probably mainly by wing, over the world. The extinct elephant birds and moas and the still existing ostrich, rhea, emu, cassowary and kiwi must have lost flight ability and have grown to big birds in a later stage of evolution.