Spotting a gaggle of Egyptian geese (Alopochen aegyptiaca) in a park in Darmstadt surprised me. I was sure of the identification: the brown mask is a giveaway, and is supported by the colour of the beak, the pink legs, and a brown patch on the breast. I’d previously only seen them in Kenya, and I’d put them down as a species from sub-Saharan Africa. I found later that my memory was not wrong. It was introduced into UK in the 17th century, but in the Netherlands only in the 1980s, and expanded from there into the rest of Europe. The expansion had just begun when I lived in Germany, so I’d missed them earlier. Breeding populations are now also found in the US. The featured photo is evidence of it breeding in Germany. The two below show it in its native habitats in Africa.

The chicks made me curious about its breeding and mating behaviour. Like most birds, there is pair bonding. Since the young develop in eggs, nests have to be defended, and brooding adults have to be nourished. This is the evolutionary pressure behind pair bonding in birds, at least for a season. But pairs of Egyptian geese bond over several seasons, often for life. In the past behaviour such as this was taken to be a moral example. But behaviour is driven by biology and its imperatives. So it is interesting to try to figure out driver behind such long-term pair-bonding.

The question has exercised several generations of evolutionary biologists. Field studies with several species show that long-term pair bonding improves fertility in long-lived species of birds. The reasons are not yet totally clear. In some species birds begin to play mating games for a year or two before they are mature enough to breed, and it is believed that long-term pairs are formed in this process. If so, it would seem to require effort and time to form new pairs, so reducing the time in each spring during which successful breeding can occur. With our very detailed knowledge of the complexity of long-term pair-bonding in one species, we can imagine that there is much more to this story. There is also interesting speculation about how long-term pair-bonding may lead to the evolution of cooperation.
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