For better or for worse?

Cathrine Glosli

Katrine Eldegard and Geir A. Sonerud have studied the behaviour of Tengmalm owls. They found that when conditions are favourable, mother takes off and leaves the work to father. Their studies showed that the better the food conditions were, the larger were the chances of female desertion.

Tengmalm's owl Photo: Geir Sonerud

Fascinating behaviour
With some species, it happens that the female deserters her offspring and leaves the male behind as the sole provider. This kind of behaviour has been given a lot of attention from scientists, especially from theoreticians. Still, the knowledge about this phenomena, and its causes, is very incomplete.

Nocturnal mouse hunter
The Tengmalm's owl (Aegolius funereus), with its weight of ca 100–150 grams, is the second smalles owl in Norway. It prefers conifer forests and lives mainly of mice that are caught during the night. Any surplus of prey is stored in the nest for later usage.

Radio-marked owls
The researchers equipped parents and offspring in 43 owl families with small radio transmitters. The use of these transmitters made it possible to document parental behaviour, seeing whether they provided for their offspring or deserted them.

The researchers followed the owls throughout their nesting period and two months after fledging, when the offspring were still dependant on their parents. The fieldwork was carried out in Hedmark during a five-year period. Rodent population densitites varied inbetween years. In one of the years the researchers also increased the amount of stored prey in half of the nests by adding dead mice.

Mother takes off
Katrine and Geir discovered that 70% of females left their partners and offspring behind. All deserted males continued to care for the offspring alone. Only in one case did the male desert the female, but in this case the female left the nest shortly afterwards, and all the chicks died.

The researchers used both rodent index and parental body mass as measures of food supply. No matter which was used, the better the food supply, the larger were the chances that the female would leave.

The experimental increase in food supply resulted in a larger desertion rate by the females. They also left earlier. The chick mortality was larger in those broods that had been left by their mother. This indicates that desertion comes at a cost.

The question is what the females gain from deserting their chicks. Much indicate that the primary motivation for female desertion is the chance to produce a second brood with another male during the same mating season.

This study has gotten a lot of media attention, read more about it here.

Read the entire article ’Female offspring desertion and male-only care increase with natural and experimental increase in food abundance’

Updated: 14.04.09
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