Corvids: Breeding collectively is latitude dependent
The corvids, which are evolutionarily very high in the bird family, developed social brood care after their phylogenetic origins, with young birds helping their parents to raise the next generation. However, this breeding aid was abandoned by some species as they spread to the northern reaches of the northern hemisphere.
This is the result of a phylogenetic tree analysis by Jan Ekman from Uppsala University and Per Ericson from the Swedish Museum of Natural History in Stockholm based on mitochondrial and nuclear DNA of the birds. In the next step of the investigation, they then included information on the distribution area, the habitat and, above all, the breeding behavior of the individual species. It is important to know that in the case of ravens, crows and jays, in addition to family help with the brood, there are also unrelated helpers who support the actual breeding pair in raising the young. Only the minority of the species does not accept external support.
This analysis then revealed a close connection between the range of the respective species and breeding behavior: Species living in the tropics and subtropics work together much more often than their relatives in the high latitudes of the northern hemisphere. They are all originally descended from primordial corvids that were paired together and first appeared in the Australian-Papuan area. From there the non-social species and genera then spread northwards. In the south, on the other hand, a second line developed, which over time acquired the cooperative breeding behavior as a secondary factor. However, there were no connections with the inhabited habitat.
The scientists suspect climatic backgrounds as the cause for the latitude-dependent differences. Corvids are exposed to greater seasonal fluctuations in food supply and greater weather extremes in the high latitudes than in the tropics. By separating families or groups early on, they try to avoid possible competitive situations. The stronger cohesion in the tropics, on the other hand, could be caused by the lack of suitable and free breeding sites in "saturated" habitats.
However, corvids in the north also show stronger family cohesion in times of need: If the separation of parents and children is delayed for a reason, the adults share their food with the offspring in times of need, while foreign conspecifics are aggressively driven away.