No rookies in art of nestbuilding

ROOKS, known as ‘crows’ in Ireland, are impatient birds; they can’t wait to begin nesting.

Some start renovating old nests as early as December and, from now on, black balls in bare trees will be a familiar sight. Building begins in earnest in February. Rooks’ are not great architects but constructing big bulky nests on thin windswept branches is quite an engineering feat. How they manage to do it is something of a mystery. There have no drawings to guide them; the blueprints are in their heads. The design is inherited and construction is by trial and error. Practice makes perfect.

Rooks tend to choose the same trees year after year. Mature fat-cat pairs build on the tattered remains of a previous nest. Young Johnny-come-latelys must find a virgin site. Crows form strong pair-bonds; some even remain together for life. But these are quarrelsome birds with big egos; partners can disagree over where to build. To spite each other, males and females have been known to embark on rival structures. Eventually, they compromise and only one nest is completed. A spinster female may build a nest on her own, presumably hoping that some Prince Charming will happen along.

Building is a slap-dash affair. Both partners share the task, although some males shirk their responsibilities, leaving all of the work to their spouses. Normally, the male gathers most of the nest material, breaking twigs from trees or picking them up from the ground. Large sticks are selected first. Most of these fall to the ground, until, eventually, a few become lodged between branches. Once the foundation is in place, fewer twigs fall and a ball of material builds up. Birds sometimes carry useless objects, such as stones and pine cones, to the site.

Nesting in colonies provides security; enemies such as hawks and pine martens will be spotted and mobbed. But there’s a price to be paid for communal living. The main problem is pilfering. Unprotected half-built nests can be raided and the materials stolen. Residents sometimes gang up on a particular pair; the nest will be targeted by several neighbours, when the owners are away, and almost everything may be taken. To human eyes, nest-raiding looks more like vandalism than looting; little of the stolen material is used in the robbers’ own nests and most of it ends up on the ground. But all is not lost for the hapless victims. Even if a nest is almost totally gutted, the large foundation sticks generally remain in place and the pair immediately set about repairing their home. One partner, usually the female, may stand guard when the other is away gathering material. As soon as the structure is well advanced and the cup is being constructed, the raids cease.

Interior décor is left to the female; a cup of flexible twigs, mud and moss is fashioned by the imprint of her body. She lines her new home with grass, leaves or scraps of paper. But how does a bird decide that the structure is complete and that it’s time to stop adding material? Experiments suggest that the crucial factor is the shape of the nest; a bird works until it has created the ideal form. When parts of weaver birds’ nests were removed, the owners immediately repaired the damage. They did so even when the damage posed no threat to the functioning, or security, of the nest. The birds were not satisfied until their nests conformed to accepted weaver fashions.

When the nest has been built, the male regurgitates food for his mate. He fed her during courtship in the autumn, but stopped doing so in the winter. Now, the extra food he presents will help her to form eggs. Mating also takes place at the nest and here another disadvantage of communal breeding is encountered; cuckolding. Adultery is rife among rooks and some of the eggs in a nest won’t have been sired by the father listed on the birth certificate. Oddly, promiscuous shenanigans are commoner after the eggs have been laid. In one Cornish study, 94% of females who engaged in such coupling, were already incubating. Just what ecological advantage the behaviour confers is anybody’s guess. Rooks seem to fornicate for fun. Nobody is perfect.