Crane (Grus grus) Science Article 1
Juveniles and immature birds normally have less foraging ability and a lower food intake rate than adults. This-it has been presumed-is compensated for by parental care and investment during juvenile development. Studies of time budgets and parental investment of Common Cranes were carried out in the years 1995 to 1999 in Northeast Germany. Having first marked young Cranes (colour rings, radio transmitters), we analysed the behaviour of parents and offspring to correlate this with reproductive success. Parents with young (39.6 %) were four times more vigilant than non-breeders. The investment of pairs with two young was significantly (p < 0.001) higher than in pairs with only one young. Females profited from the high vigilance rate of their males (males mean 43.9 %, females 35.3 %) and were thus able probably to compensate for their investment in the clutch faster because of higher food intake. Defence of the territory against other cranes and defence against predators were tasks mostly undertaken by males. If the males participated in the rearing of the young, the pair were able to rear two juveniles to fleding. The young profited from parental care. They were able to feed most of the time (67.7 %) with lower vigilance costs (11.9 %; 27.7 % less than parents), and thus possibly compensated for their lower feeding success. Rearing of two young, however, must be the upper limit of possible investment of adults. So far there has been no evidence of families with three grown-up young in a stopover region or on the wintering grounds. The survival of juveniles from the date of ringing in June up to the migration to the wintering sites in mid-November amounts to 77.9 %, with 84 % for families with one young and 75 % for those with two young. Accordingly the reproductive success was higher for pairs with two young (1.32 juv./pair) than with one young (0.84 juv/pair).
Gunter Nowald, J. Ornithol. 142, 390-403 (2001)