Manipulating brood size experiments of two species passerine birds——Testing Lack's hypothesis

The experiments were carried out at Haibei Alpine Meadow Ecological Research Station, the Chinese academy of science, in northern Qinghai province from 1997 to 1999. Through experimentally manipulating brood size, the effects of var ious broods on nestling development and adult survival of small skylark (Alaud a gulgula) and twite (Acanthis flavirostris) were analysis to test Lack's hy pothes es. We performed manipulating brood size experiments by adding or removing one, two, or three nestling in selected nests having similar hatching time, while kee ping the original number in other nests to serve as control. According the predi ction of Lack's hypothesis, we were going to test the following problems. (1) Wa s the common clutch size the maximal production clutch size actually? (2) Did the manipulating brood size affect on nestling quantity and parents effort? (3) Were the patterns responding to manipulating brood size of birds the same? The resul ts shown: ① the common clutch sizes of small skylark and twite were 3 and 5, re spectively, and those were stable and changed insignificantly among years. The f ledging rates decreased to some extent with enlarging the natural brood sizes by increasing 1 or 2 nestlings. If the productivity was assessed through the fledg ing rate of the young, the most common clutch size would mean the most productiv ity （the highest fledging rate）.② the growth parameters of small skylark's ne stling were varies insignificantly with the change of brood size, but those of t wite had the significant change. The result showed that the manipulating brood s ize had more significant effect on the latter than the former.③ the feeding rat e of small skylark parents was increase with the increasing of brood size, but i t's nestling period didn't vary. On the contrary, for twite, the feeding rate di dn't vary, and the nestling period delayed with the increase of brood size.④two species parents took on different patterns responding to enlarging brood size. Small skylark took on improve the feeding times per hour. However, twite took on delaying the period of parents care. The two patterns descended offspring or pa rent fitness through the effect on nestling quantity or parent survival. The abo ve results supported that natural selection adjusted clutch size to the maximal number that the parents fed offspring as possibly as they can, i.e., the results supported the Lacks hypothesis the common clutch size being actually the most p roduction clutch size.g the n