Within-clutch patterns of egg viability and paternity in the House Sparrow

One of the potential benefits of extra-pair copulation is an enhanced likel ihood of successful fertilization if the pair male is unable to produce suf ficient viable sperm to ensure the fertility of an entire clutch. Los male fertility can be a transient phenomenon associated with higher rates of spe rm usage than production, leading to a progressive depletion of sperm reser ves. Evidence of a correlation between hatching success and the occurrence of extra-pair fertilizations has been reported in House Sparrows. In this s tudy we have used single-locus minisatellite and microsatellite profiling t o investigate the distribution of extra-pair fertilizations within House Sp arrow broods to determine whether there are changes in the level of hatchin g failure or extra-pair paternity during the course of producing a clutch. Embryonic material was collected at mid-incubation to minimize loss of data through pl-c-sampling mortality and to ensure that laying order was known. A significant clustering of both infertile eggs and extra-pair young was n oted in the earliest eggs in a clutch. Possible causes of this phenomenon a re discussed.