Size matters, well in the case of zebra finch sperm it does. Zebra finch males with longer sperm fertilize more eggs, and farther more chicks, than males with shorter sperm. In the study, they sired 64% of the embryos, even if the shorter sperm males had a head start. The longer sperm probably swim faster, end up in more of the female’s sperm storage spots for later use and are better at staying there.
“Our findings are important because they demonstrate for the first time in birds, using a controlled competitive scenario, that sperm length can influence the fertilization success of a particular male. The results also add to the body of evidence suggesting that the final outcome of sperm competition may be partly dependent on the female, and that the chance of a male siring offspring may not be an outcome of a simple ‘race to the egg,'" says Dr. Clair Bennison from the University’s Department of Animal and Plant Sciences.http://www.alnmag.com/news/2014/12/sperm-usurpers-seen-zebra-finches?utm_medium=twitter&utm_term=%23aln+%23laboratoryanimalscience&utm_source=twitterfeed