Longer sperm means a greater chance of fertilisation, according to scientists.
Experts observed that male zebra finches with longer sperm cells were more successful with fertilisation than their counterparts with smaller cells.
Although the study was conducted on mate-swapping birds, the findings are said to have important implications for other species, including humans.
The research from the University of Sheffield highlights a key aspect of so-called "sperm wars" - the way sperm from different males compete for fatherhood.
Longer sperm are thought to have a competitive edge over shorter sperm because they are faster and more energetic swimmers.
They tend to possess longer flagella, the whip-like tails that beat from side to side to drive the sperm on, providing stronger forward propulsion.
In addition they have larger "power plant" mid-sections between the tail and head, which produce more energy.
A series of experiments reported in the journal Proceedings of the Royal Society B found that significantly more long sperm from male finches reached the ova of females than short sperm.
Males with long sperm also sired a greater number of embryos - 64 compared with 36 for short sperm individuals.
But the scientists discovered that sperm length was not the whole story. In some way still not well understood, the females also had an influence on which sperm they were fertilised by.
The team led by Dr Clair Bennison wrote: "Males producing long sperm were more successful in terms of the number of sperm reaching the ova and fertilising those ova.
"Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important."