The following table illustrating Kosovo's demography through the years is culled from Appendix A of "Kosovo: how myths and truths started a war", by Julie Mertus.
1838 data "Muslim" carried as Albanian.
1905 data "Albanized Muslim Serb" carried as Albanian.
Other minorities omitted for simplicity's sake.
| YEAR | Serb | Muslim |
| 1838 | 42% | 58% |
| 1905 | 33% | 66% |
| 1921 | 21% | 66% |
| 1931 | 27% | 60% |
| 1939 | 33% | 54% |
| 1948 | 24% | 68% |
| 1953 | 24% | 65% |
| 1961 | 24% | 67% |
| 1971 | 18% | 74% |
| 1981 | 13% | 77% |
| 1991 | 10% | 82% |
As to present conditions, it remains unknown, as the 2011 census in Kosovo was boycotted by the Serbs in the North, but 10% is probably ball park correct, if a little on the high side.
Thanks! Looks like percent of Serbs in Kosovo had declined long before the NATO involvement.