Why start another clutch before finshing the last?

[GregH]

It seems fairly common with my cut-throats that they start another clutch before they've even fledged the previous lot. Does anyone know why or beter still how to stop it? At lunch-time yesterday I decided to investigate why a new nest was being built in the porch of the Gouldian box occupied by a pair of cut-thoats that had 4 X ten day old young. I hadn't heard much calling over th past few days as I'd been buisy at work and hadn't had time to investigate. Well the smell was enough to tell me something was wrong. There were only 3 nestlings - one was still alive and two others that had died progessively over the last 3 days. I found the cut throats had renested in cane basket (4 eggs) so I shoved the remaining nestling into it and hoped. I can hear it begging this morning and the adults are still sitting. Often my cutties will start laying in the same nest at the same stage but this time they go out and abandoned the young. Given that they are ~$100pr here, I don't want to lose any. Has anyone got any suggestions?