[R-sig-ME] False convergence lmer

[Brickhill, Daisy]

Hi,
I am modelling how the movements of birds effects their reproductive success. I have 7 groups of nests which have differing mean brood sizes (e.g. group A: brood size 3.5, group B: brood size 3.0 etc). A bird dispersing from A to B therefore would lose 0.5 of a chick.What I want to know is if the difference in brood size gained (or lost) by movements varies with nest availability in 'better' areas (i.e. all those groups that have higher brood size than the group of origin). I have constructed a mixed model using lme4 with a gaussian distribution
Brood size difference between origin and destination~number of better nests available +number of nests available in the group of origin + (1|group of origin) + (1|group of destination).
However I get the warning message:
Warning message:
In mer_finalize(ans) : false convergence (8)
Looking at the mailing list I have tried standardising the predictor variables, I have tried dividing them by 100. The only thing that works on the model is removing either the random effect of  group of origin or group of destination (removing the fixed effects still gives the error message). I wondered if the problem is that the same combinations of groups of origin and destination always have the same difference between them?
The other thing that occured to me is that birds are quite likely to remain in their group of origin and for some groups this is as high as 67% - might that be a cause?
If it's not likely to be this can anyone give me any pointers as to what to look for, to work out what it is in my data that is causing this problem?
Many thanks,
Daisy


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