# GLM for proportional data

I'm hoping someone can help me sort out how the proportion comparisons using a GLM works in R.

I'm comparing hatch success among multiple years (and later, sites). I've used a GLM to compare among years by making a txt file where the success column contains the number of chicks that hatched, and failures are the number of eggs in each nest that didn't hatch. Each row corresponds to a nest. (I did this according to descriptions in "The R Book" by Michael Crawley)

The code looks like this:

y<-cbind(success,fail)
hsmodel1<-glm(y~year,binomial)


This tests for differences in hatch success among years as #of chicks hatched/eggs laid, correct? Not # of chicks hatched /nest?

Secondly, if my species lays up to 2 eggs, is using this proportional method still valid, since there could be 0, 1 or 2 successes or failures? I'm pretty sure it is, but I am starting to doubt myself from a question a colleague asked today.

Thanks! Mog

You can (with predict(type="response")) get the probability of an egg hatching, conditional on the year from this type of model (technically that is not exactly the same as a rate, but for most practical purposes, it is).