We have data on the day in which a butterfly pupates (forms a cocoon) in the summer/fall of 2 different pairs of years (2005-2006 vs 2009-2010). At the time that the pupa forms it can either be in diapause (suspended animation) or not.
Individual butterfly caterpillars were randomly selected from a wild population and observed in outdoor enclosures until they pupated. At this time the pupa was determined to be either in diapause or not.
We would like to know if there is a shift in the probability that a butterfly pupa will be in diapause or not on a given day between years. In other words, does a butterfly pupa have a significantly different probability of being in diapause later (or earlier) in 2005 vs 2006?
We have analyzed each pair of years using separate multiple logistic regressions with Day of pupation and Year as factors and are interpreting a significant Year effect as a significant shift in diapause probability on a given day between years.
Is this the appropriate way to approach this problem?
Individual Diapause Pupation_Day Year 1 1 200 2005 2 0 201 2005 . . . . . . . . . . . . n 1 300 2006