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Growth rates

Posted by zebra77 on 23 Jun 2008 at 14:41 GMT

First, the conclusion on differential growth rate of the nestling based on the first few days of growth might be limited if some nestling adopt different growth strategies and are able to catch up at later ages.
Second, the author’s decision to restrict assessment of offspring growth to only four days of age seems a bit arbitrary. On the one hand, I understand the logic of analyzing offspring growth to the point when most of them are still alive. However, that aim was not entirely fulfilled by the current restriction, as in the last category there were still nestlings that died before the fourth day. On the other hand, there should be available statistical tests that deal with repeated measures even if not all individuals have a complete set of data. For example, SAS can be used to analyze growth rate despite premature nestling mortality. If that is not possible because offspring were weighed at different ages, an alternative approach would be to calculate a single value representing growth rate for each nestling or report their size on the last day of measurement.

RE: Growth rates

kwsockman replied to zebra77 on 04 Jul 2008 at 21:48 GMT

That is an interesting possibility you raise that some nestlings (specifically late-hatched nestlings) could adopt a different growth strategy that was missed by analyzing growth over the first 4 days only. However, the growth analysis is only meant to provide a possible explanation for differences in fledging probability. As I mention in the discussion, other traits that vary with laying order (perhaps some aspect of the egg itself) may contribute.

You mention that despite my restricting the growth analysis to only the first four days, there were still nestlings in the last category that died before the fourth day. This is true. However, their early death would not have caused the statistical differences between nestlings that I report. Rather, the differences are in spite of these early deaths. Those that died early were the slowest growing of all of them, yet their death prevented their equal representation in the analysis, thus reducing my ability to show differences at all.

I will have to look into the SAS function you mention. Do you know what it is called? Also, thanks for the suggestion about reporting size of each nestling on the last day of measurement. That too may have been a good approach.