[R-sig-eco] "random" lme syntax

[Rafael Maia]

Hello everyone!

I am helping a friend analyze her data on the effect of helpers-at-nest 
on feeding rates ("feeding") in a species of bird. Her data is as follows:

- two groups ("helpers"; helpers/no helpers)
- observations conducted at several development stages of the nestlings 
("age"; repeated measures at each nest, days 1, 6 and 12)
- during 2 years ("year"; 2006/2007, as a covariate; each couple and 
nest only observed during one of these years)

She's also interested in the interaction between helpers and age of the 
nest Intuitively, using lme, I'd guess the formula should be:

lme(feeding~helpers*age+year, random=~1|nest_id, data=tanagers)

However, in Michael Crawley's "The R Book" example of "Mixed-Effects 
models with Temporal Pseudoreplication", page 642, the explanation goes 
as follows:

"Because we have a continuous random effect (weeks) we write ~week in 
the random-effects formula rather than the ~1 that we used with 
categorical random effects (above)."

It may be because I am not so familiarized with lme models, but this 
didn't make much sense to me... Only due to the repeated measures being 
continuous or categorical, I should change the syntax of which portion 
of my model has random effects? If I'm measuring the same individual 
through time, shouldn't that be specified on the model as an 
hierarchical design (say, random=~1|nest_id/age) rather than earlier?

This got me quite confused, so any comments would be greatly appreciated...

Best,
Rafael Maia
---
"A little learning is a dangerous thing; drink deep, or taste not the 
Pierian spring." (A. Pope)
M.Sc. Student
Animal Behavior Lab - Dept. of Zoology
Universidade de Brasilia, Brazil
http://www.unb.br/ib/zoo/rhmacedo/