influence du climat et de la prédation sur l'utilisation de l'habitat et la ...

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two modali ties, Table 4.1, models 2 to 4). We first compared "den" to other microhabitats
merged into a single category (Table 4.1, model 2). Among the non-den microhabitats, we
then compared arboreal ("deciduous", "conifer open", "conifer covered") to terrestri al
("ground open", "ground covered") microhabitats (Table 4.1 , model 3); and fi nall y open
("ground open", "deciduous", "conifer open") to covered ("ground covered" , "conifer
covered") microhabitats (Table 4.1 , model 4).
Calcul ating T ~ from other meteorologica1 variables
Because we did not measure Tc continuously during the study peri od, we used
synchronous measurements of Tc, Ta, wind speed and net radi ati on to relate Tc to other
meteorological vari ables. We considered only measures made during daytime (i.e. 30 min
before sunrise to 30 min after sunset) because Tc measured with a black bowl integrates the
effects of wi nd only when solar radiation occurs. We averaged measurements over one-ho ur
periods and considered only periods when Tc varied by ~ 3°C. We obtained 160 one-hour
peri ods from 37 occasions of measurements. We used general linear mixed models with
normal distributions (proc MIXED) and included hour of measurement as a repeated effect in
our analyses. We fitted air temperature, the square root ofwind speed (Kreith & Black 1980),
net radi ati on and their fi rst order interacti ons as fixed effects to test how these vari ables
influenced Tc.
Activity patterns
We wanted to test for an effect of Tc on acti vity patterns. Our data set incl uded repeated
measurements made on the same individual at different dates so we fiUed general linear
mixed models with porcupine identity as a repeated factor. We used mi xed models with
normal di stributions (SAS software, proc MIXED, SAS 2002) to analyse the effects of Tc on
time spent outside of the den, durati on of activity bouts, and nocturnality index (i.e.
continuo us vari ables) and mi xed models with binomi al di stributions (proc G LlMM 1X) to
analyse the effects of Tc on the number of activity bouts per day. This last variable had two
categori es (l acti vity bout; > 1 acti vity bout, range: 2 to 4) in order to test if changes in Tc
influenced porcupines to shi ft from unimodal to plurimodal patterns of acti vi ty. Because
sampling peri ods were different between 2004 and 2005, we tested the effect of year (fixed
factor) before fi tting other vari ables. We removed the effects of confo unding va ri ables on
acti vity pattern by including as fixed factors Julian date, snow penetrability (dail y measure in