Daily survival analysis
The daily survival of forest versus shoreline nests was compared for
predation risk, based on 175 nests (88 forest and 87 shoreline nests
surrounding in total of 46 water bodies) that survived the entire study
period or with the known depredation time (68 nests depredated).
Depredated nests where memory cards of the cameras failed were excluded
because of unknown predation date (N = 2 in 2017). Furthermore,
the daily survival of shoreline nests around permanent lakes and ponds
was compared, based on 87 nests (41 with exact depredation time).
The GLMM framework was used to calculate daily nest survival probability
by using modified logistic regression, which incorporates the number of
exposure days (seven, each beginning at 12 pm) into the link function
(Shaffer, 2004). The logistic exposure method is a modification of
logistic regression and maximizes the use of nest survival data by
treating each measurement day as a discrete trial. Daily nest fate was
analysed as a binary response variable (1 = survived, 0 = depredated).
In the forest-shoreline comparison explanatory variables were “DATE”
(1,…7) and “HABITAT” (factorial: shoreline, forest). As nests
were established in pairs around the water bodies, one in the shoreline
and one further away from the shoreline, water body (“WETLAND_ID”)
was used as a random factor. For larger lakes with more than one nest
pair, an individual ID was given for every pair of nests. When comparing
water body types, the explanatory variables were “DATE” and “TYPE”
(factorial: lake, pond). “WETLAND_ID” was used again as a random
factor, but this time it only meant shoreline nests. Field percentage
(“FIELD”) around water bodies was used as an explanatory variable in
both analyses. Year effect was found to be negligible during the data
exploration and was thus discarded from the analysis.