Data from: The trade-off between clutch size and egg mass in tree swallows (Tachycineta bicolor) is modulated by female body mass

Egg production is a costly component of reproduction for female birds in
terms of energy expenditure and maternal investment. Because resources are
typically limited, clutch size and egg mass are expected to be
constrained, and this putative trade-off between offspring number and size
is at the core of life history theory. Nevertheless, empirical evidence
for this trade-off is equivocal at best, as individual heterogeneity in
resource acquisition and allocation may hamper the detection of the
negative correlation between egg number and mass within populations. Here,
we investigated how female body mass and landscape composition influences
clutch size, egg mass, and the relationship between these two traits. To
do so, we fitted linear mixed models using data from tree swallows
Tachycineta bicolor breeding in a network of 400 nestboxes located along a
gradient of agricultural intensity between 2004 and 2011. Our dataset
comprised 1463 broods for clutch size analyses and 4371 eggs (from 847
broods laid between 2005–2008) for egg mass analyses. Our results showed
that agricultural intensity negatively impacted clutch size, but not egg
mass nor the relationship between these two traits. Female mass, on the
other hand, modulated the trade-off between clutch size and egg mass. For
heavier females, both traits increased jointly, without evidence of a
trade-off. However, for lighter females, there was a clear negative
relationship between clutch size and egg mass. This work shows that
accounting for individual heterogeneity in body mass allows the detection
of a clutch size/egg mass trade-off that would have remained undetected
otherwise. Identifying habitat and individual effects on resource
allocation towards reproductive traits may help bridging the gap between
predictions from theory and empirical evidence on life history trade-offs.