Egg mass in an asynchronously hatching parrot: does variation offset constraints imposed by laying order?

Area of Science:

• Ecology
• Evolutionary Biology
• Ornithology

Background:

• Life history theory posits trade-offs between offspring quantity and quality.
• Intraspecific variation in avian egg mass is common, with larger eggs potentially enhancing offspring fitness.
• Empirical evidence for a trade-off between egg size and clutch size remains limited.

Purpose of the Study:

• To investigate facultative adjustment of egg mass in Green-rumped parrotlets (Forpus passerinus).
• To identify factors influencing variation in egg mass within and among populations.
• To assess the relationship between egg mass and offspring fitness components.

Main Methods:

• Analysis of 5,743 Green-rumped parrotlet eggs over a 15-year period.
• Examination of variation in egg mass in relation to population, season, maternal effects, and laying sequence.
• Evaluation of the impact of egg mass on hatching, fledging, and postfledging recruitment.

Main Results:

• Heavier eggs significantly increased the probability of hatching and fledging.
• Egg mass varied between populations and showed potential seasonal adjustment linked to environmental factors like seed density.
• Egg mass exhibited moderate heritability (h² = 0.42) and was strongly influenced by maternal effects, female age, and laying position, independent of clutch size.

Conclusions:

• Egg mass is a key determinant of early offspring survival in Green-rumped parrotlets.
• Facultative adjustments in egg mass are influenced by environmental conditions and maternal factors, rather than clutch size.
• Laying sequence-dependent egg mass adjustment may serve as a mechanism for brood reduction in this asynchronous species.