# Reduced chick performance makes supernormal clutches maladaptive in a shorebird

## TL;DR

This study shows that adding an extra egg to shorebird clutches harms chick development and survival, explaining why clutch sizes remain fixed.

## Contribution

The study experimentally demonstrates post-hatching costs of enlarged clutches in shorebirds, offering new insight into clutch size constraints.

## Key findings

- Chicks from enlarged clutches were smaller and had higher mortality rates.
- Enlarged clutches had slower embryonic growth and longer incubation periods.
- Negative effects of extra eggs outweigh potential benefits, making supernormal clutches maladaptive.

## Abstract

Life-history theory predicts a trade-off between the number and quality of offspring, and that clutch sizes may be adjusted in relation to environmental conditions. In many bird species, however, clutch size is remarkably consistent, in which case factors constraining clutch size evolution are more debated. A classic example is found in shorebirds (Charadriiformes), where clutch size is typically fixed at four eggs across species and environments. Here, we experimentally enlarged clutches of Common Ringed Plovers Charadrius hiaticula to test the hypothesis that clutch sizes are constrained by parental incubation capacity. While most previous tests of this idea compared hatching success between clutch sizes, often with mixed results, we also investigated potential post-hatching effects. We found that chicks from enlarged clutches were consistently smaller during the first two weeks of their lives and showed higher mortality rates compared to control chicks. Although hatching success was relatively unaffected, enlarged clutches experienced reduced mass loss indicating slower embryonic growth, required longer incubation periods and hatched more asynchronously than controls. These findings suggest that laying an extra egg would be a waste of resources, with negative effects outweighing potential benefits. Our results highlight the potential importance of previously overlooked post-hatching constraints in shaping clutch size evolution in shorebirds.

The online version contains supplementary material available at 10.1038/s41598-026-37872-6.