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Chicks From Experimental Five-Egg Common Ringed Plover Clutches Faced 3.5 Times Higher Mortality Risk

Juvenile Common Ringed Plover photographed at Lista, Norway
Research · Reproduction & development

An experiment on the Varanger Peninsula in north-eastern Norway found substantial costs when the normal four-egg clutch of common ringed plovers (Charadrius hiaticula) was experimentally enlarged to five. Chicks from enlarged clutches were smaller after hatching and had a 3.5-times higher risk of dying before ten days of age than chicks from control clutches.

Fieldwork was carried out from late May to mid-July 2022 in a roughly 35 km² study area on Varanger. The researchers followed 46 nests. All experimental nests initially contained the species’ normal four eggs; one member of each matched pair was enlarged with a model egg with thermal properties similar to a real egg, while the other served as a control. Adults continued to incubate the enlarged clutches, allowing the team to test whether the extra egg changed incubation and later chick performance without increasing the number of chicks that parents had to rear.

Larger clutches slowed development

Hatching success itself changed little, but enlarged clutches had longer incubation periods, more asynchronous hatching and lower egg mass loss, which the authors interpret as consistent with slower embryonic development. Hatchlings from enlarged clutches were lighter and had shorter head-and-bill measurements. During the first 15 days after hatching they remained lighter and had shorter tarsi than control chicks.

Body mass and morphometric comparisons for Common Ringed Plover chicks from four-egg and five-egg clutches
Body mass and structural measurements of chicks from normal four-egg clutches and experimentally enlarged five-egg clutches during the first 15 days after hatching. Figure 2 from Heggøy, Wanders & Lislevand (2026), Scientific Reports, CC BY 4.0.

Chick mortality rose sharply

Mortality produced the clearest difference. The minimum proportion surviving the first ten days was 78.9% in the control group and 51.3% in the enlarged group. When the researchers combined clutch size, nest survival, egg survival, hatchability and chick survival, treating the model egg in the calculation as if it were a real fifth egg, the enlarged clutches had an 11% higher calculated reproductive value at hatching, but this advantage had become a 36% deficit by day ten.

Survival curves for Common Ringed Plover chicks from four-egg and experimentally enlarged five-egg clutches
Chick survival during the first ten days after hatching was lower in the experimentally enlarged five-egg group than in the four-egg controls. Figure 3 from Heggøy, Wanders & Lislevand (2026), Scientific Reports, CC BY 4.0.

The authors conclude that the costs of incubating a larger-than-normal clutch can carry over well beyond hatching. They did not directly measure egg temperatures, but describe the longer incubation, altered embryonic development and poorer chick performance as broadly consistent with reduced incubation efficiency in oversized clutches. The results provide a mechanism that can help explain why a four-egg clutch is so strongly conserved among many waders.

Sources and image credit

  1. Heggøy, O., Wanders, K., & Lislevand, T. (2026). Reduced chick performance makes supernormal clutches maladaptive in a shorebird. Scientific Reports, 16, 7305. https://doi.org/10.1038/s41598-026-37872-6Primary open-access research article published 4 February 2026 under CC BY 4.0. The experiment followed 46 common ringed plover nests on the Varanger Peninsula in north-eastern Norway and compared normal four-egg clutches with clutches experimentally enlarged by one model egg. Chicks from enlarged clutches were smaller and had a 3.5-times higher mortality risk before ten days of age; minimum ten-day survival was 51.3% versus 78.9% in controls. Enlarged clutches also had longer incubation and more asynchronous hatching. Cumulative reproductive value was 36% lower than controls by day ten.

Image: Marton Berntsen / Wikimedia Commons — juvenile Common Ringed Plover (Charadrius hiaticula) photographed at Lista, Norway, 1 September 2026. Illustrative Norwegian juvenile photograph, not a chick from the Varanger experiment. · CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/ · Source

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