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Deeper Nest Tunnels Were Linked to Larger Clutches; Higher Nests Were More Likely to Succeed in Asian Green Bee-eaters

Asian green bee-eater perched on a seed head against a soft green background
Research · Reproduction & development

A field study of Asian green bee-eaters (Merops orientalis) at Ballar Stream in Pakistan’s Mardan District found two clear relationships between nest architecture and reproduction: deeper burrows were associated with larger clutches, while nests placed higher on the bank were more likely to succeed. Researchers followed 38 natural nests during the 2024 breeding season.

Along about 10 km of stream bank, the team measured tunnel depth, entrance size and nest height, recorded distances to nearby grassland, farmland, towns and electrical infrastructure, and followed eggs and chicks through the breeding attempt. The aim was to test whether nest structure or the surrounding landscape was more closely related to clutch size and breeding success.

Deeper tunnels were linked to larger clutches

The nests averaged about 87 cm deep and contained 3.9 eggs on average. Clutch size increased with tunnel depth, whereas entrance diameter and nest height were not clearly related to how many eggs were laid.

Nest height mattered for a different outcome. Nests placed higher on the bank had a greater probability of breeding successfully, while broader measures of cavity size showed only weaker evidence of a relationship. The measured distances to grassland, farmland, towns and power lines also showed only weak relationships with breeding outcomes.

The study therefore points to the nest itself — particularly tunnel depth and vertical placement — as more informative for reproduction than the surrounding landscape variables measured here. Across the monitored nests, hatching success was 77.5%, and about half of the chicks reached fledging age.

The proposed mechanisms remain hypotheses

The authors suggest that deeper tunnels could provide a more stable microclimate or make nests harder for predators to reach. Higher nests may likewise be less accessible to terrestrial predators and less exposed to human disturbance. These explanations are biologically plausible interpretations offered by the authors, but the study did not directly test which mechanism caused the observed relationships.

All recorded nests were excavated in sandy stream banks. The authors therefore emphasise the value of retaining suitable sandy or earthen nesting banks and reducing disturbance around active nesting areas. They also caution that the study covered one stream system and one breeding season, so the same relationships should be tested at additional sites and across more years.

Sources and image credit

  1. Sadam, A., Awais, M., Hu, H., Yu, D., & Hu, Y. (2026). Nest Depth and Height Are Associated with Breeding Outcomes in the Small Bee-Eater (Merops orientalis): A Preliminary Field Study from Pakistan. Animals, 16(2), 186. https://doi.org/10.3390/ani16020186Primary peer-reviewed open-access study, published 8 January 2026; CC BY 4.0. Researchers monitored 38 natural nests at Ballar stream in Pakistan. Cavity depth was positively associated with clutch size and nest height with breeding success; the authors note the study covered one site and one breeding season.

Image: Imran Shah — Asian green bee-eater at Samianwala, Balloki, Kasur, Punjab, Pakistan, 16 May 2018. Wikimedia Commons; CC BY-SA 2.0. Representative species photograph, not from the Ballar Stream nest study. · CC BY-SA 2.0 — https://creativecommons.org/licenses/by-sa/2.0/ · Source

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