American Society of Naturalists

Adam E. Mitchell, Jordan Boersma, Anthonio Anthony, Kanehiro Kitayama, and Thomas E. Martin (Oct 2020)

Slow growth and development of a montane songbird reflects developmental plasticity in response to abiotic conditions

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Slow growth and development of a montane songbird reflects parental responses to harsh weather

Animals living at high elevations often have fewer offspring per breeding attempt, slower growth and development, and higher survival rates. These patterns have been shown in mammals, reptiles and amphibians, birds, and even insects. The Mountain Blackeye (Chlorocharis emiliae), a small songbird endemic to a few of the tallest mountains in Borneo, provides a clear example of these patterns. On Mt. Kinabalu in north Borneo, Mountain Blackeyes breeding at ~3200 meters elevation lay a single egg and their nestlings take 14-15 days to fledge, whereas the closely related Black-capped White-eye (Zosterops atricapilla) breeding at ~1500 meters lays two eggs and fledges in 11-12 days. The causes of slow growth and development at high elevations remains puzzling to scientists, but collaborative research led by scientists at the University of Montana has improved our understanding of these fascinating patterns.

Previous studies have suggested that prolonged development at high elevations might reflect an evolved shift to enhance offspring quality in the face of harsh montane weather conditions. However, authors of this study test an alternative hypothesis. They posit that parents need to spend more time warming young in colder and wetter mountaintop environments, reducing time available to provide young with food, thereby causing slower growth and development. To test this hypothesis, the authors conducted an experiment where they simultaneously covered Mountain Blackeye nests from rain while adding supplemental heat to nests. Compared to unmanipulated nests, parents in heated and covered nests spent less time warming young while feeding them more often. Nestlings from heated and covered nests also fledged from their nests earlier as well as grew their wings and gained body mass faster. While slower growth at high elevations may also have an evolved component, results from this study emphasize the influence harsh montane weather can have on growth and development rates through time constraints on parental care.

Abstract

Organisms living at high elevations generally grow and develop slower than those at lower elevations. Slow montane ontogeny is thought to be an evolved adaptation to harsh environments that improve juvenile quality via physiological tradeoffs. However, slower montane ontogeny may also reflect proximate influences of harsh weather on parental care and offspring development. We experimentally heated and protected nests from rain to ameliorate harsh montane weather conditions for Mountain Blackeyes (Chlorocharis emiliae), a montane songbird living at ca. 3200 m asl in Malaysian Borneo. This experiment was designed to test if cold and wet montane conditions contribute to parental care and post-natal growth and development rates at high elevations. We found that parents increased provisioning and reduced time spent warming offspring, which grew faster and departed the nest earlier compared to unmanipulated nests. Earlier departure reduces time-dependent predation risk, benefitting parents and offspring. These plastic responses highlight the importance of proximate weather contributions to broad patterns of montane ontogeny and parental care.