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Multifocal optical systems and pupil dynamics in birds.

Olle E Lind1, Almut Kelber, Ronald H H Kröger

  • 1Department of Cell and Organism Biology, Lund University, Helgonavägen 3, 223 62 Lund, Sweden. Olle.Lind@cob.lu.se

The Journal of Experimental Biology
|August 30, 2008
PubMed
Summary

Birds possess surprising multifocal eyes, challenging previous assumptions. Their circular pupils, combined with adaptable pupil constriction, allow for enhanced vision across different light conditions.

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Area of Science:

  • Comparative physiology
  • Animal vision
  • Ophthalmology

Background:

  • Longitudinal chromatic aberration (LCA) causes visual defocus, especially in animals with shallow depth of focus.
  • Multifocal optical systems, common in vertebrates, use concentric refractive zones to correct LCA.
  • Slit pupils in some species ensure light passes through all optical zones, regardless of pupil size.

Purpose of the Study:

  • To investigate the optical systems of bird eyes, specifically addressing the assumption of monofocal systems due to circular pupils.
  • To determine the prevalence of multifocal versus monofocal optical systems in avian species.
  • To explore potential mechanisms enabling multifocal systems in birds with circular pupils.

Main Methods:

  • Videorefractometry was employed to examine the optical properties of bird eyes.
  • The study included a diverse sample of 45 bird species across 12 orders.
  • Pupil responses to varying light levels were observed in select species like parrots and owls.

Main Results:

  • A significant majority of bird species studied (29 out of 45) exhibit multifocal optical systems, contrary to expectations.
  • Only five species demonstrated monofocal systems; results for 11 species were inconclusive.
  • Parrots showed pupil-switching behavior, suggesting a mechanism to utilize multifocal optics, while owls maintained wide pupils.

Conclusions:

  • Bird eyes commonly feature multifocal optical systems, enabled by pupil dynamics that switch between multifocal and pinhole principles.
  • This pupil switching mechanism allows birds with circular pupils to effectively use multifocal optics.
  • Differences in pupil response between diurnal (parrots) and nocturnal (owls) species correlate with their visual sensitivity and light adaptation strategies.