Birds' eye size reflects habitat and diet, may predict sensitivity to environmental change

Date:
August 9, 2021
Source:
Florida Museum of Natural History
Summary:
A new study shows the eye size of birds can reveal broad patterns
of their biology and behavior, including where they live, what
they eat and how they hunt, providing a potential roadmap for
future conservation efforts.



FULL STORY ==========================================================================
A new study shows the eye size of birds can reveal broad patterns of their biology and behavior, including where they live, what they eat and how
they hunt, providing a potential roadmap for future conservation efforts.


========================================================================== Birds have some of the largest eyes relative to their bodies of all
vertebrate land animals, second only to frogs. With a limited range of
taste and smell, birds primarily rely on vision to navigate, find food
and avoid predators. Yet surprisingly little is known about how eye
size in birds influences their behavior compared with other traits,
such as beak shape and body size, which scientists have meticulously
studied since Charles Darwin's classic work on finches.

"I was really shocked to find out while doing literature searches that
there was no definitive publication on how eye size in birds relates to
their environment," said Ian Ausprey, a recent doctoral graduate of the
Florida Museum of Natural History's Ordway Lab of Ecosystem Conservation.

Previous studies on bird eyes have been limited in scope, typically
including only a few dozen species or birds in specific regions. This gap
in scientific knowledge was all the more glaring given that a graduate
student measured the eyes of more than 4,000 species of birds in museum collections in the late 1970s, creating the largest dataset of its kind.

Ausprey relied on this resource to analyze eye size for 2,777 species
-- about one-third of the world's bird diversity -- revealing that this
single trait more powerfully predicts where birds live and how they behave
than better- studied characteristics such as size, anatomy and movement.

Large eyes increase sensitivity to deforestation Ausprey had the idea
for the study while conducting fieldwork with colleagues in the Andean
forests of Peru. Over the course of five years, the researchers measured
the eyes of Peruvian birds and attached small light sensors to more
than a dozen species of tanagers, finches, wrens and woodpeckers to
determine how these birds were coping with increased amounts of forest fragmentation due to agriculture.



========================================================================== Their results were troubling: Birds with large eyes avoided agricultural fields, keeping to diminishing forest habitats. But the researchers
could also use eye size to predict where these birds mated and laid eggs
and what they were eating, valuable information for future conservation efforts.

Ausprey wanted to know whether this pattern held true for all birds,
not just those in Peru. But with over 10,000 species spread out across
all seven continents, answering a question as broad as how eye size
influences bird behavior would have taken years.

Fortunately, the data Ausprey needed had already been collected in the
form of a dissertation, a nearly 2,000-page tome completed by Stanley
Ritland during his time as a doctoral student at the University of
Chicago.

"He spent his time traveling around museums, extracting eyes out of
specimens preserved in alcohol and then measuring them," Ausprey said. "He
did it for several thousand species of birds, as well as mammals and
reptiles." Ritland left academia upon graduating, however, and never
published his data in a scientific journal. Researchers have used small portions of the massive dataset, initially relegated to the stacks of
the University of Chicago library, to answer small-scale questions,
but comprehensive analyses have so far been lacking.



========================================================================== Although the data was available, the time-consuming task of digitizing
it still remained. Ausprey hired two undergraduate students, Savannah Montgomery and Kristie Perez, who spent five months transcribing Ritland's measurements into spreadsheets so they could be analyzed and shared more broadly with the scientific community.

Because eye size tends to increase with body size, Ausprey standardized
all the measurements for each species by mass and intentionally omitted
birds that operate at optical extremes, such as far-sighted raptors and nocturnal owls.

Scientists already know these species have unusually large eyes.

Instead, he focused on land-dwelling birds that hunt for food close to
the ground and are most active during daylight hours.

Light and shadow define bird vision Stark patterns began to take shape
as eye size was compared with a host of behavioral traits.

Birds with larger eyes live closer to the equator, where the planet's belt
of rainforests create dark understory habitats. Regardless of latitude,
birds that hunt or forage closest to the forest floor have large eyes
to take in as much light as possible, while those that spend more time
in the sky had correspondingly smaller eyes to reduce glare.

"Bright lights can cause something called disability glare," Ausprey said.

"When you shine a light on birds, they change the way they forage. They
also respond differently to vocalizations of experimental predators." Scientists worry that such behavioral changes may negatively affect
avian understory specialists, many of which have already been displaced
because of deforestation.

"Understory tropical birds may be especially sensitive to fragmentation
because they are adapted to dark forested environments and are unable
to cope with rapid changes in brightness associated with forest edges
and human-modified habitats," Ausprey said.

Eye size is also strongly correlated with diet. Larger eyes not only
absorb more light, but they can also confer increased focal length and resolution, the equivalent of upgrading your camera with a longer lens.

Birds that eat insects have larger eyes, which are better suited for
spotting prey at long distances, regardless of whether they lived in
the forest understory or open habitats. Birds with the smallest eyes
relative to body size were often nectar feeders, hinting that they may
rely on color more than shape when looking for food.

Ausprey also analyzed how eyes have changed throughout the birds'
evolution, finding that once eyes became larger in a particular group,
they stayed that way. This meant that closely related groups, such as the hummingbird and swift families, could have eyes of vastly different sizes.

Within a family, however, size didn't change much among species. Fly
catchers, for example, spend a lot of time sallying out and catching prey, which requires long-distance, binocular vision, Ausprey said.

"And it turns out, flycatchers tend to have larger eyes, as you'd
expect. All the finches and tanagers and such that eat fruits and
seeds tend to have very small eyes." Collections provide tools for understanding the natural world To Ausprey, the data collected by
Ritland decades ago offer an unparalleled glimpse into bird diversity
and behavior, which may help conserve species for the future.

"Nearly half a century of time has passed, and yet the same datasets
are relevant," Ausprey said.

Ritland relied entirely on alcohol-preserved museum collections, meaning
the same specimens he measured are still accessible to scientists
stitching together patterns in the natural world.

Some of the birds he encountered during his museum visits were already
of considerable antiquity by the time he began taking his measurements, including two birds collected during Captain Cook's first voyage around
the world.

"Museum collections are invaluable, indispensable and essentially irreplaceable," Ritland said in an email.

Ausprey, who knew firsthand the difficulty of collecting eye-size data in
the Andean cloud forests of Peru, gained a newfound respect for natural
history collections while using Ritland's work.

"As an ecologist, it's become extremely apparent
that collections are invaluable for providing data on
traits that we really can't collect in the field easily." ========================================================================== Story Source: Materials provided by
Florida_Museum_of_Natural_History. Original written by Jerald
Pinson. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ian J. Ausprey. Adaptations to light contribute to the ecological
niches
and evolution of the terrestrial avifauna. Proceedings of the
Royal Society B: Biological Sciences, 2021; 288 (1950) DOI:
10.1098/ rspb.2021.0853 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210809105857.htm

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