Fly away home? Ice Age may have clipped bird migration

The onset of the last ice age may have forced some bird species to abandon their northerly migrations for thousands of years, says new research led by a University of Nebraska-Lincoln ornithologist.

A study led by Nebraska's Robert Zink proposes that many bird species, such as the Canada warbler, may have 
completely stopped migrating during the last ice age [Credit: University of Nebraska-Lincoln]

Published in the journal Science Advances, the study challenges a long-held presumption that birds merely shortened their migratory flights when glaciers advanced south to cover much of North America and northern Europe about 21,000 years ago.

The study concluded that the emergence of glaciers in those regions instead acted as an "adaptive switch" that turned off migratory behavior, transforming the tropics from a cold-weather resort into a long-term residence for certain bird species.

Of the 29 long-distance migrant species examined in the study, 20 likely saw their northern breeding grounds become uninhabitable, according to models developed by the researchers. When the climate again warmed and glaciers retreated back to the Arctic, those species presumably resumed their seasonal migrations.

Lead author Robert Zink said the conclusions could alter how scientists reconstruct the history of bird migration.

"It fundamentally changes the way we study the evolution of migration and think about the migratory behavior of birds," said Zink, professor of natural resources and biological sciences at Nebraska.

Putting Migration On Ice

Researchers generally agree that, millions of years ago, many birds did not migrate from the tropics. But as the global climate began to warm, some species ventured beyond their native habitats to capitalize on better breeding and feeding opportunities afforded by the longer days and insect-rich environments of northern latitudes.

Those species eventually ventured farther and farther from their habitats, finally stopping when they reached environments that could not sustain them during the autumn and winter. They continued to migrate south when seasonal temperatures dropped and food sources waned.

In that context, Zink said his hypothesis suggests that the origin story of bird migration simply underwent multiple reboots, with the "migratory machinery" of birds halting for each of the 20 or so ice ages that have glazed Earth during the past 2.5 million years.

The University of Nebraska-Lincoln's Robert Zink has authored a new study suggesting that the last ice age 
completely halted the northerly migrations of some bird species from about 21,000 to 12,000 years ago 
[Credit: Craig Chandler, University of Nebraska-Lincoln]

"Migrations are costly and risky," said Zink, curator of zoology at the University of Nebraska State Museum. "They're costly in terms of safety, energy -- anything you can think of."

Rather than paying those costs to reach breeding grounds that the encroaching glaciers had shrunk to tiny fractions of their former size, he said, birds instead resorted to their ancestral state: tropical homebodies.

"Some of them were forced so far south that it was no longer a fitness advantage to migrate, because the extra young they could produce south of the glacier wasn't enough to compensate for the cost of migration," Zink said, "and then coming back to the tropics and re-establishing their territory.

"To some people, that's so completely off the wall that they may have trouble wrapping their heads around it -- except that it's the way they would explain to their classes the evolution of migration in the first place. So, in a sense, what I'm proposing is nothing novel. What's novel about it is that (the advent of migration) probably occurred many times."

Redrawing the Map

Zink and his co-author, the University of Minnesota's Aubrey Gardner, conducted their study using a computer model that linked the modern-day distribution of bird species with climate variables -- temperature, precipitation, seasonality -- that characterize their habitats. By comparing those climates with conditions that existed during the last ice age, the model mapped the regions that likely could have supported each of those species from about 21,000 to 12,000 years ago.

In many cases, Zink said, the model either found no habitable regions beyond the tropics or located habitats so miniscule that they would have struggled to support sizable populations of the species.

"Some species were probably just forced (slightly) south of the glaciers, and their habitats were extensive enough that they would maybe maintain some migratory ability," he said. "But for others, I think there was so little predicted habitat that they just ceased migration all together.

"This evolution of migration is a very (variable) thing. Normally, when we think of evolution, we think of singular, unique events in evolutionary history. But in this case, the ability to migrate is entrenched in birds. They have the ability to navigate using the sun, the stars, the (Earth's) magnetic field. They have the ability to put on large amounts of fat and sustain trans-gulf migrations. Birds are (adaptive) enough in their behavior and physiology that this wasn't a reinvention of some incredible phenomenon."

And if some species did transition back and forth from sedentary to migratory states, researchers should consider pruning certain evolutionary trees accordingly, Zink said. Many evolutionary trees currently treat migration as an irreversible trait rather than a variable behavior, he said, and that assumption could be misinforming discussions of when and where it evolved.

"I wanted to point out that this was a real danger and fallacy that's being committed: mapping something onto an evolutionary tree where the feature -- migration or sedentariness -- changes faster than new species evolved," he said. "You would have constructed the history of migration totally differently."

Author: Scott Schrage | Source: University of Nebraska-Lincoln [September 20, 2017]

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Scientists track the brain - skull transition from dinosaurs to birds

The dramatic, dinosaur-to-bird transition that occurred in reptiles millions of years ago was accompanied by profound changes in the skull roof of those animals -- and holds important clues about the way the skull forms in response to changes in the brain -- according to a new study.

These are CT scan images of the skull roof (front bone in pink, parietal in green) and 
brain (in blue) of, top to bottom, a chicken, the birdlike dinosaur Zanabazar, the primitive 
dinosaur Herrerasaurus, and Proterosuchus, an ancestral form that diverged before 
the bird/crocodile split [Credit: Yale University]

It is the first time scientists have tracked the link between the brain's development and the roofing bones of the skull. The findings appear in the journal Nature Ecology and Evolution.

"Across the dinosaur-bird transition, the skull transforms enormously and the brain enlarges. We were surprised that no one had directly addressed the idea that the underlying parts of the brain -- the forebrain and midbrain -- are correlated or somehow developmentally related to the overlying frontal and parietal bones," said co-senior author Bhart-Anjan Singh Bhullar, an assistant professor of geology and geophysics at Yale University and assistant curator of vertebrate paleontology and vertebrate zoology at the Yale Peabody Museum of Natural History. Matteo Fabbri, a graduate student in Bhullar's lab, is the first author of the study.

The developing skulls of an alligator (top) and a chicken (bottom) 
[Credit: Fabbri et al., 2017]

Although previous studies have shown a general relationship between the brain and skull, associations between specific regions of the brain and individual elements of the skull roof have remained unclear. This has led to conflicting theories on some aspects of skull development.

Bhullar and his colleagues set out to trace the evolution of brain and skull shape not simply in the dinosaurs closest to birds, but in the entire lineage leading from reptiles to birds. They discovered that most reptile brains and skulls were markedly similar to each other. It was the dinosaurs most closely related to birds, as well as birds themselves, that were divergent, with enlarged brains and skulls ballooning out around them.

The skull bones and brain shape of an American alligator and a chicken 
[Credit: Fabbri et al., 2017]

"We found a clear relationship between the frontal bones and forebrain and the parietal bones and midbrain," Bhullar said. The researchers confirmed this finding by looking at embryos of lizards, alligators, and birds using a new contrast-stained CT scanning technique.

"We suggest that this relationship is found across all vertebrates with bony skulls and indicates a deep developmental relationship between the brain and the skull roof," Bhullar said. "What this implies is that the brain produces molecular signals that instruct the skeleton to form around it, although we understand relatively little about the precise nature of that patterning."

Bhullar added: "Ultimately, one of the important messages here is that evolution is simpler and more elegant than it seems. Multiple seemingly disparate changes -- for instance to the brain and skull -- could actually have one underlying cause and represent only a single, manifold transformation."

Author: Jim Shelton | Source: Yale University [September 11, 2017]

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