Coloniality Are the robins we see in our front yard every year the same robins? They could very well be the same individuals



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Coloniality

  • Are the robins we see in our front yard every year the same robins? They could very well be the same individuals

  • Birds often show site fidelity, or philopatry, returning to the same territory year after year to breed or feed

  • 74% of the robins banded in a nation-wide study by Hickey (1943) were found within 16 kilometers of their capture point

  • 113 female Mallard Ducks (Anas platyrhynchos) banded in artificial nest baskets in the North Dakota marshes (Doty and Lee 1974)

  • Nearly half returned to their natal marsh, and two/thirds of those returning went back to the exact same nest basket

  • Of birds returning to nest, 52% had successfully nested earlier

  • Only 16% of birds who had failed to nest there ever returned

  • Colonial waterbirds also show strong site fidelity in the location of their colonial breeding grounds

  • Previous nesting success, which is directly related to local food abundance, seems to be the ultimate cue for site fidelity

  • Good evidence that at least some species of their dinosaurian ancestors were also colonial breeders

  • Showed strong site fidelity in their choice of a communal breeding ground

  • Fossils of extinct animals tell us many things about the size and shape and ecological niche of extinct animals

  • Most fossils are “hard parts” – there is a preservation bias in the fossil record

  • But the tracks and traces these animals left behind tell a far more interesting story

  • They can even tell us how extinct animals behaved, if we are smart enough to ask the right questions

  • Study of fossil tracks and traces = ichnology

  • Ichnologists spend their time studying

  • footprints, and trackways

  • burrows and marks of invertebrates in the soil

  • fossilized dung (coprolites)

  • gastroliths (stomach stones)

  • Trackways are especially informative

  • Provide data on speed, gait, behavior

  • Distance between each footprint, and the distance between alternate prints of the same foot, lets us estimate:

  • height

  • stride length

  • speed

  • normal gait (walking, trotting, running…)

  • Trackways also reveal predator/prey interactions, and social behavior such as herding

  • We know that at least some dinosaurs travelled in herds, with young in the center, adults surrounding them

  • Coprolites help us to reconstruct diets of dinosaurs

  • Gastroliths tell us that dinosaurs swallowed large stones to help them grind their food

  • Same as birds do today, adding bits of sand and gravel (grit) to their crop

  • Ichnologists also study the eggs and nests of ancient reptiles

  • John Horner’s brilliant work in the Badlands of northern Montana tells a remarkable story (Digging Dinosaurs)

  • Dinosaurs behaved very much like modern colonial waterbirds

  • Site was a communal nesting ground for large herds of a new species of the herbivorous duck-billed dinosaurs

  • Maiasaura, which means “good mommy lizard”

  • Dinosaurs that lived 80 million years ago nested very much like gulls, terns, herons, egrets, and ibises today

  • For one thing, they returned to the same location year after year to nest

  • Horner burrowed down through several layers representing nests built atop nests

  • Dinosaurs showed site fidelity

  • Location modern birds select for nesting colonies is usually sheltered or protected

  • Find modern heron colonies in the middle of deep swamps, where ground predators can’t get at them

  • Except for snakes, the major nest predators of modern wading birds

  • Horner's dinosaur nesting sites were both on small islands in a shallow alkaline lake, Egg Mountain and Egg Island

  • The water was shallow enough to be easily crossed by adult dinosaurs, but deep enough to discourage some terrestrial predators

  • The spacing of the individual dinosaur nests is very regular, like that of modern waterbirds

  • Each nest is positioned just far enough away from other nests, out of reach of aggressive nesting neighbors

  • Condition of the eggshells in the nest also tell us that these dinosaurs were very similar in their behavior to modern birds

  • Some birds have young that are born “ready to party”

  • Certain types of ducks hatch out of the egg, hop out of the nest, and plunk into the water to swim away after mama

  • Precocial strategy reduces the time the young are exposed to nest predators

  • Other birds, like robins, or herons, are born naked and defenseless

  • Must be constantly fed and fussed over for several days or weeks before they are ready to leave the nest

  • Altricial strategy increases the amount of time the young are exposed to predators while in the nest

  • Puts a big energetic burden on the parents, but when the young finally fledge they are fully equipped and trained to survive

  • One nest held the remains of 15 baby dinosaurs, each ~3 feet long

  • Lots of eggshell fragments

  • Baby’s teeth were worn, indicating they had been feeding for some time

  • This suggested that the baby dinosaurs must have spent some time in the nest, being cared for by their parents

  • They were altricial

  • He also examined nests of a different species, a small hypsilophodont dinosaur

  • Horner found that the top parts of the eggs were broken into fragments, but the bottoms of the shells were intact

  • These newborn dinosaurs must have immediately left the nest

  • They were precocial

  • These amazing similarities between Maiasaura colonies and colonies of modern colonial wading birds suggest that coloniality is an effective evolutionary strategy

  • Many species of birds that flock together to feed and roost are also colonial breeders

  • Joanna Burger (1981) defines a colony as a group of birds nesting in close proximity, regularly interacting with one another, and feeding outside the breeding territory

  • Lack (1968) estimated that only 13% of all birds were colonial

  • Most of these colonial species are waterbirds

  • 98% of marine species are colonial - limited nest sites, patchy food resources

  • 26% of species that feed in flocks also nest in colonies

  • Only 1% of solitary feeding species are colonial nesters

  • Disadvantages of coloniality

  • colonies attract more predators

  • more competition for nest sites and materials,

  • more competition for mates

  • more time spent mate guarding once pair bonds are established

  • more competition for food

  • The evolution of coloniality must represent a tradeoff between advantages and disadvantages

  • Some of the advantages of flocking also apply to colonies, such as increased predator vigilance, and the potential role of the colony as an information center

  • Another advantage of coloniality is that colonies provide more potential mates

  • Weaverbird colonies with less than 10 males attract significantly fewer mates than do males in larger colonies

  • Colonies also decrease the risk of predation by predator swamping

  • Synchronize reproduction so that most young are hatched at the same time

  • Nisbet (1975) found that predators of Common Terns took the same biomass of prey regardless of colony size

  • A hundred-fold increase in colony biomass did not result in higher loss of biomass to predation

  • Synchronized breeding results from what Darling (1938) called social facilitation

  • The sights and sounds of nearby birds performing courtship displays or constructing nests seems to have a stimulating effect that brings other birds into breeding readiness

  • This social facilitation may have spelled doom for the extinct Passenger Pigeon

  • Once nested in colonies that numbered in the billions of birds, and stretched over several square miles

  • So numerous that in April 1873 in Saginaw MI a single flock took from 7:30 AM to 4:00 PM to pass overhead

  • Flocks were commonly observed over a mile wide during migration

  • Flocks were so dense that one shot could bring down 30-40 birds!

  • Their roosts covered up to 5x12 mi2, with 90 nests or more in a single tree

  • Branches frequently broke under the weight of the birds - droppings covered the ground several inches deep!

  • In a single hunt in 1896 the last surviving flock was destroyed

  • Over 200,000 birds were slaughtered and 40,000 mutilated and left for dead, with tens of thousands of chicks left to starve to death

  • By the time the slaughter of these birds was stopped, colonies were reduced to the point where birds were insufficiently stimulated to breed together, and may have been picked off individually by predators

  • Coloniality probably evolved independently in several taxa

  • Siegel-Causey and Kharitonov (1990) proposed several stages in the evolution of coloniality

  • Birds are solitary nesters, foraging in their breeding areas

  • Birds move to habitats inaccessible to terrestrial predators, relatively few places for murres, kittiwakes, penguins…

  • True colonies form when resources become variable and unpredictable

  • Unpredictable resources are the most likely cause for the evolution of coloniality

  • Coloniality is an adaptation for exploiting resources which are ephemeral (temporally unpredictable), or scattered in patches (spatially unpredictable) Ward (1965), Ward and Zahavi (1973)

  • Erwin (1977, 1978) observed that when resources were temporally and spatially unpredictable, wading birds responded by forming larger colonies




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