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The Altruistic Raven

Why is it that ravens share their food? When and why does this happen?

Kyla Norcross

on 25 May 2011

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Transcript of The Altruistic Raven

The Altruistic Raven
by Kyla Norcross Ravens (Corvus Corax) Quick Facts: Omnivorous- eat carrion, insects, small animals, berries, & food scrapes/waste Average weight 2.6 lbs
25 inches in length. Polyamorous, but after some time, more maturity, they do tend to nest with one partner only! BUT HOW DO THEY DIFFER FROM CROWS? Bigger beak! Larger neck! roosts larger than crows roosts Ravens soar more than crows and do arial acrobatics Highly intelligent and opportunistic! Study One: Researchers in Maine
Bernd Heinrich and John Marzluff Do single individuals or crowds of ravens usually find carcasses??
1,000 hours
Put out 25 carcasses Began to notice a pattern! A lone raven or two would fly by, notice carcass, but not eat. Fly away (sometimes going 30 miles away) and coming back at dawn with a flock of ravens from the same direction to feed! al·tru·ism noun \al-trü-i-zm\
Definition of ALTRUISM

: unselfish regard for or devotion to the welfare of others
: behavior by an animal that is not beneficial to or may be harmful to itself but that benefits others of its species Why?!?!?! Goes against Darwin's theory of reproductive fitness! Carcass hard to come by in winter.
Is it strategic? Built on a trust system, a raven finds food and shares with others, and it will be reciprocated? Foraging Cooperatively? The Raven's Yell Ravens call out to other ravens, alerting them to the carcass. Study #2
Which birds make up a crowd at a carcass?
Captured 463 ravens – marked them with colored/numbered wing tags and radio transmitters. What they found:
New birds came, old birds left on hourly basis.
500 total, up to 50 birds at a time at the food bonanza they put out for the birds.
Through DNA testing, they discovered that the birds not related! The Wanderers! 2 types! The Residents Residents eat in their domain- feast on carcasses locally!
Mostly adults.
Roost alone. Wanderers have no home (one bird that was marked was found 200 miles away!)
Mostly juveniles.
Form hoc gangs to overwelm adult residents at a carcass.
Roost together (20-60 birds) *side note: Ravens need other animals to prep their food. What does this mean? They are not propagating their own genes. Goes against kin fitness. They travel so far so is it likely they will meet the same ravens in order to have the reciprocal effect of sharing food? Study #3
To test the gang hypothesis...
Outdoor aviary – 7,100 Cubic meters
20 subadults in main house
2 sidearms to the house, one of which housed a pair of resident type adults captured from wild. Found that subadults eat more when in the presence of territorial adults – territorial adults pick on the more dominant subadults and this distraction allows subordinate subadults to eat. What causes the yelling behavior?
Function of hunger when a subadult see food. Resembles the yells young offspring use when their parents come near them....
Female bird makes to partner wanting food while sitting on the nestThe yell subordinates make that there is food to other subordinates.
Young birds, out of the nest, yell to tell parents where they are, also where food can be found.
In subadults, the call attracts other vagrants, not parents.
Social status affects yelling- more dominant raven suppresses yelling in other birds.
Lone subadult didn't yell when adults were near food. Instead, suadult made a begging call that attracted the rest of the subadults.
Begging calls reduce adult aggression, probably because an adult wants to limit the calling that recruits other subadults. The Ravens' Yell Part 2 Subadults roost at dusk a few miles from the carcass they are feeding on. Leave in mass at dawn to feed, some leave to possibly find other food for the gang. In evening the ravens come back from many directions to roost. Often roost with different groups. The size of the carcass seems to relate to the roosting size. For instance, if it’s a deer that will only be room for about 15 ravens, or a moose-40 ravens.. If too many, a lot of fighting for the carcass. Roosts up to 50 observed, but in US, where ravens eat insects and grain in open rangeland there can be roosts of 1,000!Feed for only a few hours but many come back days, weeks later Study #4 Watched 328 roosts -72 times they saw 3 to 103 ravens circle over a roost and then the rest rose from the roost and go off into the distance. Ravens Roosts! Ravens Soaring! Observed that when done with a carcass, instead of coming back to roost they would spend up to two hours circling, noisely, other ravens join them can go up to 2,000 feet in large “kettles”Kettles-noisy birds flying , diving and tumbling. Stop circling and fly off into the distance together in the same direction to a new location with a new carcass nearby. Kettles! Observed -Single bird discover a fresh carcass the return following dawn with 20+ Radio-tagged 10 birds- attempted to find them on a daily and nightly basis- found most of the birds ranging in a 1,000 square miles. Another experiment spread 10 carcasses linear distance of 30 miles- 9 of the 10 marked birds ate from these carcasses. Other studies- Altruism about serving the common good- combo of self-interest and serving common good leads to their success in finding resources for food. Other birds aren’t as successful at this, and don’t have this system. The End! The Residents & The Wanderers
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