Re: Phorusrhacids killing large mammals in National Geographic Channel

[Augusto Haro <augustoharo@gmail.com>]

Ronald Orenstein send me this important information which he could not
send in plain text:

On behalf of Ronald Orenstein:

To my eye, the bird with the most similar beak to a phorhusracid today
is Steller's Sea Eagle (Haliaeetus pelagicus); see
http://animals.nationalgeographic.com/animals/birds/stellers-eagle.html.
  For a paper on this bird's diet see
http://www.wbsj.org/nature/kisyou/eagle/pdf/diet.pdf (but it says
nothing about its beak).  The abstract reads:

Qualitative data on the diet of adult and young Stellerâs Sea Eagles
Haliaeetus pelagicus in North Okhotia during spring (incubation
period) and summer (chick rearing period) have been analyzed. The
total of 177 prey samples containing 551 prey items from nests located
on rivers, seacoast and on islands with large sea bird colonies were
analyzed. The diet of Stellerâs Sea Eagles consists (in descending
order of importance) of birds, ïsh, mammals, and carrion. Birds
dominate the diet of the coastal pairs (73%, N = 107), especially in
the pairs breeding at the sea bird colonies (91%, N = 211). The
proportion of birds in the diet of eagles nesting on rivers is much
lower (11%, N = 38). In summer ïsh is a dominant component of diet
only in riparian pairs (77%, N = 78). In coastal pairs, as well as in
pairs at the seabird colonies the proportion of ïsh was lower: 26% and
7% (N = 28 and 19) respectively. Carrion is very important for
Stellerâs Sea Eagles in spring. In nests along rivers 83% (N = 6) of
prey in spring is carrion from traps set by trappers. In spring the
eagles occupying riparian nest sites consume mostly mammals and
carrion, whereas on the coast eagles feed on birds. In summer the
riparian pairs switch to ïsh, whereas coastal nesting pairs consumed
mostly birds, although the ïsh component increased also. The
composition of the diet of chicks was dramatically different between
habitats. Chicks reared in riparian nests have ïsh-oriented diet,
whereas chicks reared in coastal nests eat mostly birds. At sea bird
colonies the Stellerâs Sea Eagles selected species that were extremely
abundant and were relatively less manoeuvrable in ïight.

However, a paper from the same symposium (The morphology of the bill
apparatus in the Stellerâs Sea Eagle, by Alexander Ladyguin) examines
the bird's skull morphology and illustrates its skull; see
https://www.wbsj.org/nature/kisyou/eagle/pdf/morphology.pdf.  The
author notes:

"...The upper jaw in Stellerâs eagle is extraordinarily massive when
compared to those of other sea eagles, its depth accounting greatly
for the massive appearance.

"Birds jaws are powerful âtoolsâ used for feeding, especially in the
birds of prey. The Stellerâs eagleâs strong, very curved bill is the
perfect implement for food ripping and tearing large carcasses into
small pieces that are easy to swallow. The main food of Stellerâs
eagle are large ïsh, sometimes weighing about 6-7 kilograms, similar
to the eagleâs own mass. In the Bering and Ochotskoe seas the main ïsh
species upon which they feed are the anadromous salmon. Fish skin is
tough and difficult to tear. But ïeld observations suggest that
Stellerâs Sea Eagles can consume about 900 g of ïsh in 3-4 minutes. In
comparison, White-tailed Sea Eagle feeding at the same locations spend
about 18 minutes to consume the same amount, and Golden Eagle requires
28 minutes (Ladygin 1994, 1996)."

'Food ripping" involves holding food down with the foot and using the
beak to tear off chunks of flesh.  The author suggests that rapid food
handling may be advantageous in competing for food with other eagles
at feeding aggregations.  If this is in any way analogous to
phorhusracids, perhaps these birds used their beaks not so much for
killing but for ripping already-killed items (or scavenged carcasses)
to pieces, and if the birds tended to squabble over carcasses the bird
with the best ability to tear chunks of food away in a hurry may be at
an advantage.


Ronald Orenstein
1825 Shady Creek Court
Mississauga, ON L5L 3W2
Canada