[CT Birds] Redpoll weight

[David.F.Provencher at dom.com]

Bob,

According to Birds of North America, captive redpolls consume about 5.1 g/d
(22.4 kcal) at 22°C and eat 31–42% of body mass each day, depending on
season (White and West 1977). This 31 to 42% of body mass was Alaskan birch
seed with a digestive efficiency of 70% (Brooks 1968). A different food
source would likely result in either higher or lower % of body mass per day
value depending on its calorie content and digestive efficiency. Presumably
wild birds should be using more energy than captive birds due to exposure
to lower temperatures and greater physical activity. Therefore they require
more food calories than captive birds and so they would eat at a little
higher % of body mass per day value.

So I would guess the 142% of body mass per day value you calculated is
probably too high based on the data I quoted above. There were likely some
other factors in play with your rate of seed use. Perhaps there were more
birds rotating to your nyjer feeder on a daily basis than your high count
would lead you to believe. Additionally some seed was almost certainly
knocked to the ground as the birds extracted it from the feeder. This could
be a significant loss and this dropped seed would then likely either be
taken by mammals such as voles, eaten by ground feeders such as Juncos, or
simply add to the compost around the feeder. The layer of composting
material around a feeder can be surprisingly deep and slowly and quite
inconspicuously raise the ground up in the area.

I have to say your curiosity about this does you credit. Rather
"un-layman-like" thinking I would say.

Dave




                                                                           
             "Robert J.                                                    
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             04/09/2008 05:51                                              
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Paul and Dave---thanks for the info.  1/2 oz does seem like nothing at all.
But they really are tiny birds.  So, based on my observations, an average
of 15 birds in the visiting flock adds up to less than 1/2 lb of birds
consuming 20 lbs of seed, or each 1/2 oz bird eating 1 1/3 pounds (though
they spill a lot too) in about two months.  If my math isn't screwy (it
often is), this seems to work out to 0.71 oz of seed per day per bird.
Which means each bird is eating 1.42 times its body weight per day at my
feeder alone (and I assume they're feeding elsewhere too).  The only other
factors are the few juncos and goldfinches that also take nyjer, but I
think that's a tiny portion of the consumed seed.  Not being a scientist I
have no way of evaluating this rate of consumption, but from my layman's
perspective it seems amazing.

Bob
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