[CT Birds] redpoll weight

[Robert J. Bitondi]

Dave has an excellent point.  I've had up to 22 redpolls earlier in the winter, now averaging about 10 in a flock.  But I have no idea whether they stay in the same group or come in separate and varying groups.  It's certainly possible I have more than I think coming here each day.

Steve's observation about hulls was pointed out to me by another alert reader, Sarah, who notices this when goldfinches feed at her nyjer feeder.  I told Sarah there wasn't this kind of mess below the particular feeder the redpolls use, but I spoke too soon.  I just looked (on my hands and knees) and apparently it lands farther down the slope the feeder is on and I haven't noticed how much there is, plus the juncos and sparrows are constantly kicking up the area directly under the feeder so it doesn't seem to build up as much.  Anyway, Dave's numbers for redpoll consumption might fit my observations if the hull weight---whatever it is---could be taken into account. Even subtracting a lot for hulls, and keeping in mind that it's very likely my redpolls are feeding on other things when not here, it's a lot of food for a 1/2 oz bird.

Thanks to all for your astute comments.

Bob



Message: 8
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

Date: Wed, 9 Apr 2008 17:16:20 -0700 (PDT)
From: Steve Morytko <smorytko at yahoo.com>
Subject: Re: [CT Birds] Redpoll weight
To: ctbirds at lists.ctbirding.org
Message-ID: <215717.86881.qm at web53503.mail.re2.yahoo.com>
Content-Type: text/plain; charset=us-ascii

I don't see where anyone accounted for the nyjer husk. Who is going to
 figure out how much the average husk weighs compared to the actual
 seed? Dave might have access to some sub-atomic scales ... :)

Steve
 
Steve Morytko