I'm not sure a conclusion can be drawn concerning the differences between Mountain Bluebird and Western Bluebird survival rates. There were likely several thousand Mountain Bluebirds present and perhaps 200-300 Western Bluebirds present (perhaps 15% of MOBL). Birders do not usually see/find all birds that die. Birders just may not have found the dead Western Bluebirds as they were in much smaller numbers (and are not as brightly colored...which helps birders find them).
I also noticed that Mountain Bluebirds were "sheltered" in fairly obvious, open ended places (next to a retaining wall, or next to a restroom). These places are fairly exposed to the elements (and visible to birders). IF (big 'if') Western Bluebirds better shelter themselves, they MAY have higher survival rates and/or not be easily seen by birders if they die. Lots of factors involved and lots of speculation can be made...very little in the way of answers. :-) Joey Kellner Littleton, CO On Apr 20, 11:23 am, "Mitchell, Christina" <[email protected]> wrote: > Hello, list- > > A portion of Ira Sander's message on Saturday about birding at Chatfield > caught my interest. > > <snip> > There were bluebirds almost everywhere with the biggest concentration of > Western Bluebirds east of the marina... > > As Tammy was rescuing wet and cold Mountain Bluebirds that were trying to get > out of the rain and snow near the outhouse east of the marina... > > 2 of the bluebirds were already dead but she picked up 2 still living and we > took them to a rehabilitator in Lakewood. Unfortunately 1 had alread died by > the time we got there and we hope Shelby can save the one that was still > living. > <snip> > > I was interested in why the Mountain Bluebirds (MOBLs) might have succumbed > to the wet conditions more so than did the Western Bluebirds (WEBLs). (With > so many unknowns, it's all speculation, of course.) Since these species are > so closely related, it's hard to believe that their feather structures would > differ dramatically, resulting in differential weatherproofing. Perhaps the > MOBLs had been travelling further than the WEBLs had and arrived at Chatfield > in a more depleted condition. (We'll never know that one.) I believe MOBLs > are more dependent on insects than are WEBLs; Birds of North America Online > refer to MOBLs as more carnivorous than most thrushes. That write-up also > says that one (very old) study found 92% of the stomach contents to be > insects across the year. I couldn't find similar statistics about WEBL diet, > but in general it seems that they may more commonly eat fruit in addition to > insects. (We have a lot of MOBLs during breeding season on our property in > central CO; I can't recall ever seeing them eat the most obvious fruit > there-juniper berries-although the Townsend's Solitaires and American Robins > do in the winter.) So I'm wondering if perhaps the WEBLs were able to find > fruit to eat while the MOBLs, more dependent on insects, might not have been > able to find sufficient nourishment. > > Does anyone know more about relevant differences in these 2 species or have > other thoughts about the differential vulnerability? > > Tina Mitchell > Lakewood/Coaldale, CO --~--~---------~--~----~------------~-------~--~----~ Join us at the 2009 Convention in Alamosa: http://cfo-link.org/convention/index.php You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.as/group/cobirds?hl=en -~----------~----~----~----~------~----~------~--~---
