Fred Kaluza wrote:Mary, I'm confused...on page 5 of 6 in the first referenced .pdf, the PMCA says they've learned some things...regarding site fidelity, "Adult and subadult breeders have strong site-fidelity and about half of them will return to their previous colony site the following season. How do we get to "0% to 13%"?
The 50% return rate is for birds with breeding experience - those who bred as ASY and SY birds return to same breeding site the next season as ASY birds. The 0-13% return rate is for hatchlings who choose their natal site to be their breeding site for their SY year. I would expect these rates could be measured pretty reliably with banding techniques.
Fred Kaluza wrote:
And then, "The other half presumably died or breed elsewhere". Further down on First-year mortality they say they've recorded only a 15% to 30% first year survival rate.
Throughout the years, I believe the PMCA literature has pretty consistently estimated a 75% annual mortality rate for the first year of martin life and a 50% annual mortality rate after that. The second year of martin life has an especially high mortality rate because of a parasite problem, according to martin researcher Dr. Eugene Morton - see http://www.purplemartin.org/forumarchiv ... inHogs.htm
So, the mortality rate for ASY birds should be less than 50%. I've read somewhere that 3-5 year old martins have a very good survival rate, comparatively. So, return rate of ASY birds depends very much on the age of your colony and how successful your prior breeding season was.
However, I doubt there is any way to measure mortality rates reliably with banding techniques. So, researchers must be using some sort of statistical magic, possibly Breeding Bird Study results or other bird survey. PMCA always states that their published mortality rates are estimates.
Fred Kaluza wrote:
Again, let's run through the numbers...
If a typical landlord has 24 breeding pairs that produce maybe 100 successful fledges and I use the median value for first year survival, that leaves 22.5% or 23 survivors. If half of these birds then return to their natal site, that leaves about 6 surviving pairs to go seek NEW accomodations or more likely go to some other nearby existing colony. Perhaps the "0% to 13%" figure is already taking into consideration the first year motality losses. Yup, if I started with 100 fledges and 13% came back to where they were born, that would be the 12 or 13 I would expect. Perhaps it's more clearly stated that "Of all surviving first year birds, half will go back to their natal site." I had no idea the mortality rate was so high. So, again to do the math, in a typical "super-colony" with 100 breeding pairs I think it would go like this...500 fledges become 113 survivors of which 56 go back to where they were born and the other 28 pairs go out to form new colonies OR more likely if room permits, fall in with another nearby established colony. We need more Martins!
The 0-13% figure is percent of total fledged. So if you fledge 100 birds, expect at most 13 to return. As recall, it was only in Andy Troyer's colony that the high (13%) return rate was recorded and that was in a year when he had lot's of available housing. A more typical number is probably less than 5%.
Fred, your calculated return rate of last year hatchlings to their natal colony as SY breeders sounds way too high. Also, your fledge rate is too high.
Let me use my colony as an example, instead:
Last season 47 pairs fledged 195 young which is 4 fledglings per pair and is higher than the average. (Last season PMCA's 90 pairs fledged 131 young, less than 1.5 fledglings per pair, according to their season report in the Fall 2004 Update.)
I rarely add housing, so my site usually fills up early with ASY birds. If roughly 50% of my breeders from last season have survived, then where do all the ASY birds come from each season? My best guess is that they have abandoned other sites where they had breeding failure the prior season. So, my site is an ASY "sponge" - most of my breeding martins have come to me after abandoning another site. They are very successful here and I believe that most of them return year after year until they die.
I get relatively few SY breeders here and can easily believe that only 2, 3, or 4 of my fledglings from the prior season claim gourds here. Some SY females fill in vacancies here in late May and early June. I usually have only 1 or 2 SY males in my colony. Only a few banded birds have ever bred in my colony and those I've identified were SYs from natal colonies in Indiana. So, I know that some SYs that breed in my colony are from other sites as far as 140 miles away. So, my site is NOT an SY "sponge" - I get few SYs and likely very few that were born here.
Using the 75% mortality rate, then of my 195 fledglings from last season only 49 will survive their first full year. Probably more than that return to the U.S. as SY birds - maybe 60 or 70?. Maybe 2-4 of them are in my colony right now. Possibly 55 - 65 of them are in Michigan attempting their first season of breeding at someone else's colony. This is all speculation, wildly inaccurate.
Yes, its tough to increase the population of a bird that produces only one brood a season. Clearly the breeding martins in Michigan are not replacing themselves - they are dying faster than they are reproducing. That's what the Breeding Bird Survey (BBS) results tell us. Here's the latest population trend chart for Purple Martins in Michigan:
http://www.mbr-pwrc.usgs.gov/cgi-bin/pl ... /06110.mic
The BBS is now counting about half the martins that they were counting in Michigan when I started my colony back in the early 1990's. I can easily believe that my area has lost half its martins during my years as a landlord. I also believe that without my years of extreme colony management, including very aggressive supplemental feeding, martins would have virtually vanished from my immediate neighborhood. That's the only good news - a single landlord can sustain a martin population in his/her neighborhood.