Hypothetical on returning Martins

[SMJohnson]

For those who may have an understanding of what research has shown. What percentage of birds at a given colony would you expect to have been there the previous year? Consider a 10 year old colony in upper Midwest with 60 pair. Next year colony has 64 pair. Of those 128 birds. How many on average might have been there the previous year?

[Craig Dyer]

Great question! I'll be waiting for the response from someone much smarter than me.

[Ed Svetich-WI]

50% mortality of ASY martins is expected. About 10% of last year's fledglings may return to their natal colony. As few as 25-30% of fledglings will live until the next season. The loss of that many adult martins is the reason that a colony has a better chance of remaining viable once it reaches at least 12 successful nesting pair(24 birds). If only 12 return, you can understand why small colonies are challenged to remain successful.

This makes it critically important to do everything possible to make a colony produce successful clutches. Successful parents will return and last year's number of fledglings will determine how many of those will return "home" as that possible 10%.

Good luck in 2018

Ed

[Craig Dyer]

Ed,
Do we know why ASY mortality rates are so high?

[Ed Svetich-WI]

It is a cruel world out there. Predators of various types, disease, ingested poisons from insects, wind farms, old age, storms along the migration route, much of it over open water, loss of habitat here and in South America, misguided attempts to harass martins at roosts both here and in South America are some threats that come to mind. I am sure that similar threats occur with most migrating birds.

Females are at greater risk for mortality because they spend more time on the ground during the nesting season collecting nesting materials as well as addition calcium as egg laying approaches here in the US.

There may be other causes that I missed.

Ed

[paule]

50% mortality of ASY martins is expected. About 10% of last year's fledglings may return to their natal colony. As few as 25-30% of fledglings will live until the next season. The loss of that many adult martins is the reason that a colony has a better chance of remaining viable once it reaches at least 12 successful nesting pair(24 birds). If only 12 return, you can understand why small colonies are challenged to remain successful.

This makes it critically important to do everything possible to make a colony produce successful clutches. Successful parents will return and last year's number of fledglings will determine how many of those will return "home" as that possible 10%.

Good luck in 2018

Ed

Ed,
You always cover everything almost to perfection. I used to tell people I am trying to shoot for at least 15 large cavities to help guarantee a successful colony next year. I also explained I thought that 1 out of 4 fledglings survive. Always got strange looks. I like your explanations better. Thank you for your time and expertise.
Paul

[Lizzie]

Ed,
Do we know why ASY mortality rates are so high?

I suspect that's fairly normal among several species of birds.
I have a pair of red tail hawks that nest here, and they hatch 1-2 babies per year. Last summer, I managed to find their nest and watched as the baby grew. At that point, I started reading about hawks, curious as to what and where the little guy would likely end up, when I read that their mortality rate is very high. (iirc, 75% mortality rate is common)

[Dave Duit]

It basically comes down to a numbers game. All we can do as landlords is practice good management to boost the numbers.

[vman]

Ed, correct me if my math is wrong but if 50% of asy martins die and only 30% of the fledglings survive,and only10% of those return to their native colony how can any colony grow?
Jim

[Ed Svetich-WI]

Good question. That is the challenge. As an example, 24 pair fledge an average of 4 young means that 48 adults and 96 young means that 144 leave in the Fall. 24 adults survive the round trip to South America. 29 fledglings survive to return. The total from this colony returning in the Spring is 24 ASY and maybe up to 9 SY birds and that is from a well managed colony !

All a bird has to do to assure that it's genes survive is to replace itself at least once. That theoretically maintains the population. Any excess young is a bonus. Across many areas of the country, martins are just holding their own. I will not go into the many challenges that are beyond our control that can negatively impact martin production.Production is a strange way to look at our martin colonies but as someone else here said, "It is a numbers game".

How do you grow a colony? The first question is to determine how large a colony you can manage. If you find that the number you have is more than you can effectively manage with nest replacements, frequent monitoring and constant attention, that will impact "production". The second part of the equation covers everything else and encompasses the housing, racks, predator control and record keeping. Use the Martinwatch Nest Data Sheet or one of your own design. How many Data sheets are received by the PMCA every year?

RECORD KEEPING is the key to growing a colony. Every time I hear someone say that they have "about" 20 pair of martins or I fledged "about" 60 young, it is obvious that record keeping is absent or at best deficient. Recently, someone here on the Forum posted that they put up the housing and take it down in the Fall and they always raise a bunch if martins. Really? How do they know? How many eggs were laid? How many eggs hatched? Were any vermin present? Did any nestlings die in the nest? How many fledged? Just because the nests were empty means what?

In 2017, my 24 martin pair laid 132 eggs, hatched 128 and fledged 116. I know this because I manage my colony to PRODUCE martins.On 6/27, I found 3 dead young in a nest. I immediately checked all gourds and found 1 additional dead nestling. These nests were wet and were immediately replaced. All other nests were checked and several other nests were replaced. Had I just raised the gourds and lowered them in the Fall, what would I know??

How do you grow a colony? Have a colony that every other martin wants to live in. I leave 4 gourds plugged every Spring until the SY arrive. Last year I still had 23 pair of ASY and only 1 pair of SY. Where did the ASY come from if only 24 ASY individuals of mine from 2016 returned? They came from someone else's colony. Where do the rest of the SY that were produced at my colony go? Banding has shown that they go to other well managed colonies to grow them.

We are all in this together. Well managed, appropriately sized colonies grow the colonies of other martin enthusiasts.

Have a productive year.

Ed

[vman]

Thank you Ed,that data should be an encouragement to those who are trying to start a colony, we are often told that sy birds are the only martins that start new colonies, personally, the three different times we had colonies start up here on our farm they were always started by asy males, hoping for # 4 this year!
Jim

[Lizzie]

Thank you Ed,that data should be an encouragement to those who are trying to start a colony, we are often told that sy birds are the only martins that start new colonies, personally, the three different times we had colonies start up here on our farm they were always started by asy males, hoping for # 4 this year!
Jim

I suspect that last year's pair was also an already-mated pair, who had lost their cavity for some reason last season (probably bad storms, if I were to guess) , and they arrived here relatively late in the season, then nested again.

[Dale D]

Numbers just don't add up. I have been giving some serious thought to Purple Martin Colony statistics and something just isn't right in my mind.

I think we can all probably agree that according to growth statistics a colony in 2017 of 100 pairs or 200 Martin's could grow to 120 pair or 240 Martin's next year, right? Well if you go by what we believe are the correct % rates of survival then something doesn't add up.

Statistics say:

50% of Adults will make the round trip to South America and back.
30% of fledglings will survive the trip.
10% of the fledglings will return to their natal site.

Now using that same colony of 100 pairs lets say they produced 500 fledglings in 2017. Of the 500 fledglings only (30%) or 150 will survive the trip and of the 150 only 15 or (10%) will return to their Natal site. So based on these numbers below is what a Landlord could expect back in 2018.

100 adults will return in 2018
30 of the sites fledglings will return in 2018

Total returns would for 2018 would be 130 Martins or 65 pairs which means this colony would have to pick up an additional 55 pairs or 110 of SY's from other colonies to increase to the estimated gain of 20 pairs to increase from 100 pairs in 2017 to go to 120 pairs in 2018. I don't buy into that. I believe the mortality rate of adults has to be higher than the believed 50%.

Keep in mind that all the numbers above would have to be based on 50/50 split of male to female Martins which is also highly unlikely.

[randyM]

Quite the interesting discussion. I like to look at things from a slightly different angle - the overall breeding population. According to Partners In Flight and The American Bird Conservancy there are estimated to be 7 million breeding purple martins in the world, which indicates roughly 3.5 million breeding pairs. According to the most recent PMCA Project Martin Watch summary, over the past 23 years of data that was analyzed, 328,041 young fledged from 97,620 nests. So, that means an average of 3.36 martins fledged per nest over the past 23 years. If we multiply this number by the 3.5 million breeding pairs, that equates to 11.76 million young fledged each year. If 30% of those young survive until the following breeding season that means about 3.528 million SY birds are added to the population the following year. If 50% of ASY birds survive each year, that means 3.5 million ASY birds make it back north to breed. The 3.528 million SY birds will replace the 3.5M ASY birds that perished since last breeding season, which equates to adding 28,000 overall birds to the breeding population (an increase to the breeding population of 0.4%). Given that, we should expect the growth of an existing colony to also expand the next year by 0.4%. So, for a colony of 250 pairs it hypothetically would be expected to grow by 1 SY pair the following year (i.e., 250 x 0.004 = 1). If we assume the number of fledglings making it back for the next breeding season to be at the lower value of the survival range at 25%, and 50% of the ASY birds making it back, the overall breeding population would decrease by 560,000 individuals (-8.0%) and that same colony of 250 pairs should hypothetically decrease by 20 pairs. Using the same ASY population survival rate and # of young fledged per nest that are listed above, if 29.762% of the young fledged in the previous year made the trip back north to the breeding grounds, the current year's breeding population would remain unchanged from the previous year and the colony of 250 pairs would hypothetically remain stable. One should take into consideration that in any given year the average # of young fledged and/or the annual survival rate of ASY or SY may be higher or lower than the averages used in the above calculations, thus resulting in some years where there is an overall population increase and some years with an overall population decrease.

So, getting back to the conversation of what to hypothetically expect for growth and composition of individual birds of a given colony in any given year...as other's have stated in this thread, "it's a numbers game", and these numbers are constantly changing from year to year and locality to locality. The overall statistics used to describe average annual survival rates, average # of young fledged, percent of young returning to their natal colony, etc. are values that reflect the mean of the extreme values of these categories across space and time. So it is virtually impossible to know what to expect for a specific colony's growth and individual composition of it's members until it actually happens at your specific colony...each year is unique, as is each individual colony. There area just too many variables regarding an individual colony and population dynamics within individual years to know what to expect for individual colonies from year to year. These unknowns are what makes this passion that we have for purple martins so intriguing, interesting and rewarding and keeps us coming back each year for more!

[lynnh]

Thanks Dale; Randy this thread has been interesting to say the least !And you too SM Johnson for starting this thread;some good stuff!!!

[handyman315]

Thank you Ed,that data should be an encouragement to those who are trying to start a colony, we are often told that sy birds are the only martins that start new colonies, personally, the three different times we had colonies start up here on our farm they were always started by asy males, hoping for # 4 this year!
Jim

Exactly how my new colony started, i.e., by two ASY-Ms in particular.

The numbers discussion was very interesting, although my eyes glazed over a couple times.

It seems to me that darned few landlords can look at these numbers from the macro perspective, i.e., our micro perspective is OUR colony, what WE experience, what WE count, and that undoubtedly contains quite a bit of PM "moving around". Yes, I understand site fidelity, but throw in some catastrophic losses of colonies, landlords forced to move away, and landlords unable to continue managing their colonies because of death, sickness, and/or infirmities, and what we experience may be markedly different than the "norm".

Lemme put it another way, as a newbie landlord of a tiny colony, if my site only grows by one bird in 2018, I plan to take a very mature approach - I will sit down and cry! JUST JOKING! I love my little band of intrepid travelers and recognized the anchor male right off from being here the year before. I've always said, gimme one nesting pair and I'll still be happy. Nothing has changed that!

[Dale D]

Numbers just don't add up. I have been giving some serious thought to Purple Martin Colony statistics and something just isn't right in my mind.

I think we can all probably agree that according to growth statistics a colony in 2017 of 100 pairs or 200 Martin's could grow to 120 pair or 240 Martin's next year, right? Well if you go by what we believe are the correct % rates of survival then something doesn't add up.

Statistics say:

50% of Adults will make the round trip to South America and back.
30% of fledglings will survive the trip.
10% of the fledglings will return to their natal site.

Now using that same colony of 100 pairs lets say they produced 500 fledglings in 2017. Of the 500 fledglings only (30%) or 150 will survive the trip and of the 150 only 15 or (10%) will return to their Natal site. So based on these numbers below is what a Landlord could expect back in 2018.

100 adults will return in 2018
15 of the sites fledglings will return in 2018

Total returns would for 2018 would be 115 Martins or 58 pairs which means this colony would have to pick up an additional 62 pairs or 124 of SY's from other colonies to increase to the estimated gain of 20 pairs to increase from 100 pairs in 2017 to go to 120 pairs in 2018. I don't buy into that. I believe the mortality rate of adults has to be higher than the believed 50%.

Keep in mind that all the numbers above would have to be based on 50/50 split of male to female Martins which is also highly unlikely.

[tboydshirt]

here's a thought on that. i read somewhere (forum or in the book, "a purple martin journey") that according to banding studies, most martins change mates regularly. most animal species exhibiting sexual dimorphism (males looking different than females) will not mate for life. each year the surviving 50% (of 200 birds) may well return with a new mate bringing numbers back to 200. thus 200 birds plus a few offspring returns makes the math work. .
overall, the grand total of living birds is going to be stable year to year if 50% of adults plus 33% of fledglings from a nest average of 6 survive.

[Doug Martin - PA]

In my personal experience it seems most my ASY birds return. I am not sure of the percentage nor would there be any way to account for it unless they were all tagged and marked.

However I can almost guess what birds will come in next at my site. The one to the front of the T-14 should be along any day now.

Although I have no proof or scientific data I would estimate closer to a 80% return most years. Does the math work for that?

Last year however I lost many of my oldest birds. Indeed the old 8 gourd rack probably only had 50% return in my estimation. The rest of the gang seemed to make it back however. When you are out there feeding them on a daily basis and watching them arrive daily in the spring you can't help but to begin to recognize many of them. They sure all know the routine for supplemental feeding too. They seem to go right back to their same cavities for the most part based generally on their age. When this changes you sure can tell by the vicious fighting than ensues when birds come back to find their cavity taken.

Doug

[flyin-lowe]

Another easy way to think about it is that you being a responsible land lord helps build other colonies and hopefully others near by you have a good colony which will keep you numbers growing.