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Why do Bald Eagles have 4 different plumages??? (1 Viewer)
- Thread starter TRMichels
- Start date
Farnboro John
Well-known member
Might as well ask why you don't just wear white shirts (assuming you don't).
John
John
As a wildlife reasercher myself (waterfowl, wild turkeys) I know that there is generally a "reason" for morphological diffierences, whether it be between sexes (refered to as dimorphism), or between age groups.
There is a reason. Generally the difference in coloration between juveniles and adults - is to show breeding readiness. Several gulls, and oceanic birds exhibit gradual changes (over the space of 2-4 years) between juvenile plumage and adult plumage.
But, the plumage of bald eagles goes from all dark in the first year bird - to a dark body, with a light belly, and the same area as the white head and neck of an adult being dark in the second year bird - and then a third year bird looks like a gradual change between the first year bird and the fourth year bird.
The second year plumage seems to be an anomally. But, there should be a reason for it.
I'm still watiting for answers from Cornell University, the National Eagle Center in Minnesota, and the Raptor Center in Minnesota.
Nightranger
Senior Moment
It is possible that there is not a single answer to this question because the common buzzard is similar in having two distinct plumage phases. In reality, the gradation between the dark and light phases is not always as distinct as ID description would suggest. To complicate the explanation even further, the phases do not seem to have a geographical bias that may suggest habitat or climate influences. The closest we can get is that this is likely to be a genetic insurance against climatic change given that both species have ranges throughout temperate latitudes. The famous peppered moth offers a simpler way of understanding how this process works but basically, the birds have natural variation within the species. The differences are benign when conditions are benign but almost certainly have an advantage under more extreme climatic swings. Interestingly, the genetic coding for plumage phases does not necessarily seem to have a strict dominant - recessive relationship so that if one phase was to die out in the physical sense, it may not be completely lost from the species.
Ian
Ian,
We may be talking about two different things here, but your points are well taken.
I am specifically referring to juvenile to adult plumage changes, in the Bald Eagle. It appears that the 2 year old plumage is not a "succession" from 1 year old to 3 year old, such as the 3 year old to four year old succession. I guess the question is why does a 2 year old have a white (or brown mottled with white) belly, given that what appears to be the logical progression would be for a two year old to have a brown belly, with some white mottling on a brown head.
I don't think this plumage change has anything to do with "vastly" (my word) differing genetics in Bald Eagles, such that it would lead to different colored "morphs" or "phases" as it does in Red-tailed Hawks for instance; which often have both a dark and light morph within the same subspecies.
I suspect that geogaphy does have an affect on Red-tailed Hawks, although one would think that the light phase of the Krider's subspecies would be more prevalent in northern areas (as opposed to southern areas), as in snowy areas; and that the dark phase of the Harlan's subspecies would be more prevalent in heavily forested areas (it does live in Alaska and Canada) where it would blend in with its surroundings.
It could be that light phases (in red-tails) reflect more light and heat in southern climes, and dark phases absorb more light in northern climes. We find that this is often true in mammals, where southern subspecies are often lighter in color than northern subspecies.
In mammals, many northern subspecies are larger than their southern counterparts, responding to what is called Bergmann's Law, which (if I remember corretly) basically states that the further north the subspecies is found, the larger it will (or can) be, because it has less heat loss per area of body size.
This is turning into an interesting conversation. I hope to hear from some eagle biologists soon.
God bless,
T.R.
We may be talking about two different things here, but your points are well taken.
I am specifically referring to juvenile to adult plumage changes, in the Bald Eagle. It appears that the 2 year old plumage is not a "succession" from 1 year old to 3 year old, such as the 3 year old to four year old succession. I guess the question is why does a 2 year old have a white (or brown mottled with white) belly, given that what appears to be the logical progression would be for a two year old to have a brown belly, with some white mottling on a brown head.
I don't think this plumage change has anything to do with "vastly" (my word) differing genetics in Bald Eagles, such that it would lead to different colored "morphs" or "phases" as it does in Red-tailed Hawks for instance; which often have both a dark and light morph within the same subspecies.
I suspect that geogaphy does have an affect on Red-tailed Hawks, although one would think that the light phase of the Krider's subspecies would be more prevalent in northern areas (as opposed to southern areas), as in snowy areas; and that the dark phase of the Harlan's subspecies would be more prevalent in heavily forested areas (it does live in Alaska and Canada) where it would blend in with its surroundings.
It could be that light phases (in red-tails) reflect more light and heat in southern climes, and dark phases absorb more light in northern climes. We find that this is often true in mammals, where southern subspecies are often lighter in color than northern subspecies.
In mammals, many northern subspecies are larger than their southern counterparts, responding to what is called Bergmann's Law, which (if I remember corretly) basically states that the further north the subspecies is found, the larger it will (or can) be, because it has less heat loss per area of body size.
This is turning into an interesting conversation. I hope to hear from some eagle biologists soon.
God bless,
T.R.
Nightranger
Senior Moment
Hi T R,
I deliberately mentioned the common buzzard because much the same situation exists with the juveniles. Indeed, standard bird guides struggle with the juvenile plumage phases to such an extent that they are reported as anything from ospreys, honey buzzards, golden eagles and juvenile white-tailed eagles by many less skilled birders. Dick Forsman's excellent book on European raptors goes some way to sorting out the confusion but even this does not give a complete representation. Once again, the phases are not necessarily linked to age or geographical distribution and this almost certainly means it is an insurance strategy (early snow?) shared by the entire species. In theory, first-year common buzzards should show more white plumage than older birds but this is only partly true because some birds are already significantly dark at fledging. Eurasian sparrowhawks also have juvenile plumage phases but these are more directly related to age so that you can age an individual within a few months up to around the third year (but only for a female, males are less easy to age at maturity and this usually occurs in the second year so that no juvenile plumage is visible by the second spring*). I am sure there is some geographical element to this somewhere and it would be interesting to look at the predominance of the various phases from different locations although I cannot recall seeing a study of this type.
* This is not an absolute relationship and a good photograph and appreciation of the timing can be crucial to be reasonably sure.
Ian
Again, interesting thoughts, hypothesis etc, which is why I posted it here, because I can get input from across the globe.
Now, if only our nations, leaders and politicians could be so cooperative and civil.
May God bless all of you during this Christmas season,
T.R.
I have not heard back from a single eagle "expert".
Now, if only our nations, leaders and politicians could be so cooperative and civil.
May God bless all of you during this Christmas season,
T.R.
I have not heard back from a single eagle "expert".
dantheman
Bah humbug
No eagle expert here, I'm afraid. . .
Could it be that there is no real reason? ie no evolutionary advantage is conferred and that just happens to be the way it is?
Or as Steveo alludes to it is an evolutionary advantage for 2nd year birds to have a more 'broken up' body pattern (like battleships etc) would that confer any advantage though in a predator feeding on fish or carrion though? And the juvenile is equally out of place as a plumage type (many birds can have juvenile plumages vastly different from their parents and also similar species to themselves), maybe that's equally as out of place. I'm assuming the feathers in a feather tract are coloured, rather than changing colour as they wear- ie the eagle moults all its feathers once a year.
Evolution is a weird and wonderful thing to get your head around sometimes, and yes you'd almost consider a creation scenario to be more plausible at times - maybe that's why!! (Or is that a taboo subject here on scientific BF land?
)
And assuming evolution again, maybe the gene switching light and dark is turned on in one year and off in the next in the genetic coding making up Bald Eagles- there isn't enough evol advantage for it be better one way or the other and their hasn't been enough of an advantage conferred for it to be any differnet yet. ie it's just chance they go light in 2nd year.
I'm assuming differrent genes comes into play as a different section of the chromosome gets actiavted in each subsequent year of the birds life before adulthood.
Why do birds have different phases anyway? surely they could attain full breeding plumage in the first year. I guess that is so they do not mistaken for breeders by their peers. And I guess they don't breed in their first summer because they have to grow experienced enough to hunt successfully and 'prove' that they will make it and be able to hunt for themselves and a family- an older bird is better otherwise the species could possibly die out. And since these 'intermediate plumages' confer little advantage in a fish eating species, they could be almost anything . . it's when it get sto adulthood the plumage counts (=female sexual selection? whatever)
Why do some birds eg gulls have 2, 3 or 4 year groups- the larger ones because they have to grow larger and more experienced I guess, but it does seem a little arbitrary. And I wonder if a new section of chromosome gets switched on each year. Obviously at some point it may have been copied from athe previous years, and suddenly, one time you had a 4 year gull where there was none before, and over time you saw this one gull spread out and speciate in time to what we have now.
Anyway, that's enough rambling and speculation for now . . . . .
Could it be that there is no real reason? ie no evolutionary advantage is conferred and that just happens to be the way it is?
Or as Steveo alludes to it is an evolutionary advantage for 2nd year birds to have a more 'broken up' body pattern (like battleships etc) would that confer any advantage though in a predator feeding on fish or carrion though? And the juvenile is equally out of place as a plumage type (many birds can have juvenile plumages vastly different from their parents and also similar species to themselves), maybe that's equally as out of place. I'm assuming the feathers in a feather tract are coloured, rather than changing colour as they wear- ie the eagle moults all its feathers once a year.
Evolution is a weird and wonderful thing to get your head around sometimes, and yes you'd almost consider a creation scenario to be more plausible at times - maybe that's why!! (Or is that a taboo subject here on scientific BF land?
)And assuming evolution again, maybe the gene switching light and dark is turned on in one year and off in the next in the genetic coding making up Bald Eagles- there isn't enough evol advantage for it be better one way or the other and their hasn't been enough of an advantage conferred for it to be any differnet yet. ie it's just chance they go light in 2nd year.
I'm assuming differrent genes comes into play as a different section of the chromosome gets actiavted in each subsequent year of the birds life before adulthood.
Why do birds have different phases anyway? surely they could attain full breeding plumage in the first year. I guess that is so they do not mistaken for breeders by their peers. And I guess they don't breed in their first summer because they have to grow experienced enough to hunt successfully and 'prove' that they will make it and be able to hunt for themselves and a family- an older bird is better otherwise the species could possibly die out. And since these 'intermediate plumages' confer little advantage in a fish eating species, they could be almost anything . . it's when it get sto adulthood the plumage counts (=female sexual selection? whatever)
Why do some birds eg gulls have 2, 3 or 4 year groups- the larger ones because they have to grow larger and more experienced I guess, but it does seem a little arbitrary. And I wonder if a new section of chromosome gets switched on each year. Obviously at some point it may have been copied from athe previous years, and suddenly, one time you had a 4 year gull where there was none before, and over time you saw this one gull spread out and speciate in time to what we have now.
Anyway, that's enough rambling and speculation for now . . . . .
talon_dfa
Well-known member
Many good points above.
But Keep in mind, that it is a common mistake to assume that there is adaptive value attached to all "favorable" mutations. Many mutations have persisted, not because they are "favorable", but because they are not too "expensive" (neutral mutations).
Natal down & Immature plumages were undoubtedly shaped, textured and colored by evolutionary forces. (many hypothesis - collection solar radiation, killing bacteria, cryptic coloration). These plumages are very similar to other Haliaeetus species, as expected.
Birds are notorious for development of "attractive" plumages and elaborate behaviors due to sexual selection pressures. So no doubt, Bald Eagles have also traveled this evolutionary path, which has led to the adult plumage (and courtship rituals) we see today.
It appears that increasing (intermediate) levels of adult hormones lead to Basic I - Basic III plumages. Are each of the different plumages "adaptive" for different age classes? I doubt it.
More questions raised than answered.
Does the Southern Race of Bald Eagle (true Southern, not all individuals below US-Canada border) show all the intermediate plumages?
Are the same intermediate plumages seen with White-tailed or Stellar's Eagles?
Interesting questions all.
But Keep in mind, that it is a common mistake to assume that there is adaptive value attached to all "favorable" mutations. Many mutations have persisted, not because they are "favorable", but because they are not too "expensive" (neutral mutations).
Natal down & Immature plumages were undoubtedly shaped, textured and colored by evolutionary forces. (many hypothesis - collection solar radiation, killing bacteria, cryptic coloration). These plumages are very similar to other Haliaeetus species, as expected.
Birds are notorious for development of "attractive" plumages and elaborate behaviors due to sexual selection pressures. So no doubt, Bald Eagles have also traveled this evolutionary path, which has led to the adult plumage (and courtship rituals) we see today.
It appears that increasing (intermediate) levels of adult hormones lead to Basic I - Basic III plumages. Are each of the different plumages "adaptive" for different age classes? I doubt it.
More questions raised than answered.
Does the Southern Race of Bald Eagle (true Southern, not all individuals below US-Canada border) show all the intermediate plumages?
Are the same intermediate plumages seen with White-tailed or Stellar's Eagles?
Interesting questions all.
I finally got an answer from the Raptor Center here in MN. It doesn't completely answer the question, but ...
T.R.
You are correct in stating that there can be a benefit to differences in plumage and that certainly applies to color differences in birds that differ in color depending on the maturity and sex of the bird.
Bald Eagles, as with most raptors, do not differ in color as male and female display the same color plumage but do differ in size. Raptors display a characteristic known as reverse sexual dimorphism which means that the females are larger than the males in the species. The plumage color differences represent different transitional periods in the bird’s maturation process. This could serve as an identifier for prospective breeding age birds as an older and more experienced eagle could serve as a far better breeding partner than a younger one. An older adult is more likely to be a healthy individual and perhaps a more successful hunter.
Not all organisms mature at the same rate so seeing a difference in maturation lengths and time is not all that uncommon.
We hope the information we provided you with has answered your question. If you have any further raptor related questions, please, do not hesitate to contact us again.
Thank you again for contacting The Raptor Center!
T.R.
You are correct in stating that there can be a benefit to differences in plumage and that certainly applies to color differences in birds that differ in color depending on the maturity and sex of the bird.
Bald Eagles, as with most raptors, do not differ in color as male and female display the same color plumage but do differ in size. Raptors display a characteristic known as reverse sexual dimorphism which means that the females are larger than the males in the species. The plumage color differences represent different transitional periods in the bird’s maturation process. This could serve as an identifier for prospective breeding age birds as an older and more experienced eagle could serve as a far better breeding partner than a younger one. An older adult is more likely to be a healthy individual and perhaps a more successful hunter.
Not all organisms mature at the same rate so seeing a difference in maturation lengths and time is not all that uncommon.
We hope the information we provided you with has answered your question. If you have any further raptor related questions, please, do not hesitate to contact us again.
Thank you again for contacting The Raptor Center!
Here is the answer I received from Cornell University. Again not much "in depth" information.
Dear T. R.,
The short answer to your question is simple. It's because they're
Bald Eagles and that's the way Bald Eagles have evolved.
The longer answer is more complex. Songbirds reach their adult (definitive) plumage in less than a year. Of course, they're also ready to breed in less than a year and their life expectancy is pretty short as well.
In general, long-lived species such as large raptors, gulls, and pelagic seabirds retain their subadult plumages for a relatively longer period than songbirds because
they're not ready to breed until they're 4-5 years of age.
Hope this helps.
Dear T. R.,
The short answer to your question is simple. It's because they're
Bald Eagles and that's the way Bald Eagles have evolved.
The longer answer is more complex. Songbirds reach their adult (definitive) plumage in less than a year. Of course, they're also ready to breed in less than a year and their life expectancy is pretty short as well.
In general, long-lived species such as large raptors, gulls, and pelagic seabirds retain their subadult plumages for a relatively longer period than songbirds because
they're not ready to breed until they're 4-5 years of age.
Hope this helps.
Colleen-AK
Well-known member
Neither explaination covers why there are 4 distinct plumages... rather..they treat anything without a bald head as one plumage. The differences are not subtle, 1st year plumage looks like a different species. The 3rd year eagle I posted was the largest eagle I have seen... she was huge-- I wonder if there are so many more males... Its one of the reasons I thought it might not be a bald eagle... of course, I've only gotten this close to a few and in flight or at a distance, size is more difficult to guage. I would guess her at 8-10" bigger in height, not a small difference.. and not yet mature... Please post if you get a more definitive response...
As I stated in my opening - there is generally a "reason" for morphological differences within a species, whether it is sexual dimorphism, or age-specific color or apttern differences.
Obvioulsy most birds have juvenile and adult plumages, and many birds (waterfowl in particular) have distinct "breeding plumages. And, as has been mentioned, many large )often long-lived) sea birds have 2-3 age-specific plumages.
I'll propose a hypothesis here, without knowing anyting about the age-related genetic switches that trigger these plumage changes.
Obvioulsy these differences have something to do with age/breeding readiness,
So, we start out with a basic "dark" 1-year plumage, and we have what appers to be an an "intermediate" 3-year plumage (basically dark body with basically white head). This seems to be an "understandable" plumage progression. But, then there is the basically white belly and chestof the 2-year old, that diverges from what appers to be an understandable plumage progression.
If (big if) there is a chance that "pair bonding", or at least sub-adult "dating", occurs at age three (in other words bald eagles may begin to select mates during their thrd year), then there "might" be a need (for the birds) to distinguish between 2 and 3 year old birds of the opposite sex; which could explain why there are 4 plumages.
Another question that could be asked is, "is there a "camouflage" benefit to the two year old plumage?" It is possible that light chest and belly plumage, along with mottled wings, breaks up the "overhead" outline of a 2 year old, making it easier for a 2 year old to "seek and ambush" its prey.
I've seen 1-year old birds still being fed by adults in February; which might mean they are still partially reliant on food from their parents. I've also seen a lot of 2-year birds hanging out near adult birds (impossible to tell whether the 2 year olds are related to the adults, but it is a possibility, since family familiarity might still be present after a 2-year old moult). I see the 2-year old plumage most often from November through February, because that is when the birds congregate "en masse" along the Mississippi and MInnesota Rivers here in Minnesota, so it is the best chance of seeing 2 and 3 year old birds.
Then (the hypothesis factor) - if 2-year olds start to hunt or scavenge for themselves, the "camoulfage" effect of the 2 year old plumage could be an advantage in hunting (which the 1- year old has less need of - because of being fed by adults, and which the 3 year olds do not have need of - because they have learned to survive on their own the previous year).
Comments anyone???
God bless and enjoy the great outdoors.
Obvioulsy most birds have juvenile and adult plumages, and many birds (waterfowl in particular) have distinct "breeding plumages. And, as has been mentioned, many large )often long-lived) sea birds have 2-3 age-specific plumages.
I'll propose a hypothesis here, without knowing anyting about the age-related genetic switches that trigger these plumage changes.
Obvioulsy these differences have something to do with age/breeding readiness,
So, we start out with a basic "dark" 1-year plumage, and we have what appers to be an an "intermediate" 3-year plumage (basically dark body with basically white head). This seems to be an "understandable" plumage progression. But, then there is the basically white belly and chestof the 2-year old, that diverges from what appers to be an understandable plumage progression.
If (big if) there is a chance that "pair bonding", or at least sub-adult "dating", occurs at age three (in other words bald eagles may begin to select mates during their thrd year), then there "might" be a need (for the birds) to distinguish between 2 and 3 year old birds of the opposite sex; which could explain why there are 4 plumages.
Another question that could be asked is, "is there a "camouflage" benefit to the two year old plumage?" It is possible that light chest and belly plumage, along with mottled wings, breaks up the "overhead" outline of a 2 year old, making it easier for a 2 year old to "seek and ambush" its prey.
I've seen 1-year old birds still being fed by adults in February; which might mean they are still partially reliant on food from their parents. I've also seen a lot of 2-year birds hanging out near adult birds (impossible to tell whether the 2 year olds are related to the adults, but it is a possibility, since family familiarity might still be present after a 2-year old moult). I see the 2-year old plumage most often from November through February, because that is when the birds congregate "en masse" along the Mississippi and MInnesota Rivers here in Minnesota, so it is the best chance of seeing 2 and 3 year old birds.
Then (the hypothesis factor) - if 2-year olds start to hunt or scavenge for themselves, the "camoulfage" effect of the 2 year old plumage could be an advantage in hunting (which the 1- year old has less need of - because of being fed by adults, and which the 3 year olds do not have need of - because they have learned to survive on their own the previous year).
Comments anyone???
God bless and enjoy the great outdoors.
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Here is something interesting I found on the net today, from a Yelowstone study:
Nestling and adult Bald Eagles (Haliaeetus leucocephalus) banded and observed in the Greater Yellowstone region of northwestern Wyoming and southeastern Idaho displayed previously unrecorded plumage and talon coloration. Atypical characteristics were noted in nine nestling and three adult Bald Eagles associated with the Snake River watershed between 1979 and 1989.
Atypical plumage in nestlings was characteristic of genetically based pied plumage (i.e., feathers either completely white or completely brown, not diluted, speckled, or partially albino). Pied plumage in nestlings was manifested as one or all of the following abnormal characteristics: a wedge of white feathers on the crown, white breast patch, white chin, abdomen, and crissum feathers, white epaulets, and in one case, a completely white primary feather. One parent of a brood of affected nestlings exhibited atypical characteristics while parents of another brood were normal. Pied plumage of adults was characterized by small white breast patches, white feathers on the lateral crus, white feathers on the leading edge of the patagium, and/or white epaulets. Similar atypical plumage and talons were recorded in Golden Eagles.
Pied plumage was expressed only in female Bald Eagles and was associated with white (normally black) talons in both nestlings and adults and partially yellow (normally all black) cere in nestlings. Pied plumage may be masked in definitive plumage of Bald Eagles or at least not obvious when viewed from a distance.
Pied plumage in nestling Bald Eagles may be unique to Greater Yellowstone and proximity of affected eagles exhibiting pied plumage indicated close genetic relationship among resident breeding pairs.
so, part of the pied plumage may be "sex-linked" to females.
Nestling and adult Bald Eagles (Haliaeetus leucocephalus) banded and observed in the Greater Yellowstone region of northwestern Wyoming and southeastern Idaho displayed previously unrecorded plumage and talon coloration. Atypical characteristics were noted in nine nestling and three adult Bald Eagles associated with the Snake River watershed between 1979 and 1989.
Atypical plumage in nestlings was characteristic of genetically based pied plumage (i.e., feathers either completely white or completely brown, not diluted, speckled, or partially albino). Pied plumage in nestlings was manifested as one or all of the following abnormal characteristics: a wedge of white feathers on the crown, white breast patch, white chin, abdomen, and crissum feathers, white epaulets, and in one case, a completely white primary feather. One parent of a brood of affected nestlings exhibited atypical characteristics while parents of another brood were normal. Pied plumage of adults was characterized by small white breast patches, white feathers on the lateral crus, white feathers on the leading edge of the patagium, and/or white epaulets. Similar atypical plumage and talons were recorded in Golden Eagles.
Pied plumage was expressed only in female Bald Eagles and was associated with white (normally black) talons in both nestlings and adults and partially yellow (normally all black) cere in nestlings. Pied plumage may be masked in definitive plumage of Bald Eagles or at least not obvious when viewed from a distance.
Pied plumage in nestling Bald Eagles may be unique to Greater Yellowstone and proximity of affected eagles exhibiting pied plumage indicated close genetic relationship among resident breeding pairs.
so, part of the pied plumage may be "sex-linked" to females.
Please see link on my web called the first 5 years www.loubuscher.com
Besides the plumage change it involves sexual maturity as on the average it takes 5 years for this to happen and this could be a good question to ask when you get to Heaven as Lord knows I sure have a lot of themo There are many things we still don't know about the eagle and why they do and go the places they do. For instance here in New York about 95% of our eaglets get banded and we have found that the females will come much further from their birth nest to build a home but the male don't. One nest I monitor has a female that comes from a nest I monitor about 100 to 150 miles from where she nested and the male from a short distance on the Delaware River and he was one of three. Both of these birds were born in 1996 and this was there first nesting so you could say I knew her when she was an egg:-O Their nest lasted two seasons and it started to fall out so I bugged them in Albany for some help for some dumb young eagles and they got it. Check this nest build show. http://www.flickr.com/photos/loub/sets/72157600346254857/show/
Lou
Besides the plumage change it involves sexual maturity as on the average it takes 5 years for this to happen and this could be a good question to ask when you get to Heaven as Lord knows I sure have a lot of themo
There are many things we still don't know about the eagle and why they do and go the places they do. For instance here in New York about 95% of our eaglets get banded and we have found that the females will come much further from their birth nest to build a home but the male don't. One nest I monitor has a female that comes from a nest I monitor about 100 to 150 miles from where she nested and the male from a short distance on the Delaware River and he was one of three. Both of these birds were born in 1996 and this was there first nesting so you could say I knew her when she was an egg:-O Their nest lasted two seasons and it started to fall out so I bugged them in Albany for some help for some dumb young eagles and they got it. Check this nest build show. http://www.flickr.com/photos/loub/sets/72157600346254857/show/Lou
I can probably shed some light on the "dispersal" of young birds to other "home ranges" than their parental home range.
Since I research deer, elk, turkeys and Canada geese I have a littel i nsight on this subject. Basically dispersal occurs so that "in-breeding" among family related individuals does not occur.
In many mammals it is the males that disperse, or "pioneer" (look for new habitat) to either inhabit as a "breeding range" or simply to get away from larger males who have already taken up much of the "good" or "nearby" habitat. Whitetail deer and elk generally are kicked out of the ancestral herd by older females in their second fall, when they often begin to wander until they find an "open" (of other more dominant males) habitat, where they can remain. They may not establish their own "home range", where they generally stay for the rest of their lives, for 1-2 years after they leave the herd. Due to natural mortality and hunting - there is often available habitat, where some of the older bucks have left an "open" habitat. The females often stay near their mother's home range, and if there is available habitat they breed and raise their young there, with the result that the herds are often related through the female side of the family.
It is basically the same in turkeys and geese. The young females stay near their mother's (traditional) home range, and as long as there is available habitat, they nest and raise their young there, generation after generation, with the result that the "flock" is related through the female side of the family. In the case of many ducks "pair bonding" (getting engaged) occurs in the fall or winter, and the males follow the females back (on spring migration) to the female's mother's home range.
So why (at least in your case) do the female bald eagles desperse? Due to the fact that female raptors are often larger (and possibly stronger) than the males, they may be the ones that establish a "breeding range". And the males then have to find the females.
Frow what I can see here in Burnsville, MN, where at least one pair of eagles winters on the same river where they nest (thus no migration), many of the birds are seen sitting in two's, in what appears to be pairs (one large bird and one smaller bird). As a resulf ot these observations - I suspect many "paired" eagles either stay together when they do not migrate, and some may stay together when they do migrate. Thus, pairs can and do "stay on" or "return to" the same nesting range each year.
I've been told by the people at the National Bald Eagle Center here in Wabasha, MN, that when only one of a pair of bald eagles returns to a previous nest site, a new "pair bond" can occur within a matter of hours of when the ramaining bird meets a potential "new" mate. I suspect this occurs so that breeding occurs at the optimal time for raising young. Delayed "pair bonding" could result in late mating and non-survival of the young.
I'll be sure to ask Yahweh-God about all of this when I get to Heaven.
Thanks for the input, it all adds pieces to the puzzle.
Merry Christmas and Happy New Year, and God bless,
T.R.
Since I research deer, elk, turkeys and Canada geese I have a littel i nsight on this subject. Basically dispersal occurs so that "in-breeding" among family related individuals does not occur.
In many mammals it is the males that disperse, or "pioneer" (look for new habitat) to either inhabit as a "breeding range" or simply to get away from larger males who have already taken up much of the "good" or "nearby" habitat. Whitetail deer and elk generally are kicked out of the ancestral herd by older females in their second fall, when they often begin to wander until they find an "open" (of other more dominant males) habitat, where they can remain. They may not establish their own "home range", where they generally stay for the rest of their lives, for 1-2 years after they leave the herd. Due to natural mortality and hunting - there is often available habitat, where some of the older bucks have left an "open" habitat. The females often stay near their mother's home range, and if there is available habitat they breed and raise their young there, with the result that the herds are often related through the female side of the family.
It is basically the same in turkeys and geese. The young females stay near their mother's (traditional) home range, and as long as there is available habitat, they nest and raise their young there, generation after generation, with the result that the "flock" is related through the female side of the family. In the case of many ducks "pair bonding" (getting engaged) occurs in the fall or winter, and the males follow the females back (on spring migration) to the female's mother's home range.
So why (at least in your case) do the female bald eagles desperse? Due to the fact that female raptors are often larger (and possibly stronger) than the males, they may be the ones that establish a "breeding range". And the males then have to find the females.
Frow what I can see here in Burnsville, MN, where at least one pair of eagles winters on the same river where they nest (thus no migration), many of the birds are seen sitting in two's, in what appears to be pairs (one large bird and one smaller bird). As a resulf ot these observations - I suspect many "paired" eagles either stay together when they do not migrate, and some may stay together when they do migrate. Thus, pairs can and do "stay on" or "return to" the same nesting range each year.
I've been told by the people at the National Bald Eagle Center here in Wabasha, MN, that when only one of a pair of bald eagles returns to a previous nest site, a new "pair bond" can occur within a matter of hours of when the ramaining bird meets a potential "new" mate. I suspect this occurs so that breeding occurs at the optimal time for raising young. Delayed "pair bonding" could result in late mating and non-survival of the young.
I'll be sure to ask Yahweh-God about all of this when I get to Heaven.
Thanks for the input, it all adds pieces to the puzzle.
Merry Christmas and Happy New Year, and God bless,
T.R.
HI TR Yes you are right about some staying put come the winter. There were two nesting pairs I used to monitor that do that but in their case they both have everything they need and that being open water and food. So far this year I have received about 8 to 12 band # from the guys at the Conowingo Dam in Maryland on the Susquanna River and in checking the records with the endangered species unit in Albany N.Y. (Pete Nye their leader) send me back all the info on the birds. So far over the years just about all the New York birds that get ID are young birds most from this year nesting placeing them at about 10 months of age while some are just starting to get white. So I would have to wonder where the rest go but there are a lot of eagles and a lot of places like that with open water and a lot of fish and how in the world do they comunicate with each other??? Another question for the big guy upstairso
Each winter the DEC tries to trap a few adults that come down from the north and band and place satilite transmitters on them just to see how their migration or journey north goes and how they get there. Needless to say the ideal thing would be to get a mated pair trapped and transmitters placed on them so the route each takes can be traced. (A tough one to figure) Signals are put out every 54 hours and thy are solor powered, last quite awhile. There is a link to the journey North on my home page ( www.loubuscher.com )where each year thousands of school kids pick a bird and tract the cooridinates which are posted for each bird on the journey north web.
Lou
