Tag Archives: intruder

LOST NESTLINGS AND FAILED BROODS OF EAGLETS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

Lost Nestlings

20.8% of the eggs laid at the observed nests from 2006-2020 were lost. (See discussion here.) But the number of nestlings lost before they could fledge was fewer, 16.2%. As a percentage of the number of eggs laid, the number of nestlings lost was 12.9%.

Table 5

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Losses of nestlings were roughly equivalent across clutch size:

    • 1-egg nests lost 16.7% of their nestlings.
    • 2-egg nests lost 16.9% of their nestlings.
    • 3-egg nests lost 15.2% of their nestlings.
    • 4-egg nests lost 22.2% of their nestlings.

This contrasts with the more dramatic differences among clutch sizes in the loss of eggs, where 1-egg nests were far less successful with 55.6% losses, and 3-egg nests were significantly more successful with only 16.7% losses of eggs.

Causes of nestling loss, as with egg loss, include external events, such as bad weather, a fallen nest, Bald Eagle intruders, and intrusions by other animals. But nestling losses also come about for reasons that don’t apply to eggs, including fall from the nest, injury, starvation, ectoparasites, disease, and poisoning. As with egg losses, many causes are observable on cam, but often the cause cannot be perceived from afar. If a nestling’s body can be retrieved from the nest without disturbing the other eagles, laboratory analysis might reveal a cause, but sometimes even then the reason is elusive.

The highest percentage of lost eggs were brought about by intruders (see Table 3), but it was bad weather that caused the most lost nestlings. This is no doubt due to the likelihood that nestlings are often exposed to the elements, whereas eggs remain more protected throughout the incubation period.

    • 19.7% nestlings were lost because of bad weather.
    • 13.7% fell from the nest.
    • 4.3% were predated.
    • 4.3% starved.
    • 4.3% were lost because of intruders.
    • 3.4% were injured.
    • 2.6% were victims of ectoparasites.
    • 1.7% of losses were due each to disease and poison.
    • The causes of a large plurality of losses, 44.4%, were unknown.
Failed Broods of Nestlings

There were 350 broods of nestlings at the nests from 2006-2020, and 8.9% lost all of their eaglets. Again, causes of some of the failed broods are known, but many are not. Table 6 enumerates the failed broods at specific nests (referred to by abbreviated codes, which are identified at the end of the table) and gives the cause, if known.

Table 6

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As with losses of clutches of eggs, 1-egg nests had the highest rate of failed broods of nestlings:

    • 1-egg nests lost 16.7% of their broods.
    • 2-egg nests lost 8.9% of their broods.
    • 3-egg nests lost 7.4%. of their broods
    • 4-egg nests lost none of their broods.

The number of broods of nestlings lost was highest at 4 in 2012, 2017, and 2018. But 2012 lost the highest percentage of total broods, with 17.4% lost. 2006 and 2008 had no failed broods, and only 1 brood failed in 2011, 2013, and 2019. The percentage of losses in 2019 was quite low, with only 2.9% lost.

Note that the second brood of eaglets at the Southwest Florida nest is included in the total number of broods. It is the only such second brood of nestlings in the data. (See discussion here.)

SUCCESS RATES OF CLUTCHES AND BROODS

AT WILD BALD EAGLE NESTS,
2006-2020

© elfruler 2020

Table 2 drills down more deeply into the clutches of eggs and broods of eaglets at the nests observed, showing the number of clutches of each nest size (1 egg, 2 eggs, etc.) each year, the number of clutches with hatched eggs in each nest size, and the number of broods with fledged eaglets in each nest size. The first page of the table gives numbers for clutches of eggs, which includes second clutches. The second page gives numbers for broods of eaglets and fledges.

The figures in the table refer to the number of clutches or broods of a particular size (1-hatch or 1-fledge clutches, 2-egg or 2-fledge clutches, etc.), not to numbers of individual eggs, chicks, or fledges, which are tallied in Table 1. Percentages illustrate the degree of success of a clutch or brood.

I use the term “successful” in reference to a clutch in which at least one egg hatched and to a brood in which at least one egg hatched and at least one eaglet fledged. An unsuccessful clutch is one in which no eggs hatched, and an unsuccessful brood is one in which no eaglets fledged.

I use the term “perfect” in reference to a clutch in which all eggs hatched and to a brood in which all eggs hatched and all eaglets fledged. Perfect clutches and broods are highlighted in orange.

TABLE 2

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Clutches of eggs with hatches (p. 1 of the Table)

Of 401 clutches of eggs, 87.5% were successful and 66.3% were perfect.

    • 1-egg clutches averaged a 44.4% success rate, which of course is the same percentage for perfect clutches, since only 1 egg is involved.
    • 2-egg clutches averaged a much higher success rate of 87.7%, with 70.1% perfect with 2 eggs hatched.
    • 3-egg clutches averaged an eye-popping 96% success rate, with 63.5% perfect with 3 eggs hatched.
    • 4-egg clutches hit the jackpot with a 100% success rate, 2 out of 3 of which (66.7%) were perfect with all 4 eggs hatched.

The low success rate of 1-egg clutches can be attributed at least partially to the fact that if the only egg is lost, the clutch is lost. The same could apply to the higher rate of success of both 2-egg and 3-egg clutches, with more eggs to “spare.” But the high rate of perfect 2-egg and 3-egg clutches defies this logic and perhaps points to subtle behavioral or biological factors such as parental attentiveness or the reproductive superiority of adults who succeed in laying more eggs than one.

Broods of eaglets with fledges (p. 2 of the Table)

Of the 401 clutches of eggs in Table 2, 76.8% ended up with successful broods of fledged eaglets, and 46.9% resulted in perfect broods.

    • 1-egg clutches averaged 33.3% successful and 33.3% perfect rates of fledged eaglets.
    • 2-egg clutches averaged 77% successful, with 50.8% perfect with 2 eaglets fledged.
    • 3-egg clutches averaged 85.7% successful, with 42.9% perfect with 3 eaglets fledged.
    • 4-egg clutches were 100% successful in producing fledglings, but only 1 out of 3 clutches, or 33.3%, resulted in a perfect 4 fledged eaglets.

Comparing the success rates of broods of eaglets with success rates of clutches of eggs illustrates well the challenges that hatched nestlings and their parents face in achieving the full development and growth from hatch to fledge. In all except 4-egg clutches, the percentage of successful broods dropped by a little over 10 points from the percentage of successful clutches.

    • 1-egg nests had 44.4% successful clutches but only 33.3% successful broods.
    • 2-egg nests had 87.7% successful clutches but only 77.0% successful broods.
    • 3-egg nests had 96% successful clutches but only 85.7 successful broods.
    • 4-egg nests had a 100% successful rate for both clutches and broods.

As noted in the discussion of Table 1, some fledges could not be confirmed. In Table 2 where the numbers refer to clutches and broods rather than to individual eggs or eaglets, a nest where at least 1 eaglet’s fledge is not confirmed is counted in the unconfirmed row, even if at least 1 eaglet did fledge.

Table 2, like Table 1, shows that numbers can fluctuate up and down from one year to the next, and there is no clear trend in either direction.  For example:

    • Successful clutches hit a peak of 100% in 2007, and a low of 75%in 2015. 2019 was above average with 89.7% successful, but 2020 was below average with 76.7%.
    • Perfect clutches ranged from a low of 50% in 2006 to an astounding 91.3% in 2012. 2019 was slightly above average at 66.7%, while 2020 was well below average at 53.5%
    • Successful broods were at a low 50% in 2006, with a high of 87% in 2011. 2019 was well above average with 84.6% successful, while 2020 fell slightly below average with 72.1%
    • Perfect broods were low in 2006 with 33.3%, but very healthy in 2010 at 63.2%. 2019 had an above average rate of 48.7% perfect broods, and 2020 was at the lower end of the range with 39.5%.

NUMBERS FROM THE NESTS

WILD BALD EAGLES, 2006-2020

© elfruler 2020

The video cameras that have been trained on Bald Eagles’ nests since 2006 have provided a treasure trove of information about the breeding behavior of these apex raptors. In the universe of the more than 100,000 active Bald Eagle nests in North America, the data that these particular nests yield is minuscule. A few published scholarly reports on Bald Eagle nesting success focus mainly on a circumscribed area (e.g. Florida) for 1 or a few breeding seasons. The data here from the nests on cam span 15 years of breeding from 2006-2020 across a wide geographical expanse throughout the continent, and they represent the full range of climates and habits in which Bald Eagles reproduce. (Nests included in the data are listed here.)

Over the period, adult pairs at these nests made 401 breeding efforts at 85 locations, producing 910 eggs, 721 hatchlings, and at least 588 fledglings. These numbers might be considered a fair sampling of breeding data for the species.

The pages and tables that follow break down the data collected via these cameras on multiple levels. The raw numbers of eggs laid, nestlings hatched, and juveniles fledged, from nest to nest and year to year, yield statistics and percentages that give an overall view of breeding success over the 15 years. Burrowing more deeply into these numbers reveals how many clutches are successful over time, and which clutches of a particular size (1 egg, 2 eggs, etc.) are more successful than others. The numbers open a window into losses of eggs and eaglets, and what we can learn about reasons for those losses. And the numbers help flesh out some perceptions of behaviors of nesting Bald Eagles, such as coping with bad weather, predators, and intraspecific intruders (by other Bald Eagles), and replacing a lost clutch.

The data reinforce some facts that are already known:  Bald Eagles typically lay clutches of 2 eggs, with clutches of 3 eggs less common, clutches of 1 egg unusual, and clutches of 4 eggs quite rare. A fair number of eggs do not hatch, but a healthy majority end in successful fledges.

Other details to emerge from these analyses are perhaps more surprising:  While overall averages seem consistent with what is generally believed, there is often a wide range of values across seasons and from nest to nest.  In some years the number of eggs lost far exceeds the average, while in other years few eggs remain unhatched. Similarly, the number of nestlings that die before fledging covers a wide range among the years. Three-egg nests produce a higher percentage of fledges than either 2-egg nests or 1-egg nests; the latter are least successful in producing fledges.

These pages represent a complete revision of data that I published here in 2018, which consisted of a single page and 1 spreadsheet. For this new report I have pared down the nests to include only those with the most reliable observations, mainly the ones with streaming video cams, plus a small number of nests with reliable ground observers. I have also expanded the detail and breadth of information and analysis, resulting in 8 spreadsheets, and I have provided a narrative discussing each one. I have also compiled a lengthy list of References to literature on breeding, eggs, incubation, and survival.

These tables and narratives are presented in sequence in the pages that follow:
Additional new pages also make use of the nest data:
Full references for citations in the following pages are given here:

I began collecting data when I started watching web cams in 2009. Thanks to the Hancock Wildlife Foundation, the Institute for Wildlife Studies, spreadsheets compiled by Judy Barrows, nest cam websites and Facebook pages, and numerous individuals with whom I have communicated, I have been able to stretch the data back to 2006 when streaming cams first began operating. These sources also have been invaluable in filling in gaps in my own observations. I owe all of them a great debt of thanks.

 

INTRASPECIFIC INTRUSIONS AT BALD EAGLE NESTS

© elfruler 2018

“intraspecific adj. : occurring within a species or involving members of one species.” (www.merriam-webster.com)

Bald Eagles choose their breeding territories and nest sites carefully, driven by factors that will lead to success in raising their young.  These factors include adequate food resources, a sturdy nest platform, available shelter from dangerous weather, ease of defense, and tolerable distance from disturbances.  A good location will be attractive to any Bald Eagles that come along, and it is not surprising that a resident Bald Eagle pair will be challenged by other Bald Eagles for the site, leading to competition between members of the species, or intraspecific conflict.

It is not uncommon for one or both members of a pair to be challenged even before the nesting season begins, resulting in displacement, injury, and even death.  Conflicts that occur once a clutch of eggs has been laid or a brood of chicks has hatched can cause loss of eggs and chicks, despite the fierce defense that the parents inevitably mount against intruders.  Often the parents are successful in repelling a challenge and their chicks fledge.  In rare cases an intruder is accepted by the resident pair as a cooperative breeder (Go here for a series of pages about Cooperative Breeding and its occurrence among Bald Eagles).

The Bald Eagle nests that have been viewed on camera or monitored carefully from the ground since 1992 have provided a glimpse of intraspecific intrusions of many types and with a variety of outcomes:

  • Events before, during, and after the season
  • From one to many intruders
  • Intruding males and females, adults, subadults, and juveniles
  • Replacements, disappearances, injuries, and deaths of parents
  • Unhatched or broken eggs and injured or slain nestlings
  • Double clutches
  • Rescued nestlings and rehabbed fledglings
  • Successful fledges
  • Cooperative breeding

Even careful monitoring of cams and nests don’t provide the full picture of events surrounding intrusions, which often take place out of human view.  Even happenings in full view do not always have clear-cut explanations.

  • How many intruders are in the area?
  • When did they first appear?
  • What encounters occur off-nest between intruders and residents?
  • What is the sex or age of an intruder?
  • Does a resident eagle disappear because it has been injured or killed, or because it has decided that it cannot prevail in a battle?
  • Was an egg broken by an intruder or an agitated parent, or because it was unviable or infertile?
  • Did an egg fail to hatch because intruders interrupted the reproductive cycle and prevented fertilization?
  • Why would one intruder destroy eggs or chicks but another intruder leave eggs or chicks undamaged?
  • Does a resident adult respond primarily defensively to an intruder, or might there be a trigger that precipitates an offensive response?
  • Why would an intruder become a helper and cooperative breeder rather than a threat?

Answers to such questions would provide a much better understanding of events we can see, but too often the answers elude us.

The number of Bald Eagle nests for which reliable daily reports have been made more than doubled from 2008 to 2018 – from 24 to 57.  They range from southern California to New England, from south Florida to Alaska, and many points on the continent inbetween.  The nests are found in a variety of habitats, including rural farmland, along rivers and streams, lakes and bays, in woodlands in parks and wildlife refuges, on coastal islands, college campuses, and in city neighborhoods.  Yet it cannot be claimed that these nests are a representative sampling of all of the thousands of Bald Eagle nests in North America.

Nor can we be sure that we have witnessed every conceivable behavior or outcome associated with intraspecific intrusions.  The information that I present here is illustrative of certain types of Bald Eagle behavior, but should not be taken as a statistical report on intraspecific intrusions.

This first chart (at this link) describes the intraspecific intrusions observed at Bald Eagle nests from 1992 through 2018, the nest locations and habitats, what is known about the intruders, the events, and the outcomes.  (Link opens in a new browser tab.)  Losses (of parents, eggs, or chicks) can result directly or indirectly from intrusions, or they may occur for other reasons not related to intraspecific conflict.  Not every intrusion leads to a loss.

This second chart (follow this link) summarizes the details and gives percentages to enable comparison of intrusions, losses, and fledges from one year to the next.  (Link opens in a new browser tab.)  The percentages are number of intrusion events (nest intrusions, clutch intrusions, eggs lost, etc.) compared to the total number observed (nests, clutches, eggs., etc.) in that season.  (The years 1992-2007 at the California West End nest are not included in this summary because of the outsized effect of DDE contamination on egg production there.)  Notably, for these nests there is no clear trend in the percentages over time.

  • While 2018 clearly was a bad year for nest intrusions (including both before and during a clutch), at 24.6% of observed nests, 2008 was almost as bad, at 20.8% of observed nests.
  • On the other hand, 2008 was a worse year for clutch intrusions (after eggs were laid), at 19.2% compared to 17.9% in 2018.
  • And in 2008, 4.5% of chicks were lost, compared to a much smaller 1.3% in 2018.
  • The number of eggs lost was a staggering 11.3% in 2018, but the 8.6% of eggs lost in 2008 is the second highest percentage.
  • The year 2013 was difficult, with 13.6% of clutch intrusions and 6.7% of egg losses.
  • Some years were relatively benign: 2011 saw only one intrusion and 2012 only two.  A dip in losses occurred in 2015 and in intrusions in 2016.
  • From 2013-2017 most percentages were relatively stable – nest intrusions, clutch intrusions, losses – with an overall dip in 2016. Intrusions and losses in 2018 were severe, and it remains to be seen in coming seasons whether that year was an outlier.
BRIEF UPDATE ON THE 2018-2019 SEASON: I have not yet added new information to the charts, but the number of intraspecific intrusions declined significantly from the year before. Only 6 such intrusions occurred, only 2 of which happened after eggs were laid. 2 nests ended up with no eggs laid, and a total of 4 eggs were lost. No eaglets were lost.

There is no question that suitable habitat for nesting Bald Eagles is on the decline across the continent because of human development and encroachment.  But the numbers we have for these particular nests do not necessarily mean that increasing numbers of nest intrusions point to an approaching saturation of carrying capacity for Bald Eagles across the board.  Each territory has its own conditions that may or may not be either conducive or resistant to nest intrusions.  Increasing population density in a particular area may simply drive some Bald Eagles to adapt by seeking out previously unclaimed territories, by gradually shrinking the size of their territories (over time) to allow for more nests (if the food supply allows for it), or by allowing more instances of cooperative breeding.  It remains to be seen whether intraspecific intrusions will have a negative impact on the Bald Eagle population in the long run.  Some have argued that a rise in population density ultimately could result in a state of population equilibrium by slowing the breeding productivity to offset the long period of increase that followed the banning of DDT in 1972.

While the snapshots that these observed nests provide give us some narratives about intraspecific intrusions at Bald Eagle nests and make comparisons possible, a broader understanding of the causes and effects of such intrusions, as well as a glimpse of what they may entail in the future, must await more detailed and systematic studies (such as Mougeot et al. 2013 in Saskatchewan and Turrin and Watts in the Chesapeake Bay, 2014 and 2015).

For perspective on the Bald Eagle population in North America and trends over time, Partners in Flight (PIF) estimates the number of breeding-aged Bald Eagle individuals in 2017 at around 250,000, based on data from the North America Breeding Bird Survey, an approximate 131% increase since 1970.  The U.S. Fish & Wildlife Service’s oft-cited number of about 10,000 breeding pairs (or 20,000 individuals) in the lower 48 United States in 2007 does not include numbers from Canada or Alaska (both of which exceed the number in the lower 48 states), and it represents only eagles in pairs that are actively breeding.  The PIF estimate encompasses all individual Bald Eagles throughout North America of breeding age whether they have formed breeding pairs or not.  None of these numbers include juvenile or subadult Bald Eagles, which could more than double the totals.

 There is as yet no sign that the Bald Eagle population is declining, whether because of habitat changes that lead to overpopulation and intraspecific conflict in a territory, or other causes such as contaminants, trauma, electrocution, disease, poisoning, and poaching.  In 2010, following the removal of the Bald Eagle from the list of threatened and endangered species, the U.S. Fish & Wildlife Service produced a Post-delisting Monitoring Plan for the Bald Eagle.  The Plan establishes a 20-year monitoring period (roughly four generations of breeding Bald Eagles) in the lower 48 states, with data analyzed and reported to the public every 5 years.  The Plan will yield information on changes in numbers and their causes, and it includes provisions for responding to a 25% or greater decline with corrective action by federal, state, and local agencies, Native American Tribes, and other interested partners.  The Plan specifically references the possibility of re-listing the Bald Eagle as threatened and/or endangered as a remedy to an unacceptable level of decline.

REFERENCES

Dzus, E.H. and J.M. Gerrard 1993.  Factors influencing Bald Eagle densities in northcentral Saskatchewan.  The Journal of Wildlife Management 57: 771-778.

Elliott, K.H, J.E. Elliott, L.K. Wilson, I. Jones, and K. Stenerson 2011.  Density-dependence in the survival and reproduction of Bald Eagles: linkages to chum salmon.  The Journal of Wildlife Management 75: 1688-1699.

Farmer, C.J., L.J. Goodrich, E. Ruelas I., and J.P. Smith  2008.  Conservation Status of North America’s Birds of Prey.  In K.L. Bildstein, J.P. Smith, E. Ruelas I., and R.R. Veit (eds). State of North America’s Birds of Prey.  Nuttall Ornithological Club and American Ornithologists.  Union Series in Ornithology No. 3. Cambridge, Massachusetts, and Washington, D.C., 303-420.

Grubb, T.G., L.A. Forbis, M. McWhorter, and D.R. Sherman 1988.  Adaptive perch selection as a mechanism of adoption by a replacement Bald Eagle.  The Wilson Bulletin 100: 302-305.

Hancock Wildlife Foundation Forum.

Hornby Eagle Group Projects Society.  Our Nature Zone.

Hunt, W.G. 1998.  Raptor floaters at Moffat’s equilibrium.  Oikos 82: 191-197.

Institute for Wildlife Studies.  Channel Islands EagleCAM Forum.

Jenkins, J.M. and R.E. Jackman 1993.  Mate and nest site fidelity in a resident population of Bald Eagles.  The Condor 95: 1053-1056.

JudyB.  Watching Eaglets Grow.

Mahaffy, M.S. and L.D. Frenzel 1987.  Elicited territorial responses of northern Bald Eagles near active nests.  The Journal of Wildlife Management 51:551-554.

Markham, A.C. and B.D. Watts.  Documentation of infanticide and cannibalism in Bald Eagles.  Journal of Raptor Research 41: 41-44.

Mougeot, F. 2004.  Breeding density, cuckoldry risk and copulation behaviour during the fertile period in raptors: a comparative analysis.  Animal Behaviour 76: 1067-1076.

Mougeot, F., J. Gerrard, E. Dzus, B. Arroyo, P.N. Gerrard, C. Dzus, and G. Bortolotti 2013.  Population trends and reproduction of Bald Eagles at Besnard Lake, Saskatchewan, Canada 1968-2012.  Journal of Raptor Research 47: 96-107.

Partners in Flight.

Turrin, C. and B.D. Watts 2014.  Intraspecific intrusion at Bald Eagle nests.  Ardea 102: 71-87.

Turrin, C. and B.D. Watts 2015.  Nest guarding in Chesapeake Bay Bald Eagles.  Journal of Raptor Research 49: 18-28.

U.S. Fish and Wildlife Service.

Watts, B.D., G.D. Therres, and M.A. Byrd 2007.  Status, distribution, and the future of Bald Eagles in the Chesapeake Bay area.  Waterbirds 30: 25-38.

Watts, B.D., G.D. Therres, and M.A. Byrd 2008.  Recovery of the Chesapeake Bay Bald Eagle nesting population.  The Journal of Wildlife Management 72: 152-158.