FIRST DRAFT - OCTOBER 1, 2015
STATISTICAL EVALUATION OF UNOFFICIAL DATA ABOUT BALD EAGLES NESTING AT TURTLE BAY, REDDING, CALIFORNIA, USA
Reporting Nine Seasons Beginning 2006-2007 and Ending 2014-2015
This first draft has not been reviewed for factual accuracy, statistical errors, grammatical errors, or scientific criteria. This post and the attached statistical tables are meant to provide "fun facts", possible forward looking data trends, reference information and topical discussion. This guide is written as a “stand alone” piece and is not meant to substitute-for or correct any other post. If you care to provide additions, corrections, data, answers to our questions, or other input to this article, please private message me through the HWF mail system. If you would like a PDF copy of this article, please private message me with an external email address where I can send the PDF attachment. Please do not post your email address in the forum.
ALL DATA AND STATISTICAL CHARTS ARE ATTACHED TO THE BOTTOM OF THIS POST, AND THE POST(S) FOLLOWING (A TOTAL OF SIX TABLES ON FIVE PAGES). TO VIEW THE TABLES, CLICK ON THE IMAGE AND THEN USE YOUR BROWSER'S ZOOM FUNCTION.
A statistical evaluation is made for the data presented in the table, “Unofficial Data about Bald Eagles Nesting at Turtle Bay, Redding, California, USA” as presented in Table 1 of this document. Various elements of the statistical evaluation are presented in Tables-2 through -6. The ensuing analyses and tables are presented as methods for nest watchers to anticipate various nesting activities and to see the beginning of possible data trends.
This analysis is presented by an amateur nature enthusiast who has no advanced training in biology or statistical analysis related to wildlife communities in general, or eagles in particular. As such, data grouping, terminology and other elements may be incorrectly addressed within this analysis. Due these factors, as well as limitations in data as discussed below, this analysis should be viewed less as a scientific article and more as an informational guide.
The data and statistical analyses herein address various factors in the life and nesting activities of a mated pair of Bald Eagles in Redding, California, USA. The original pair of eagles were given the names Liberty (female) and Patriot (male). They were first observed from the ground during the nesting seasons in 2006-2007 and 2007-2008. About that time, a highway construction project in the vicinity of the nest caused the local governmental jurisdictions (Caltrans, California Department of Fish & Game, US Fish and Wildlife, etc.) to install the first live web camera to observe the eagle nest during the construction process. Once the webcams went online, the eagles quickly gained supporters within the local community and from web viewers across the world. Soon, observations were being made and archived in various online forums. Through the years, the eagles built a new nest, the camera was moved and upgraded, and a second male (Spirit) replaced the first. Most recently, all nest-related observations were consolidated into one data table and presented in the principal online forum at Hancock Wildlife Foundation (see link in the “Conclusions” section). This document presents the first statistical analysis of the consolidated data.
The same nesting female, Liberty, has been present for all nine seasons. Two nesting males have been present; Patriot for the first seven seasons, and Spirit for the last two seasons. The last season when Patriot was present (2012-2013), and the first season when Spirit was present (2013-2014) were the only two seasons when no eaglets were fledged despite clutches of three eggs in each of those two seasons.
THESIS AND DATA
This statistical evaluation has no thesis, though an effort has been made to view, sort and analyze data in relation to clutch size. This has been done primarily to view the relation of incubation period as it relates to the clutch size and order of egg laying/hatching. Other data trends may become apparent based on these conditions as well.
Definitive conclusions, themes, and trends generally cannot be made from the given data due to small sample sizes, variations in conditions, variations in precision, incomplete observations and other factors. Nevertheless, data ranges, minimums, maximums, means (averages), medians (middle data points), and modes (most frequent data points) are presented as a method for nest watchers to anticipate nest activities and to see the beginning of possible data trends. In most cases, the sample size is far too small to present a mode value.
The data set includes observations beginning in the 2006-2007 nesting season and ending with the 2014-2015 nesting season, a total of nine seasons. All nesting seasons start in the early fall (late September/ early October) and proceed through the following summer.
Data from the 2006-2007 and 2007-2008 seasons includes only the number of eaglets fledged since there was no camera present at the nest. As a result, data from these years is used only to compute the total number of eaglets fledged during the study period.
Six of the next seven seasons (excluding the 2011-2012 season) included observation by web camera. While some of the observations during the first few years were made or confirmed by professional biologists, the vast majority of observations in the latter years were made by amateur wildlife enthusiasts who made on-site ground observations or viewed the live web camera. Early camera(s) did not have night vision, so nest activity that occurred overnight could not be logged more accurately than the number of hours during the span of darkness. Valuable data points are missing because of that fact, thus limiting the sample of nesting seasons with complete and accurate data.
Data from the 2011-2012 season is incomplete because the eagles moved the nest and a camera was not present to make definitive observations. A very loving and dedicated ground crew made (nearly) daily observations at the nest area, but the precision of these observations cannot be assured as if they were in full view of a nest camera. As such, during the 2011-2012 season, data regarding egg laying, egg hatching, incubation and clutch size is NOT considered. Similarly, because we cannot know the number of eggs in the clutch, or if there was an eaglet that didn’t survive, success rate calculations do NOT consider data from the 2011-2012 season. Ground observations did provide reliable information on the dates of return for the adult eagles, dates of fledge, and numbers of eaglets fledged, therefore such information is used in applicable statistical calculations.
In total, as described above, three of the nine seasons (2006-2007, 2007-2008, and 2011-2012) have no observations via nest camera and are of only the most basic use. Another three of the nine seasons (2008-2009, 2012-2013, and 2013-2014) have incomplete data sets due to various factors such as lack of night vision (2008-2009), egg failure (2012-2013 and 2013-2014), and predation (2012-2013). Only three of nine seasons have complete data sets, one for a two egg clutch (2010-2011), and two for three egg clutches (2009-2010 and 2014-2015).
Because the partial data set from the 2008-2009 season lacks only the time of day for laying and hatching of the third egg (both occurring overnight), the author deemed that excluding the available data from that season would skew results more than including it. Thus, data from the 2008-2009 season is included in calculations for clutch size of three eggs, and for clutches of any size. A footnote is added to the data tables to note the missing data points for the third egg.
As described throughout this section, the limited number of complete data sets makes sample sizes extremely small. Therefore, reliable conclusions cannot be made for the data. Though statistically unreliable, the current data can be analyzed and sorted in an effort to compare it to known parameters for the Bald Eagle species in general, and to make a forward-looking peek into possible trends for the Turtle Bay nesting pair in particular.
RESULTS OF STATISTICAL ANALYSES
Sample size is generally too small to make any conclusions on data grouping and trends. More data will be required to make such analyses. Despite the limitations in data, the following is presented for informational use and reference to possible future trends. Only a select bit of information is summarized here; much more information is available and can be learned from study of each of the statistical tables (Table-2 through Table 6).
Dates When Adult Eagles Return to the Nest Area: Refer to Table 6 for data on return of the adult eagles to the nest area. The male of the pair has returned to the nesting area before the female in four of the five seasons when such observations have been made. The male typically returns anywhere from one day to 14 days prior to the female, averaging about seven days prior. During the only season when this trend was reversed, the female returned four days prior to the male.
Clutch Size: Refer to Table 1, Table 2 and Table 6 for data on clutch size. During the six seasons when clutch size could be positively determined, the clutch size was two eggs once (16.7%), and three eggs five times (83.3%), averaging 2.71 eggs/season. The remaining three years all produced eggs and at least one eaglet was fledged in each year, but the clutch size could not be determined due to the absence of a nest camera. These missing data years would likely have lowered the average clutch size. If we take the number of eaglets fledged in the three seasons when the nest camera was absent and assume that no eggs/eaglets were lost from the time the eggs were laid to the time the eaglet(s) fledged, then the lowest average clutch size possible at the Turtle Bay nest is 2.44 eggs/season over nine full seasons covered by the study.
Dates and Times for Egg Laying in Clutches of Two Eggs and Three Eggs: Refer to Table 1, and Tables-4 through -6 for data on egg laying. On average, eggs are laid at 3 day intervals. Of the eggs laid and observed by camera (including night vision when available), approximately 93% were laid between 3:36 pm and 7:43 pm, a 4-hour window in late afternoon and early evening. This constitutes an overwhelming majority and definite data trend.
Egg Non-Viability (Failure): Refer to Table 1, Table 5 and Table 6 for data on egg non-viability. Within the six seasons when eggs could be observed by nest camera, there were two seasons when at least one egg did not hatch. Both of these seasons had a clutch size of three eggs. In the 2012-2013 season, one egg did not hatch; the egg that did not hatch was determined to be the first egg by matching incubation period of the remaining two eggs and eliminating the possibility of the second and third eggs as being the ones that failed. By that process, it was determined that the first egg failed on the 23rd day. Cause of egg failure during that year could not be positively determined, but two leading theories include non-fertilization, or damage during encounters with an third adult eagle intruding to the nest. In the 2013-2014 season, all three eggs did not hatch. Because all three failed and the eggs could not be positively identified, it was impossible to determine a single number of days when each egg failed. Instead, a range days to failure was determined by taking the three egg failure dates and subtracting the egg laying date for each egg. That range varied from a minimum of 10 to a maximum of 34 days, averaging 18-25 days. Cause of egg failure in the 2013-2014 season could not be positively identified, but the leading theory is non-fertilization. A possible alternate theory includes a damage to the eggs caused by a split-second night-time attack when a predator bird (Great Horned Owl?) physically grabbed at the adult on the eggs during a fly-by caught on camera approximately 1-2 weeks after the eggs were laid. The adult eagle on the nest at the time was physically hit by the talons of the predator, and the eagle was seen to “bounce” in a non-voluntary movement, but that was the full extent of the attack and reaction. Though there are too few eggs and data years to make conclusions regarding egg non-viability, particularly if it is due to non-fertilization, it is interesting to note that all data seems to converge on an average egg failure date approximately 21-23 days after being laid.
Dates and Times for Egg Hatching in Clutches of Two Eggs and Three Eggs: Refer to Table 1, and Tables-4 through -6 for data on egg hatching. Data is limited, but appears to show a random nature of hatching during AM and PM hours. Of eggs that hatched during seasons when a camera was present, about 80%-85% hatched during daylight hours. Tables show general trends on hatch dates, but the sample size is too small. The sample size also varies between the egg orders. Due to the data limitations, a comparison of dates and times for hatching between the egg orders is not recommended.
Egg Incubation Period in Clutches of Two Eggs: Refer to Table 1 and Table 4 for data on egg incubation period in clutches of two eggs. During the only season when a clutch of two eggs could be confirmed (2010-2011), the incubation period for the two eggs was identical at 36 days:2 hours. Due to the sample size of one, no conclusions can be made from the data.
Egg Incubation Periods in Clutches of Three Eggs: Refer to Table 1 and Table 5 for data on egg incubation period in clutches of three eggs. During the seasons when a clutch of three eggs could be confirmed, the incubation period for the three eggs, in order laid, was 38 days:7 hours, 36 days:22 hours, and 36 days:3hours. The sample size was three seasons for Egg No. 1 and Egg No. 2, but sample size was just two seasons for Egg No. 3; this was due to the lack of night vision on the camera, but we know that the overnight egg laying and hatching of the third egg in the missing data year made the incubation period 36 days, plus or minus about 10 hours, thereby closely correlating to the third egg sample average of 36 days:3 hours. Given the data and its limitations, an early (and far from conclusive) look at a possible data trend appears to show that the second egg hatches on average of 33 hours faster than the first egg; and that the third egg hatches on average of 52 hours faster than the first egg.
Dates and Times for Eaglet Fledging in Clutches of Two Eggs and Three Eggs: Refer to Table 1, and Tables-4 through -6 for data on eaglet fledging. The minimum period for an eaglet to fledge was 78 days from a clutch of two eggs (sample size of one season), and 75 days from a clutch of three eggs (sample size three seasons). The maximum period for an eaglet to fledge was 90 days from a clutch of two eggs, and 85 days from a clutch of three eggs. The overall average of each of up to three eaglets ranged from 81 days to 84 days.
Hatch and Fledge Success Rates in Clutches of Two Eggs: Refer to Table 3 for data on success rates. The success rate for both hatching and fledging was 100% in the only season when the clutch size was two eggs. [Sample size is 1 season; includes only 2010-2011 season]
Hatch and Fledge Success Rates in Clutches of Three Eggs: Refer to Table 3 for data on success rates. The success rate for hatching in clutches of three eggs varied from 0% to 100% per season, averaging 73.3% in the five seasons reported. The success rate for fledging in clutches of three eggs varied from 0% to 100% per season, and averaged 60.0% over the same five seasons. The last season when Patriot (male) was present (2012-2013), and the first season when Spirit (male) was present (2013-2014) where the only two seasons when no eaglets were fledged despite clutches of three eggs in each of those two seasons. [Sample size is 5 seasons; all seasons except 2006-2007, 2007-2008, 2010-2011 and 2011-2012].
Hatch and Fledge Success Rates in Clutches of Any Size: Refer to Table 3 for data on success rates. The overall success rate for hatching in clutches of any size ranged from 0% to 100% per season, and averaged 76.5% in the six seasons reported. The success rate for fledging in clutches of any size varied from 0% to 100% per season, and averaged 64.7% over the same six seasons. [Sample size is 6 seasons; all seasons except 2006-2007, 2007-2008, and 2011-2012].
Total Number of Eaglets Fledged: This nest has fledged 16 eaglets during nine seasons when observations have been made. Of these, all are from the same female, Liberty. Thirteen are from the first male, Patriot. Three are from the second (current) male, Spirit.
Based on analysis of the limited data, we are left with far more questions than answers. These include questions with regard to potential data trends at the Turtle Bay nest in particular, and with regard to the nest’s data compatibility with the Bald Eagle species in general.
Here are a few questions that are posed:
Our data shows that the male of the nested pair usually returns to the nesting territory before the female. Is this typical of the Bald Eagle species in general?
The Turtle Bay nest data shows that clutch size varied somewhere between 2.44-2.71 eggs/season. How does this compare to the Bald Eagle species in general?
The Turtle Bay nest data shows that the overwhelming majority of eggs are laid between 3:36 pm and 7:43 pm, a 4-hour window in late afternoon and early evening. Is this typical of the Bald Eagle Species?
Though there are too few eggs and data years to make conclusions regarding egg non-viability, it is interesting to note that all data seems to converge on an average egg failure period approximately 21-23 days after being laid. A common failure date would be more likely for repeatable conditions like non-fertilization, and not for random causes like distress due to accidents, intruder/predator confrontations and/or weather. Is there data on egg failure period for the Bald Eagle species when non-fertilization is the cause? If so, does it match the Turtle Bay nest ranges/averages?
We see from our data that second egg and third egg incubation periods are progressively shorter than the first egg in clutches of three eggs. Does this trend hold true for clutches of three eggs within Bald Eagle species in general? If such data is available, what is the average incubation period for each egg (as precisely as possible), and how do they compare to data from the Turtle Bay nest?
Our one season data for clutches of two eggs shows nearly identical periods of incubation. Does the second egg incubation period differ from the first egg incubation period in clutches of two for the Bald Eagle species in general? If so, is it longer or shorter and what are the incubation periods?
What is the minimum, average and maximum period from hatch to fledge for the Bald Eagle species?
Finally, if such data is available, what is the success rate for egg hatching and eaglet fledging in clutches of any size and/or separated by clutch size if data is available?
This amateur wildlife enthusiast has much to learn. It is fascinating to watch and to help document the lives of our mated pair and their eaglets with such an energetic and dedicated community of followers. Thank you to all of the individuals, foundations, and others who make this all possible.
To follow this nest here are some links:
Hancock Wildlife Forum
Friends of the Redding Eagles
Turtle Bay Eagle Cam
Table 1 - This is the Actual Data Collected by Each Nesting Season (No Statistical Summary)
Table 2 and Table 3 - These Tables Look at Clutch Sizes in Each Year and Summarize the Success of Hatching Eggs and Fledging Eaglets
Table 4 - This Table Looks at Averages and other Statistics of the Nest when Separated by a Clutch Size of Two
REFER TO THE NEXT POST FOR TABLE 5 AND TABLE 6