Good morning from the warm, humid Eastern Shore! It is a lovely day, even with the humidity. My thermometer is showing 77 degrees, partly cloudy and quite pleasant for outdoor activities. The water is sparkling and there is lots of activity from recreational boaters. The big excitement since our last blog involved egg #4, which has broken and is no longer visible. Dr. Spitzer was right on with his prediction that the last egg would not hatch. The exact fate of the broken egg remains a mystery, even with all of our modern technology and a bevy of dedicated nest watchers. Once again, Mother Nature has had the last laugh!
Tom enjoyed a Father’s Day fish on our boat lift this morning before heading to the nest. This is not his favorite spot to chow down, so I guess he is mixing it up a little today. The most popular spot for Tom to eat is on our neighbor’s dock, specifically on the top of the swim ladder on our neighbor’s dock. As I write this, Tom is sitting on the camera and looking around for his next source of sustenance. He does spend most of his downtime on top of the camera. Keep in mind that it is of utmost importance that Tom eat his fill before anyone else, as without him to fish and bring food back to the nest, none of the others would survive. Audrey must also eat her fill before the nestlings, for without her and Tom, the nestlings would not make it for long. Tom has been an exemplary father to his little osprey family, so we dedicate this blog to Tom and all of the other fathers out there.
We had some communication with Dr. Spitzer a couple of days ago, and inquired about the types of fish upon which our ospreys feast. Dr. Spitzer advised that according to Jim Uphoff, a Maryland DNR (Department of Natural Resources) biologist, menhaden are the most prevalent fish consumed at our site. According to Dr. Spitzer, other types of fish that ospreys will eat in this area are catfish (3 species), white perch, yellow perch, oyster toadfish (marginal prey as they are very well defended) and needlefish. He did not mention rockfish (striped bass to those of you who don’t live around these parts), although one of our astute Facebook watchers was sure he saw a rockfish brought to the nest today.
Dr. Spitzer also had a comment about Chick #3 that I will share with you for informational purposes. I quote Dr. Spitzer: “Whether the runt will make it may remain unresolved for quite a while. Quite a drama.” Dr. Spitzer has provided another chapter in his “Ospreys Explained” reports. This one is about Osprey Population Dynamics. It is longer than some of the others, so we will post it in two parts. Here is Part One:
OSPREYS EXPLAINED, by Dr.Paul Spitzer
Chapter #5: Osprey Population Dynamics, or “Life in the Osprey Garden”
Ultimately, this is about the balance between reproduction (the “credit” side of the osprey ledger) and mortality (the “debit” side of the osprey ledger). An increasing breeding population means ospreys are “in the black”, a profit of sorts. A stable population is “break-even”, or non-profit. A declining population is “in the red”, with a net loss of ospreys.
That’s what happened during the DDT era, when DDT and the related pesticide dieldrin snuffed out reproduction by destroying egg viability, and also poisoned some adult breeders directly, increasing the adult mortality rate (which is normally about 15% each year). Local populations in heavily-contaminated CT and RI “crashed” at 30% annual decline, with almost no reproduction. The Chesapeake was far more lightly and locally contaminated, so it fared much better, and was primed for the big osprey population growth we have recorded post-DDT. The present-day Chesapeake is truly An Osprey Garden, with 3500 active nests.
Although stable, DDT eventually broke down in local ecosystems, and contamination was gradually reduced. Then the fundamental question arose: What level of reproduction is necessary for a stable population? This break-even point is known as the “Replacement Rate”: It is a core concept in Population Biology, including that of human beings. Several components of osprey life-history contribute to replacement rate. They are known collectively as “parameters”, and some are easier to measure than others. During the 1970’s, as osprey reproduction recovered, I undertook to make those measurements in the field, from the wild (but unwittingly cooperative) ospreys, combining them in a population model to estimate osprey replacement rate. I did this with a decade-long field study on the remnant breeding population between New York City and Boston, which fell as low as 109 active nests in 1975 and 1976. Pre-DDT, there were over 1,000 nests active in this region, where much DDT was used. Today that population has mostly regained it’s numbers, and in some locales such as MA is far more abundant than previously (but see discussion of the “Gardiners Island Anomaly” that concludes this chapter).
Osprey nests aren’t too hard to find on a populated coastline. Every year I counted the active nests, and thus the total breeding population. The 8-week nestling period enabled a total count of nestlings, with some help from volunteer Citizen Scientists. (This participatory learning approach is in much wider use today.) Most of the later counts were done by state nongame wildlife biologists (another innovation), with years of patient data collection. “Reproductive Rate” is expressed as mean (average) number of young fledged per active nest (Y/AN), or “Productivity”.
These were blissful springs for a young biologist, with sea breeze in my nostrils. I trudged over the firm peat of gem-like northern salt marshes, rode ferries to offshore nesting islands, ran ladders up high nest poles, ducked the talons of defensive female ospreys, trapped and color-banded breeders to measure their annual survival, color-banded entire regional annual “cohorts” of young, then watched with binoculars and telescope for their return as breeders later in the decade. These measurements taken in nature required patience, accuracy, and sample size, because I submitted the population study in 1980 as my Ph.D thesis at Cornell University. I had the privilege of directly living my work; as I have tried to do ever since. I bonded intimately to that fine old historic post-glacial coastline, LI-CT-RI-MA, where various cultures of man have lived for millennia. During a follow-up Chesapeake osprey study in the 1980’s I bonded to the Bay region; and I have now lived here for thirty years. Study of living things gives benign and creative form to my own life. And perhaps I have gained some ability to think like an osprey.
I estimated the replacement rate of both these osprey populations at 0.8 young fledged per active nest (0.8 Y/AN). How did I get there?
We need to return to discussion of fledging life-history (end of Chapter #4), and combine it with the very different roles of male and female breeders (Chapter #2). The learning curve of fledging ospreys includes recognition of local nest sites, local habitat types, local prey species, fishing methods for those species, and handling methods for those species. Catfish skulls, for example, are equipped with barbed, defensive spines that could potentially blind an osprey that fed too close to the head, or wound the feet of an osprey that held the catfish wrong. When people ask me how ospreys catch catfish, I answer “very carefully”.
Fledging male ospreys are going to be the providers of food for future breeding efforts, and their local learning period before their first southbound migration probably marks a critical beginning for their local foraging skills. Females, by contrast, are going to be insulated from local resources by male provisioning for needs to avoid inbreeding.much of future breeding seasons. Thus it is not surprising that males tend to be “homebodies”, returning to breed in proximity to their fledging habitat. Females are much more prone to dispersal, thus providing the gene flow that a healthy population
How did I demonstrate the tight connection between male return and population dynamics? First, I color-banded the limited young produced in CT and LI in 1972 and 1973, and scoped all active nests for the rest of the 1970’s so I could relocate them as breeders. This also allowed me to quantify another absolutely critical population parameter: Age at first breeding. In that northern population, artificially depressed relative to resources, the commonest first breeding was at age 3 (the “mode”), the range I observed was ages 3 to 5, and the mean was 3.7 years of age. Please remember this very important, hard-won data set.
Certain biologists love to band nestling ospreys with the aluminum, numbered bands issued by the USFWS Bird Banding Laboratory at Maryland’s Patuxent Wildlife Research Center. They have done this passionately for decades, going forth late every spring to band the annual “cohort”. These saturation banding efforts have yielded a windfall of population data and understanding, which I have been able to harvest. Sometimes it’s enough just to see what percentage of breeders of each sex wear these bands. Ospreys’ long pale legs, and their attachment to the nest, make this easy. For example, in the spring of 2005 I returned to my northern “Ph.D.” population, now recovered from the old DDT days. Biologists Gerald Mersereau and Henry Golet had banded 1,684 nestlings in CT (but none across the Sound in LI) during the 15-year period 1988-2002, and I wanted to have a look at the dispersal of the breeding survivors. The highest nest density and core banding area is the Connecticut River estuary (70 nests). There I found that 70% of the males I checked were banded (32 of 46), but only 28% of females (17 of 61). Sampling to the south across Long Island Sound, only 10-30 miles away, I found only 3% banded males (1 of 31), but 16% banded females (7 of 43).
Thank you, Dr. Spitzer for another fabulous report. Although the camera is fascinating to watch, your insight and expertise add another whole dimension to our osprey experience! The rest of Chapter #5 will be posted in the next blog.
That’s it for now. Happy Father’s Day to all of the fathers in our midst, regardless of your genus and species!
Until next time, we remain,
Crazy Osprey Man and Mrs. Crazy Osprey Man
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