During nine or ten years my wife Helen and I had the opportunity to observe the behavior of the ospreys that nested on navigation daymark #8, just fifty feet or so from the stern of our boat docked in Town Creek in Oxford, MD. During that time we answered many questions about the birds’ behavior as visitors wandered down the pier to look at what was going on at the nest.
I thought it might be helpful for other interested osprey watchers to have a guide to what was happening in the nest near their house or boat.
Ospreys belong to the raptor family of birds and have been shown by DNA studies to be closely related to hawks and eagles, but one look at the beak and the talons and the fearsome countenance will tell you that.
Fossil evidence shows that ospreys have been around for 13 million years – well before man made his appearance.
As in most hawks the male is somewhat smaller than the female. The average male osprey weighs about 3-1/4 pounds, the female about 4-1/4 pounds. Sometimes this size difference is difficult to see unless they are standing next to one another for comparison.
Another clue to the gender of a bird is that the female usually has more prominent brown “strings of pearls” strung across her lower neck and upper breast. The breast of the male is almost always closer to an unmarked white.
But, as the season progresses, their behavior at the nest will be the best way to determine which is which.
Their life during the winter:
When the Chesapeake Bay ospreys are not here they are most likely in Venezuela, Colombia or Panama. They generally reach those destinations by flying down the east coast to Florida, then to Cuba, Hispaniola, and Puerto Rico. From there they make a great over-water flight of about 350 miles that ends on the Caribbean coasts of South America. A few may go into Brazil and other South and Central American countries.
The male and female, and offspring, from a nest here do not migrate south, or north, with one another and may not see each other at all while they are down south.
The juveniles stay in South America through their first summer before they return to the Bay to start the courting process. It then takes 3-5 yrs of practice before they get the nest building, food sharing, domestic relationships right and finally participate in a successful nest.
It is amazing to consider that an osprey fishing in a creek here in the Chesapeake Bay was doing the same thing only a few weeks before in South America.
Predators and other threats:
Ospreys may lose the occasional fish to a bullying bald eagle. This must be an irritating inconvenience but probably is not a significant long term problem.
Nests in trees may be raided by raccoons and the occasional large snake. Nests over water are obviously less accessible to predation but are raided by great horned owls which have been known to kill a sitting adult female during the night. Great blue herons are a threat to the very young nestlings. But these raids are unusual and few ospreys are lost because of them.
Nests on official navigation marks should not be destroyed unless they present a danger or a hazard to navigation. When it is necessary to remove a nest this task should be done between October, after the ospreys leave the Bay, and February, before their return. Nests are much more likely to be successful if all the birds have to do is repair work and not a complete rebuilding from scratch.
Return to the Bay:
There are now (2020) probably about 10,000 nesting pairs [5,000 nests] of ospreys in the Chesapeake Bay region representing about 20-25% of the USA population. At their low point during the DDT crisis in the 1960’s and 1970’s there were only 1500 pairs.
If a nesting pair had established a strong and successful bond in the previous year, they will both return to that nest site. It is fidelity to a proven, successful nest site, not to one another, that brings them back and keeps them together year after year.
The male returns to his nest site of the previous year some time during the middle two weeks of March. They often fly in on the winds of a southerly storm so it is not unusual to see a very bedraggled bird sitting at the nest site the morning after such an event. After all, he may have made this 2000 – 3000 mile trip in only 3 – 4 weeks.
After a day or two of rest and light subsistence fishing, he starts to gather sticks to refurbish an old nest that may have been damaged by the winter storms or to start a new one if the old nest is no longer there.
The female usually arrives within a week or two after the male. She alights on the nest and they acknowledge each others’ presence and get reacquainted. He will present her with a few sticks which she will add to the nest. Later he will bring in a fish or two and present them to her. If the bond is made they will both start collecting sticks, cornstalks, and other items that might make a good nest. They know intuitively to collect bigger sticks for the early foundation and repair stages, then smaller material to form the bowl of the nest, and finally softer material for a lining. Unfortunately, this also includes plastic bags and fishing line and other potentially dangerous trash.
If you have a lawn or other open space near a nest, try putting out some nest sized branches for them to collect. You might get to see some dramatic, close up “touch and go’s” as they swoop down to pick them up. They may make as many as 100 trips a day to collect sticks and other building material.
As the collecting and nest building process continues, you will see how their roles begin to differentiate. This all becomes very anthropomorphic. The female will begin to spend more time on the nest. She makes it her job to arrange the sticks he brings and wedges them into the nest where she thinks they will do the best job. She has already started doing less fishing for herself and depends more and more on the male to provide food for both of them. She is the one who stays at the nest and emits the shrill begging call we all associate with these birds. In fact, the ability of the male to provide sufficient food determines the strength of their bond and the success or failure of the nest.
Copulatory activity is also going on with increasing frequency during this time. The male hops onto her back or glides in from above. If she is receptive she elevates her tail and moves it to the side so their cloacal openings can approximate one another and sperm can be transferred.
Depending on the state of the nest when they returned to it, it will take a week or two, or more, to bring it up to the level she wants for egg laying. During this time you will have seen the female standing on the nest, walking around it and arranging and rearranging sticks. Meanwhile the fertilized eggs have been moving down the oviduct, a process that takes 3-5 days, gathering calcium in the shell and acquiring their variegated brown, tan, buff, spotted coloring in preparation for laying.
The eggs are laid 10-30 days after her arrival, one each day or two, until three are in the nest. Sometimes there are only two, sometimes as many as four. The eggs are the size of a large hen’s egg with each successive one being significantly smaller. At this time you will note a dramatic change in the female’s behavior. She hardly ever walks around the nest. She is now almost always hunkered down in it, only her white head visible, incubating the eggs. The male continues to bring in sticks which she arranges while sitting on the eggs. More importantly he must bring fish. After the eggs are laid she will never leave the nest to fish. She will leave however, to stretch her wings and take a bath, often wading into the water on a nearby sand spit and dunking and splashing the water over herself. After this brief respite she shakes the water from her feathers and flies back to the nest to relieve the male who has been sitting there incubating the eggs while she took her break. He then will be off to do more fishing.
Brooding lasts 35-45 days. Remember that during this time you have not seen much of the female; she was always hidden, well down in the bowl of the nest. All of a sudden you see her again, standing on the side of the nest peering down into it and nudging around with her beak. The eggs are starting to hatch. This process takes 1-2 days for each egg. The male continues to bring fish and the occasional stick.
The eggs hatch in the order in which they were laid, one every day or two. Just as with the eggs, each hatchling is significantly smaller than the one before. Now when she receives a fish from the male, the female can be seen ripping off little pieces and reaching them down into the nest to the still unseen young. They are able to eat within a few hours of hatching. When you can first see the young they are pathetically tiny bald little things. There is an obvious size difference. The oldest is the biggest. If fishing is good they will all survive. If fishing is poor the smallest one or two may not.
About ten days after they hatched, the young will start to poke their heads above the rim of the nest. They also are toilet trained by then. It isn’t pretty, but you will see them aim their bottoms over the rim of the nest and squirt a mighty stream of white excrement into the water.
When the summer sun is blazing down the mother protects her young by mantling. She stands, back to the sun; wings slightly extended, shading her chicks from the searing heat. She gains additional cooling for herself by ruffling the feathers on her back and panting with her beak open.
When a classic Chesapeake Bay thunderstorm comes roaring through she takes her nestlings under her wings, ducks her head down, and sinks as far down into the nest as she can. As the storm rolls off to the east she lifts her head, shakes off the rain water; then lifts her wings to free her chicks.
Growth and development:
During the next 45 days or so activities at the nest become increasingly more interesting. The male continues catching and providing fish. That is his job. A breeding male must bring in six to eight, half pound fish a day to sustain this family of four or five birds. He will swoop in from over the horizon, fish held securely in his talons, aerodynamically headfirst, while the female squeaks her begging call from the nest. He may glide in over the nest and display the fish to her. Sometimes he will land on the nest and both will examine the catch. He will usually fly to a perch a little bit away from the nest where he eats the head and the entrails of the fish. Then he delivers the “fillet” to his mate who eats some herself interspersed with feeding fragments to the young. On other occasions he will float in to the nest, turn the fish over immediately to his mate who, with a few twists of her head, tears off pieces and feeds the morsels to the hungry nestlings. After feeding time you may see the adults glide low enough to drag their slimy, bloody talons and beak for several yards through the water to clean them.
As time goes on you will see more and more of the babies. They grow more feathers. They are very curious and increasingly explore the nest and the goings on around it. They become able to tear their own pieces of fish from the fish body held by the female. Thirty days after hatching they will have achieved 70-80% of their adult size.
Then, sometime in July or August you will see them stand on the edge of the nest when the wind is blowing just right down the river. They will spread their wings and feel the lift. A few days later they try a feeble flap or two. Exercising continues, strength is gained. The first flight is for a short distance, away from the nest, turn around and fly back. Fledging (leaving the nest) occurs when they are about 50-55 days old. By now it is mid-August. Bolder and bolder excursions follow. But always they return to the nest for feeding and roosting during the night. Within a week they start to learn to fish but are only successful 30-50% of the time and need supplemental feedings.
Heading south for the winter:
The female generally leaves first. Her job is done. She helped build the nest, incubated the eggs, and fed the nestlings. She has resumed fishing for herself. During the nesting time both the male and female lost about 10-15% of their weight in the effort to raise the family.
The male stays around to continue feeding the young their supplemental feedings. It will take two to three weeks of practice after fledging before the young can fish well enough to be independent. Finally he disappears from the scene and the young are left to fend for themselves. It is sink or swim time for them. Finally they too start south. Now it is September and by October it’s hard to find an osprey. The squeaking is gone; the honking begins. The Canada geese will be here soon.
Helen and I made the observations.
Alan F. Poole supplied the scientific background in his book: Ospreys: a natural and unnatural history: Cambridge University Press, 1998.
And later in his follow up book: Ospreys: the revival of a global raptor: Johns Hopkins University Press, 2019.
Since I first wrote this essay tracking technology and the information learned from it has increased dramatically but the biologic essentials remain the same. All of this new information is reflected also in Poole’s new book.
These web sites are informative:
Originally written April 8, 2005 and updated 2013/2014/2016/2018/2019/2021
Several photos used here were provided by Adkins Arboretum photographer, Kellen McCluskey. You can reach her at kmccluskey@adkinsarboretum.