Why are female Red-tail Hawks almost always 30% larger than the males?

It is called Reverse Sex-size diamorphism. There are several theories as to why this happens with Red-Tail Hawks.

The first is that the females had to be larger to protect themselves from the feisty males.

The second is that the females selected smaller males to be their mates because the size difference allows each of them to hunt different prey and reduce any competition for food between the pair. Raptors that hunt birds are generally smaller and faster with the female specializing on larger prey. At the beginning of the nesting season, the female becomes an active hunter and again when the nestlings are larger. Generally the adults tend to partition the prey resources in their territory.

Another theory is that the females need to be larger because they must accumulate reserves in order to produce eggs. When they are incubating eggs and brooding young, they rely on the male to feed the family. Red-tail hawks are usually born in April when the prey are sluggish and just coming out of hibernation. Small males can make quick turns. In the case of the Red-tails on the Cornell Campus, Arthur W, the male, is known as the ‘stealth bullet.’ He is quick, fast, focused, and quiet when he hunts. Big Red’s former mate, Ezra, was like Arthur W a great hunter but he was also about 30% smaller than Big Red as is Arthur.

Ezra has his wings spread and his legs tucked tight. If you look closely you will see that he is delivering two items to Big Red. The first is a snake and the second is a flower!
It is very difficult to get Big Red and Arthur to stand up straight at the same time. They certainly do not listen to requests from humans!This is the closest I have come to finding an image that might be able to show you the difference in their size. Big Red is on the left and has her head leaning back preening and she is a little more slumped than Arthur who is on the right.

In terms of the nestlings, the only way to positively known the gender of the bird is through either a DNA sample or you see them laying an egg when they are older. Everyone likes to guess the sex of the chicks growing in the nest cup. Using reverse sex-size diamorphism, we speculation whois a female by their overall size as they grow in the nest and the size of their feet. But again, no one can be absolutely sure. In the past one of the small nestlings was always believed to be a male. This particular bird injured its wing and, as a result, had to be taken to a vet. The bird’s DNA was tested and, to the surprise of many, it was determined to be a female. So not every small Red-tail Hawk is a male!

In terms of the relative size of each J, J1 is the largest of the three. Early in its development, everyone noticed the large size of its feet. Now as a juvenile, J1 remains the largest of the three siblings. But there is something else about J1 that I personally find fascinating. J1 is very much interested in nest maintenance. She can be seen, even today, mimicking Big Red in attempts to vent the nest bowl. She rearranges the nest twigs and on the second day after she fledged, she brought a branch to the top of the Rice patio. There seems to be 100% agreement that J1 is going to be a good mama like her mother, Big Red.

J3 in the front and J1 behind watching J2 fledge. Look at the difference in the size of their legs as a starting point.

J2 on the other hand is also a large bird, not as large as J1 but larger than J3. J2 was actually the first egg to be laid but the second to hatch and is, therefore, actually the oldest. His fledge was more like a fludge when he climbed up on the light box and then slipped but his flight was still remarkable. J2 has continued to be a very, very strong flyer already interested in hunting. The shape of “his” head – you will notice that I continue to use the term “his” – is also different than that of J1. He has an intense focus and besides bugs and insects he has already been interested in small birds in the territory. To my knowledge he has not caught one. He is also very aggressive. A few of us believe he is truly a male and will be a great hunter like his dad, Arthur W.

J3 eating his first prey drop after fledging.

J3 is the problem child that everyone loves. . J3 was born four days after the other two and it was also the last egg to be laid. Taken together, J3 is actually a week younger than its siblings. That is a lot in the life of a Red-Tail Hawk chick. It may also account for the fact that Arthur W brings food to the nest tower in an apparent effort to feed J3 away from the two larger siblings. Let me try to explain what has been happening. Normally the juveniles are spread out being individuals. This trio is quite different according to the people who have monitored all of the broods on camera (since 2012). It is because they tend to congregate together. To hang out. If one is on the Rice patio, all three might be there.

All three on what is called the Rice patio. Arthur W often makes food drops here but it is also a safe and flat site where they can sleep in the sun or practice their skills.

They might all be on one of the towers including the nest tower. Still, on occasion, they go their separate ways.

J1 in one of the oak trees near the Fernow Building looking at a squirrel.

A prey drop is just that – one of the parents dropping lunch. The juveniles will, unless they have recently eaten, fight for the food with the most aggressive mantling the prey. Unless the prey item is too large for the chick to eat all at once, sharing doesn’t seem to happen. So, because of J3s size and a seemingly lack of aggressiveness in comparison to J2, J3 is somewhat at a disadvantage. This is the reason that I believe Arthur W still supplies food on the nest to the little guy.

Arthur has been dropping lunch off to J3 for several days now. This was around 3pm. He seems to be looking for J3 who arrives almost immediately.
J3 mantling the prey that Arthur W dropped on the nest. Remember mantling is a way of protecting the food item so that no one else can steal it.
J3 finishing up his afternoon snack.

For now, the gender of the three juveniles is sheer speculation. There are no banding practices and no GPS monitors on the Js. In other words, no identification. It is only when one of the juveniles might find its way to the vet and be both recognized and tested that the sex would be determined.

Tomorrow we are going to talk about the importance of preening.

It’s Nearly Mother’s Day and I tip my hat to Big Red, a 17-year-old Red Tail Hawk for “Mother of the Year 2020”

If my mother were alive, I hope that she would understand why I am so adamant that a Raptor Formel should be nominated for Mother of the Year 2020.  It was, after all, my mother who carried the duck my father had given me to my grandmother’s every day on her way to work.  There the duck lived in a specially designed ‘cage’ or stayed in the hen house.  On occasion, the duck would join my grandmother and me for a swing on the porch. I know that my grandmother would approve as she had a fondness for all living creatures, as did my dad.

2020 is a very unusual year.  Since the end of 2019, the international community has been paralysed by COVID-19 that has killed nearly three million people as I type this.  Many are without jobs or health insurance.  Entire countries and cities have been under various levels and length of lockdown.  The funeral homes cannot handle the number of dead.  Hospitals have run out of protective gear for healthcare employees.  And there remains uncertainty from world leaders on how to continue to manage this virus.  Is it safe for people to be outside amongst one another?  or should we be locked down longer? When will a vaccine be available? When will people be able to travel? When will schools open? Will people have jobs? Will there be enough food?  The level of anxiety, coupled with the number of people working from home, has caused people to seek solace in cooking, reading, and learning.  Many have turned to nature with the number of individuals watching bird cams sometimes more than five times the norm.  I am one of those people.  I have a fondness for hawks ever since I first stood about a half metre away from a female Sharp-shinned hawk in our garden three years ago.  That moment had a transformative impact on my fondness for these regal birds of prey.

In early March I began following the exploits of a pair of Redtail hawks with their nest on the ledge outside the office of the President of New York University.  They were Aurora and her new mate, Orion.  Having laid three eggs, the pair took turns incubating them so the other could eat.  On the morning of March 26, Aurora did not return.  She did not return that evening nor the next day.  Everyone was in tears and devastated beyond belief.  That pair of hawks symbolised hope for the people watching who were living in the hardest-hit area of the United States at the time.  The virus was so harmful and so many people were dying that the parks were being turned into field hospitals and temporary burial grounds.  One of the members suggested that we switch our attention to the Bird Cams run by Cornell University.  And that is how I met “Big Red.”

Big Red is specifically a Buteo jamaicensis.  Technically this is the order Accipitriformes, and the family is the Accipitridae.  Leaving the fancy language aside, Big Red belongs to one of the most common hawk families in North America.  There are approximately two million.  The birds, along with their nests, eggs, and feathers are protected by treaties on migratory birds throughout the Americas.

Big Red was born in 2003 and was banded in Brooktondale, New York that fall.  When you look at pictures of her, you will notice that she has a large dark red head, nape, and throat and the most magnificent red tail feathers.  She currently resides on the campus of Cornell University in Ithaca, New York with her mate, Arthur.  Redtail hawks mate for life.  Arthur was born and fledged in 2016, making him a whole thirteen years younger than Big Red!  Arthur and Big Red completed their first breeding season in 2018; this is their third year for successfully raising chicks.  Arthur has a real pale head, chest and nape, not unlike the notorious Pale Male from Central Park in New York City.  For the last two months, this pair of hawks taught me so much and inspired so many others at a time when the world needed something beautiful.

Big Red is the epitome of dogged persistence and dedication to the task of taking care of her nest, incubating her eggs, brooding and feeding her chicks, and being a model for them for their successful life as raptors.  Since the first records at Cornell in 2012, Big Red has successfully raised twenty-one chicks!  There could be almost that many more uncounted – before the cameras – from 2006 (?) through 2011.

Big Red and Arthur are often seen in the late fall inspecting their nest which is eighty feet above the ground on a lighting tower on the campus of Cornell University.  Hawk nests can get very quiet high and wide as the couple continue to refurbish and redecorate annually. Typically, hawks have one nest, but Big Red and Ezra actually have two. For the last couple of years, they have favoured their current nest.

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Sticks and twigs ranging in size between eight inches and fourteen inches are carried from the ground to the site in anticipation of eggs being laid.  Redtail hawks lay between one to four eggs depending on the local food supply.  Typically, Big Red has a clutch of three eggs. Redtail hawks usually incubate their eggs from 28-35 days although in 2012 Big Red sat on her eggs for 35-38 days with the longest being 42 days in 2013.

Observing the weather in Ithaca, New York made me quite happy, actually, to be living in Winnipeg.  There were quite a few days where it was frosty with snow, but on April 17, Big Red found herself encased in ice and snow as she incubated her eggs, ensuring the survival of her chicks.

Below are the three eggs in the clutch. Big Red laid egg number 3 at 1: 23 pm on March 24.  She immediately “told Arthur” and began incubating the clutch.  Both Big Red and Arthur take turns sitting on the eggs until they hatch.

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There are several things to notice in this image.  The first is the nest bowl which is lined with soft materials.  Big Red and Arthur will continually maintain the nest bowl, making sure that it is big enough to hold the checks and that they cannot harm their tiny legs and talons.  Second, notice the pine.  The hawks bring these into the nest in preparation for the hatching of the chicks.  They help keep flies and their larvae away and protect the chicks from disease caused by flies.  And third, the bottom left egg has a pip, and the chick is beginning to use its body to crack the shell.  Pipping is when the chick first breaks through the shell with its “egg tooth”.  Sometimes it takes the chick up to twenty-four hours to completely break out of its shell.

pipping red tail hawk

It takes a lot of energy to hatch, and the newborn chicks are often tired for the first day of their lives.  Before long, however, their feathers will have dried off, and they will be covered with white fluff.

J1 hatching and J2 pipping

At first, the chick’s eyes do not focus well, and they do not quite understand what “food” really is and who is feeding them.  There is an awful lot of pecking and bonking that goes on with the siblings.  This settles down after about four or five days.

The chicks are not given names.  They are assigned letters of the alphabet.  In 2012, when Cornell University first installed its hawk cameras, the chicks were given the letter C after Cornell.  In 2013, the chicks were given D and so on until 2020 when the chicks have been given the letter J.

In the image below, the first hatched chick, J1, is trying to take a bite out of J2’s head!

Two chicks

J1 and J2 tiny fighting over a small piece of meat

3 chicks

It is up to Arthur to bring food for Big Red and the chicks.  Arthur’s territory is abundant.  Until the chicks fledge, he will bring chipmunks, squirrels, pigeons, Starlings, snakes, voles, and rabbits to the pantry.  There is never a shortage and viewers have been surprised – shocked even – at the plentiful supply of animals and at the talents of both Arthur and Big Red at hunting.  She has, in fact, brought some meals back with her when she has gone off the nest for a bit.

Arthur filling up pantry on May 1

These chicks have a lot of food security, thanks to the excellent hunting skills of Arthur and Big Red.  The rails to keep them in the natal nest are made out of their dinner.  Pine is scattered about to keep away the flies, and sometimes you could see the chicks sleeping with their head on a furry pelt.

all the food

Big Red fed and kept her chicks warm during a period of dangerous wind and heavy rain on May 1. I don’t think anyone slept that night and there was certainly a lot of emotion, even tears.

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wet2

And she has patiently made sure that each and every chick, from the first to hatch J1 to the tiny J3, is fed.

Here the three of them are lined up for an afternoon meal.  Little J3 is front and right with J2 front left.  J1 is behind both.  The trio managed to eat an entire chipmunk!  That was just one meal.  Big Red is feeding them a lot.  She still spends the night brooding, keeping those chicks toasty warm.  Soon they will sleep on their own at night.  Eventually, they will jump and flap their wings, preparing to fledge.  By then they will also be eating on their own, and Big Red and Arthur will courier food to the nest throughout the day.

3 lined up for a feeding

By the middle of June, all of the 2020 chicks will have fledged.  They will spend the summer learning how to hunt, and by fall they will be gone to find their own territory.  At the age of two, they will get their distinctive red tail feathers, and by three, they will have families of their own.

In the meantime, Big Red and Arthur will enjoy being empty nesters, and by late fall they will again repair their nest on the Cornell campus in preparation for 2021!

All of us who have gotten to know this hawk family and to learn a little about raptor behaviour have been inspired by the sheer dedication Big Red has maintained during the most horrid of weather.  We have watched J1 grow to be four times the size of “Little J3”. We have worried that the little one might be left out.  This was decidedly not the case!  J3 is right up there, and Big Red makes sure each is fed well, that they are safe and warm.  She is currently teaching them to preen their feathers and by observation, J1 today began flapping its wings.