Continuing Broodmate Aggression at the UFL-Gainesville Osprey nest

2 May 2022

I have not spent the time I should have observing the UFlorida-Gainesville Osprey nest since the third hatch starved to death because of siblicide. The nest appeared to have ‘calmed’ down after that. I am, thus, very grateful to ‘R’ who took the time to write to me telling me about the change to aggression.

While my research focuses on that third hatch, it appears from the instances of aggression that we have seen this season – that ‘something’ is happening out there in the nests. Before we chat about these sorrowful avian behaviours, I want to go back to the book I am reading, The Eagle Man. It is about Charles Broley, a Canadian, who spent his time between here and Florida observing Bald Eagles and banding thousands. Broley’s observations are from direct observation or information provided to him by others. He did not have the benefit of the streaming cams and data collections that we have today. Still, some of his observations continue to hold true such as the ‘Northern’ Bald Eagles are larger than the ‘Southern’. In another instance, he notes that the ‘Southern’ eagles are more aggressive than their ‘northern’ counterparts. In terms of nest aggression, Broley said: “The more demanding of the babies gets the greatest amount, however, for the weaker, smaller one is always neglected for the more aggressive one. As a consequence on that is a little smaller, a trifle weak, perhaps, tends to grow weaker through lack of food, abuse from the older bird, and general neglect, while the stronger one, because it gets all the food, increases in size and strength rapidly (52).

In their article, “Why do some siblings attack each other? Comparative analysis of aggression in avian broods” by Alejandro Gonzalez-Voyer, Tamas Szekely, and Hugh Drummond (Evolution, 2007), the authors list eight traits that they are testing to determine if any, some, or none drive aggression on nests. Those traits are:

• feeding method (monopolization of food). Direct feeding when food passes directly from the adult beak to the chick’s beak. This allows the dominant bird to violently exclude any competition. The authors believe that when food is placed on the nest floor in indirect feeding it is more difficult to monopolize. The feeding method theory predicts that “a greater use of aggression in species in which direct feeding predominates throughout the nestling period).
• effective weaponry (pointed or sharp beaks)
• limited escape possibilities (nest site typography)
• differences in age and size of broodmates (hatching asynchrony)
• large food parcel size
• small brood size. As brood size increases the authors observe that it is more difficult to intimidate and attack.
• aggressive potential (maturity and body size)
• slow food transfer

The researchers studied and compared several species including the Accipitridae (eagles, hawks, and buzzards) along with Boobies, Anhingas, Ibises and Spoonbills, Egrets and Herons, Pelicans and Kingfishers. I was, of course, disappointed that they did not include Ospreys. Following lengthy observational study, the authors concluded that indirect feeding, small broods, and long nesting periods are significantly correlated with broodmate aggressive competition. They also agreed that more studies needed to be conducted because exchanging or removing even one factor might alter the results of the research. I did find it interesting that out of 58 Eagle nests studied compared to 52 falcon nests, the eagles were 81% more inclined to nest aggression. They thought that factors such as larger clutches, a shorter nesting period, and one other factor — that the broodmate aggression being eliminated through evolution- could be at the heart of why falcon nests have little siblicide. It is rare to find siblicide in falcons and hawks while, as we have seen recently, it is much more common in eagles and ospreys.

Within this season, we have seen siblicide, the deliberate killing of a sibling, at several nests. These include the third hatch, DH16, at the Dale Hollow nest on the border of Tennessee and Kentucky; the third hatch at the UFlorida-Gainesville Osprey nest; the second hatch at the MN-DNR nest who was physically abused (with holes in its head) in the nest and, ultimately, successfully shoved off the nest on the third attempt; and the biological chick killed by the foster chick at the Pink Shell Osprey nest in Florida. In addition, it is difficult to know what the outcome would have been at the Captiva Osprey nest if the oldest sibling had lived. Big did prevent the female and the two younger siblings from eating for a period of 72 hours before it literally dropped dead. Would the death of the youngest sibling at the Duke Farms Eagle nest be classified as siblicide?

The situation at the UFlorida-Gainesville Osprey nest has not stabilized. The information I received this afternoon from observations on the nest indicate that the eldest sibling is relentless in stopping the middle sibling from eating. Observations of the behaviour include – the mother feeding the dominant sibling, the dominant sibling abusing the younger even if its crop is full to overflowing, the younger one having to expend energy and become aggressive to even get a few bites of fish. I do not know the number of deliveries in a day or the size of the fish. The camera re-wind is such that it is not conducive to discovering this information. Tomorrow I plan to write to Gainesville and see if I can get the data which someone must be keeping – and also to find out precisely how old these osplets are. While there is no truly ‘safe period’ against broodmate aggression, it is normally expected to stabilize around the age of one month. That said, the dominant aggressive osplet in the Port Lincoln nest did throw its sibling over the nest to its death when the chick was 65 days old. This is why we cannot predict what will happen.

In the top image, the eldest chick, Big, has learned to get between the mother’s legs preventing Middle from getting food and from the advantage of having the adult in between the two birds. Thank you ‘R’ for drawing this to my attention.

Middle had to expend a lot of energy flapping its wings and trying to mantle to get a few bites. The ratio was more than 10 bites to 1 with Big having the advantage. I do not know when the Middle sibling last had a good meal with a crop.

We must remember that the more food Big gets the bigger and stronger that bird gets to the detriment of the younger. It is hot on top of that light stand in Florida and the fish is the only hydration the chicks get.

Middle is once again rushing at the Mum to try and get some food.

Another small fish comes on the nest later. In the image below, Big has been eating and has a huge hard crop. Middle has no crop. Big has been eating but Middle moved slightly and caught Big’s attention who went on the attack.

Big goes back to eating.

Eventually Middle gets a few bites of fish once Big passes out in a food coma. In essence, Big ate almost two entire fish with the Mum and the Middle sibling getting very little.

One thing that is missing in this discussion is the fact that the Mum also needs to eat. It was 25 degrees C today or 77 F on the ground and later in the day (9pm). It is very hot on top of that light stand. I cannot predict what will happen on this nest or on Middle’s survival. Normally I would say not to worry but Big has tried to push Middle off the edge – when I see this behaviour my antennae really begin to alert. It is going to depend on many factors some of which we just do not know – will there be an increase in size and number of fish being delivered? is there a genetic disposition towards aggressive behaviour at this nest?

I am sorry to bring you this sad news. I had so hoped that the aggressive behaviour would have ended with both chicks thriving. I will be keeping a closer watch on this nest and I am very grateful for the letter alerting me to the change. If you know of other nests where siblicide has occurred since 1 July 2021 to the present, please do let me know. It would be much appreciated.

Thank you for joining me. Take care everyone. See you soon.

Thank you to the UFlorida-Gainesville for their streaming cam where I took my screen captures.