What’s up at the Arb?

 For those looking for updates on what is happening on the Arboretum Osprey cam….we have entered the window for hatching. During the early days of the reintroduction project that I worked on, we had so few nests that I was able to check them all every day. It was during this period that we determined that most of the time eggs hatch on day 39 of incubation. The earliest restoration project in Pennsylvania also came to the same determination. I helped put up the current Nestpole with Xcel energy way back in 2001 and have been monitoring it ever since. Ospreys originally tried to build a nest on an active power pole, so that power pole was retrofitted to prevent nesting and this current Nestpole was erected just for them. The ospreys were circling as we put up the pole and when it was up and we moved away, the ospreys landed on it immediately! It has been occupied every year since tho the occupants have changed.

This nest has had some difficulty in recent years with many changes to the ospreys that have settled there. ( I have the entire history of who the birds were every year since we put up the Nestpole.) The pair this year are new and both are unbanded so we know nothing about their history. It is unknown how old they are, and this may be their first breeding attempt. They have already run into problems when two eggs were tossed out of the nest cup (covered in an earlier post). They have continued to incubate the one egg that remained in the center of the nest, and ignored the one that was now lying outside the nest cup. In recent days I have noticed that they are sometimes leaving the remaining viable egg uncovered, even in the rain. The embryo undergoes rapid growth towards the end of incubation so it’s far more troubling to see an egg uncovered at this stage of development than earlier in the incubation process. How much time uncovered is too much is unknown. We will see. I also observed the male copulating with the female yesterday which is also unusual for this stage of breeding. In my 32 years of experience watching these birds, I have seen them start copulating again when they know a nest is failing….as if they know something has gone wrong and they want to start over. I don’t know if this nest is failing or not, but these are just some behavioral observations I thought you might be interested in. I also noted that the female has sometimes been eating on the nest, which is not the normal place for them to eat during incubation. Usually when the male,brings a fish to the incubating female, she takes it and leaves to eat, many times just to the perch on the Nestbox. The male incubates in her absense. They usually only begin eating on the nest when chicks have hatched or are hatching. So we have some very mixed messages on this nest.

With Ospreys, eggs are laid sequentially, about 2-3 days apart. Egg laying takes a lot of energy and laying them a few days apart allows the female to recoup some of her energy in between eggs. Ospreys usually begin incubating from the day the first egg is laid. This means that the eggs will also hatch sequentially, ( asynchronous hatching) which sets up a hierarchy in the ages of the chicks with the third chick significantly younger and smaller, and puts younger siblings at a disadvantage. The latest hatched chick is more likely to die in the nest, which has been seen on this nest. But it also gives the older chick a headstart and increases its chance of survival to adulthood, especially if food supply is unpredictable. Some ospreys do use a strategy called delayed incubation, which means they do not begin full time incubation until all eggs are laid. This will cause eggs to hatch closer together, ( synchronous hatching). Many birds delay incubation until all eggs are laid but ospreys usually do not. There is a lot written about asynchronous vs synchronous hatching….too much to go in to here. Feel free to google that! I will put a link to a great article about it below…

Having explained all that, we only have one egg and it is not known if it was the first, second or third egg laid….so it’s harder to know when it should hatch. Day 39 has passed, so we will wait and watch to see if it hatches later. So stay tuned! I just thought some of this info might help people understand and enjoy watching this nest.

https://scottishwildlifetrust.org.uk/2021/05/what-is-asynchronous-hatching-and-why-do-ospreys-hatch-their-eggs-days-apart/