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Posts Tagged ‘Predators’

It has already been over two weeks since the Great Horned Owl roamed the surface of Ship Island looking for a late-night snack. You would think that over time, the terns would settle down and begin to behave “normally.” But that’s not the response we’re seeing. Even today most of the colony begins to sweep high above the island soon after sunset, then disappear quietly out over the ocean. It seems that small numbers do come back to warm their chicks and eggs, but the majority aren’t seen again until sunrise.

The owl caused full colony abandonment during the nights on the island. This occurred for over a week straight, which might have led to some long-term effects on chick physiology. Many of the eggs didn’t end up hatching since they weren’t incubated during the nights. But, some eggs were still able to make it. Typically, eggs hatch after 21-23 days of incubation. With the owl disturbance, incubation length increased, which is why our chicks arrived slightly behind schedule.

Although we have many healthy chicks all around the island, there are a select few that are showing what we assume to be the negative consequences of this over-exposure to the cold and wet nights on a Maine island. When terns incubate, they are constantly rotating the eggs around. This allows for even nutrient and heat distribution throughout the egg as well prevents the embryo from sticking to the shell, allowing it to float in the middle and develop successfully. Without this constant rotation, it’s possible that the chicks could have developed certain physical defects.

 

Not only are we seeing odd chick appearances, but we are also seeing a huge change in colony behavior. The terns have been extremely sensitive to any presence that might seem or sound threatening. This even includes species that are not considered predators. In order to protect themselves, terns often mob, dive-bomb, or attack the predator. They also might flee, just as they did with the owl. Their actions depend on the level they feel threatened themselves versus how threatened their young are. We’ve observed terns going after Common Eiders, Dowitchers, and Harbor Seals. They were even frightened by the sound of a nearby fishing boat. Although we can see that these species are here to do no harm, it’s still good to see the terns working hard and being extra protective.

These actions displayed by the tern colony isn’t uncommon among populations who are or were at risk of nocturnal predation. In fact, it has been witnessed in several other Common Tern studies where owls were present. Looking at a well-known colony observed by Monomoy NWR in particular, you can read how they experienced very similar results years ago (Nisbet and Welton, 1984).

It’s amazing how a single bird can influence an entire colony in only a few days. This owl left an impression on the terns to last the entire season. The fate of this years fledgling was greatly altered and we can only hope that next year the colony works to make up for this years loss.

On a better note, we’re still waking up to a few more chicks every day, and we’ve already seen a few fledge! Based off of our provisioning efforts and weight measurements, our current chicks are growing at a steady rate and being fed a healthy diet, which mostly consists of Atlantic Herring. Some chicks are being fed so much that they actually have to lose weight in order to lift themselves off the ground and fly! We’re glad to finally start seeing our chicks transform into successful adults!

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One of my healthy provisioning chicks. Not quite ready to fledge yet!

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Chicks that were recently born. Only a couple days old!

Only one more week until the island closes. This season really flew by! I’ll keep you updated on any more unusual or exciting events happening on Ship!

-Amanda

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This past week on Ship has been very similar to what has been going on at Metinic and PMI. Most of our days have been pretty dreary. On these foggy and rainy days we spend our time reading (A+ to Morgan for reading 8 books so far), eating snacks, catching up on sleep, writing letters, drawing, and staying updated on what’s going on in the real world. It is relaxing, but we’re anxious to get back out there and get a closer look on how our terns our doing.

When it’s not too foggy out, we are able to sit outside and watch the colony. We don’t sit too close because we don’t want to surprise or scare them. We’ve been doing this frequently to deter the Peregrine Falcon who has been stopping by from the island.

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Sunset view while watching for predators

Before the bad weather, we spent most days attempting to re-sight bands, making productivity plots, and trapping adult terns to band, measure, and weigh. To trap the terns we use a Treadle Trap. We first need to replace the eggs with fake eggs. This prevents the tern from damaging their eggs once he/she is trapped. After this we place a wired box over the nest with one end open. When the bird steps through the opening onto the pad, the door will shut and the tern is unable to escape. We quickly retrieve the bird to collect our data, put back the original eggs, set him free, and repeat. It was pretty cool when I got to hold and release my first tern!

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One of our productivity plots

The results from our GOMSWG census indicated that we have about 620 nests in total. Hopefully we’ll start seeing some chicks soon!

-Amanda

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