Signs of Spring

Despite the chilling temperatures this week, spring is in the air. I was glad to hear the chickadees singing their “Spring’s Coming!” song as a reminder while I shoveled an inch of fresh snow off the sidewalk.

Some migrating birds are returning, including my first-of-year (or “FOY” to serious birders) red-winged blackbird singing last week. I also saw my FOY American Kestrel as Aldo and I drove to Ashland for a program on Monday. That kestrel looked like a fluffy cotton ball as he balanced on a powerline, feathers puffed out to stay warm. I wondered if he regretted coming back so early!

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Peregrine Falcons in southern Wisconsin are well into their nesting season with many pairs already sitting on eggs. I caught a quick glimpse of an egg on this camera in Pleasant Prairie, Wisconsin and have been checking back regularly to try spotting it again. While we don’t know how long our snow will last, birds farther south and optimistic migrators like the kestrel remind us that “Spring’s Coming!”

 

 

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Foot Tuck

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I am always looking for signs that the birds are relaxed. Any time their head is down (while eating) or covered (while preening their wings or sleeping), they are more vulnerable to a threat. A predator might take that opportunity to launch a surprise attack, so they only risk covering their eyes when they feel safe. Other comfort behaviors, like Carson’s foot tucked in the photo, indicate that they are comfortable and and not worried about their surroundings.

Perch Variety

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Aldo tried out two new perches this week to spice up his time indoors. First he sampled a flat block perch for an afternoon, then he tried out a small hemlock branch that I bent over his normal perch. New substrates keep him engaged, test his balance, and promote foot health. Wild raptors perch on a variety of branches, wires, fence posts, and ledges, constantly changing how weight is distributed on their feet. Captive birds also need options; otherwise they risk developing pressure sores that can develop into swelling called bumblefoot. Aldo has one favorite perch, but it is beneficial to switch things up every once in a while.

Falcon pellets

Learning about digestive systems and pellet schedules is interesting (at least for me!) and it is also useful to know when caring for the birds. This week Aldo the American Kestrel wasn’t interested in training and wouldn’t take any food from me. This odd behavior could have multiple explanations. Did he suddenly decide to boycott quail? Was he overweight or sick? Or did he just need to eject a pellet? Soon enough, he opened his beak. With a little cough, he shook his head and flung this tiny pellet at my arm. He simply had to empty out his digestive system to make room for more food.

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Occasionally Aldo’s pellet isn’t a pellet at all. Sometimes he coughs up a few pieces of gravel. Oddly enough, this is a normal behavior, especially for falcons that often pick around feathers, fur, and bones on their prey. Without these materials to form a pellet, falcons replace them by ingesting small stones to clean out their crop. Knowing this tidbit of their natural history keeps me from being alarmed whenever it happens with Aldo. Instead I marvel at a bird’s ability to take care of itself, even if it means eating gravel from the floor of the mew.

 

Full Crop

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I couldn’t resist taking this picture of Carson yesterday, even though it creates a rather backwards story. Last week’s article described pellet formation in raptors; now I’d like to hit the “rewind” button to see what happens to the food right after it is swallowed.

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After swallowing a rat, Carson’s meal slides down to rest in her crop, a very elastic portion of the esophagus that is used as temporary storage. The crop becomes apparent after a large meal as it fills and forms a bulge under her throat (see the arrow in the photo).

The food will slowly make it’s way into the bird’s two-part stomach. First is the proventriculus, where digestive juices start breaking down meat and bones. It will next pass into the ventriculus (also known as the gizzard). This section is known to be the grinding stomach for many birds, where powerful muscles crush food to aid in digestion. The meat in a raptor’s diet is relatively soft compared to tough seed diets of some birds, so the raptor’s ventriculus is less developed. They rely more heavily on chemically breaking down their food than mechanically crushing it.

The disintegrated meal will continue into the intestines where nutrients will be absorbed. The undigested fur stays in the ventriculus where muscles wring out extra liquid, then compact the fur into a pellet to be cast later.

 

References:

Pellet Schedules

Ready for a rousing game of “Who Cast that Pellet?”  Take a guess with this first pellet:

Pellet #1:

Theo pelletSee the bone fragments poking through this pellet? Theo the Great Horned Owl left this after eating a furry white rat. His stomach muscles and juices broke down the meat, allowing the meal to continue into the intestines. But indigestible fur and bones remained in the stomach where they were pressed together and, eventually, coughed back up. The time required for this process depends on the size of the meal: larger meals take a longer time to process.

Pellet #2:

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This pellet has smoother edges and no visible bones. If I split this pellet open, we would only find fur. Carson the Red-tailed Hawk ate the same meal as Theo, but hawk stomach acid is much more acidic and can disintegrate bone. The bones of her rat passed through, but the tough fur was still cast back up as a pellet.

While owl pellet schedules depend on when and how much they eat, hawks adhere to a circadian rhythm. In one study, Red-tailed Hawks cast a pellet at the first light of the day regardless of when they were fed the day before. They are even known to cast a pellet after a fast day with no food in their system. Their routine pellet casting at dawn may reflect their hunting strategy. Hawks require daylight to hunt, so emptying their stomach in the morning will prepare them for a fresh meal. Owls, on the other hand, can hunt during day or night, so their system simply empties after the food has been digested.

Learn more about raptor pellets in this Raptor Physiology chapter.