Saturday, July 26, 2014

Farm Bulletin: Sparrow Hawks

As organic farmers, contributor Anthony Boutard and his wife, Carol, are caretakers of the 144 acres of land that comprises Ayers Creek Farm and that grows the crops that they sell at the Hillsdale Farmers' Market. An integral part of that stewardship is protecting and promoting its natural systems, as well as contributing to a greater understanding of how those underlying systems work.

Ayers Creek Farm is 144 acres, almost a half-mile square, actually more a trapezoidal-rhomboidal hybrid than a simple square. At the northern end of the property we have about 22 acres of oak savannah and the farm buildings. It is a raptor-rich area, supporting a nesting pair each of great horned owls and red tailed hawks, two pairs of barn owls, as well as two to three pairs of kestrels. 

Prof. Van Buskirk and student banding baby kestrel.

Kestrels—those of us over 50 most likely grew up calling them "sparrow hawks" and we like that name better—are the smallest of the falcons and nest in holes. They have narrow, swept-back wings and a long tail, and the head markings often adopt the style of ancient warriors' helmets. These traits are shared with their larger brethren, including the merlins and peregrines that linger during migration. These birds of the savannah and grasslands are permanent residents in the Willamette Valley, and you often see them hunting from utility wires. We build 20-foot tall perches to make their wintertime perch-and-pounce hunting method easier across the property. On the lighter summer and autumn air, kestrels will hover above the field. The slighter males have slate blue wings which distinguishes them from their mates that have brown wings.

Baby kestrel.

At Ayers Creek, the kestrels have utilized both natural cavities in trees as well as boxes we build. However wholesome "natural" may sound, as with us, a fine stickbuilt with an ample floor plan is preferred by the birds over a dank cave built by a woodpecker in a rotten tree. The boxes have a 10" x 10" floor, and are approximately 12" deep, with a 3" entry hole. In the British Isles, they use a box that acts more like a ledge than a cavity. Our boxes are used by flickers and starlings, as well as kestrels. All three are welcome birds, even the much-maligned starling which may eat a few cherries and grapes, but is first and foremost an insectivore that eats many thousands of grubs. 

Remnants of a meal.

When offered a selection, the kestrels change nest boxes annually. Just as in organic farming, the rotation interrupts the buildup of pests and diseases. We have come to the conclusion that it is important to offer at least three boxes per nesting pair. The kestrel, starling and flicker belong to three different bird orders, just as humans, dogs and cats belong to different mammal orders, and the three different birds move nest to new locations each year. Last year, as soon as one of our  kestrel families left their nest, a pair of flickers moved in and raised their family over the remaining days of the summer. We have three nesting pairs of kestrels close to one another and we suspect this is due to a high density of suitable nest sites. 

Their diet in the winter consists of mice and voles. These they carefully eviscerate, abandoning the guts but eating the rest of the animal. We find the entrails left on top of the smaller nesting boxes where the kestrels perch to enjoy their prey. Occasionally they will eat a bird; this spring I watched one grab and consume a junco. They also eat insects, especially grasshoppers, as well as worms and small snakes. The preference is for mice and voles, but in the summer grasshoppers probably assure their survival.

Young kestrel's first "flight."

The southern half of the farm provides no natural nesting cavities, nor are there any in the vicinity. There is a grove of trees, but it consists of Oregon ash and hawthorns, neither of which provide a good source of cavities. The ash falls apart before cavities appear and the hawthorn is too small of a tree. As a result, kestrels are rarely seen on that part of the farm during breeding season. This year we added four nesting boxes to the area; a pair settled in and the young are now out of the nest and hanging out in the shelter of the ash and hawthorn grove. One of the considerations in locating the boxes is having cover for the young who cannot fly when they plunge out of the box and are quite helpless for the first few days. Even when they do get aloft, their attempts to land are more akin to a crash onto a limb, and perhaps not the one intended.

Rich Van Buskirk, Associate Professor and Chair of the Environmental Studies Department at Pacific University, has been studying kestrels in the valley, including the four pairs nesting at Ayers Creek. The adults have been trapped and banded, and the chicks are banded as well. The female nesting at the southern end of the farm was from one of the clutches Van Buskirk and his students banded here last year. His work will give us a finer-grain picture of the kestrels on the farm over time, and their relationships to one another. He has also been studying the success of birds raised in natural cavities compared to nesting boxes. We enjoy seeing the Professor and his students at the farm on their regular visits. We are, after all, just naturalists who happen to grow some food on the side, and happy when people notice.

No comments: