Saturday, January 26, 2013

Another Day, More Birds.

The icy trails meant for treacherous hiking and the dense fog meant for poor visibility and lighting, but there were birds to be seen. These wasn't very many people out on the trails. Enjoy.

Lesser Goldfinch.
Lesser Goldfinch.
Great Horned Owl.
Female Adult Sharp-shinned Hawk.

Thursday, January 17, 2013

Birds of the Day

Great afternoon hike today in 15 degree weather. The air quality wasn't too great, but it was great to be out anyway. Lots of birds to watch as well. We hiked our favorite loop through Hull's Grove.

Male Downy Woodpecker.
Virginia Rail.
Song Sparrow stirring up the mud.
Female American Kestrel eating dinner.
It's not all about the birds!

Sunday, January 13, 2013

Great Horny Owls

For more than eight years Karyn and I have been watching the Great Horned Owls that occupy our nearby trail system. There are a number of owl nests in the foothills near Boise, but the Hull's Gulch Owls are probably the most visible. The owls have been a near permanent fixture in the area, so much so that the Ridge-to-Rivers trail system managers named the trail "Owl's Roost".

Days are getting longer. While you and I may not have noticed, the days in the northern hemisphere have been getting longer since the winter solstice back on December 21st. The owls may not consciously realize this, but physiologically they have known it for some time. Shortly after the solstice, the increasing day length triggered a response in the endocrine system of the owls (and other birds, mammals, etc, probably including ourselves). The day length is sensed by the pineal gland which causes the hypothalamus to trigger the pituitary gland. the pituitary gland then produces two hormones - luteinizing hormone (LH) and follicle stimulating hormone (FSH). LH triggers other reproductive hormones in both males and females and induces ovulation of mature egg follicles in the female. FSH stimulates sperm production in males and egg development is females (Gill 2007). It's a complicated process but the linkage is clear - longer days trigger the drive to reproduce!

We observed the Hull's Gulch Owls paired up the day after the solstice. They were likely paired prior to this. I doubt they sensed that the day after the solstice was 20 seconds longer then the previous day. But the lengthening day has since modified their behavior. On January 11th Karyn and I watched as the pair carried on with their mating ritual. These early mating sessions will not likely produce eggs, but are believed to help reinforce pair bonds and act as a sort of competition with other males. The thought being if the male is mating with the female, no other male can do that. This will cause the pair to mate possibly hundreds of times over the next two months.

Claiming the territory. In diurnal raptors and owls, the nesting territory is usually first claimed by the male. The male then presents the territory to prospective females. The females are therefore evaluating the male and the territory as a single package. Long term pair bonds are common in raptors, but this territory ownership by the male can lead to a split. For example, after a nest failure the female is much more likely to switch territories the following year. This often leads to a split as the male is less likely to leave. In goshawks the female has a 50% chance of abandoning an area after a nest failure, whereas the male only leaves in 10% of the cases (Kenward 2006). This has not been an issue for the Hull's Gulch Owls as their nesting attempts have been successful every year that we have observed them, successfully fledging at least three young and often four. We do not know if there has been any turnover in the adult owls during this time.

Our January 11 observations. Last week, on January 11th Karyn and I went out at dusk to see what the owls were up to, although we had a pretty good idea! We arrived near the nest to see the male perched within. Remember, that the males present the territory and the nest to the females. This ritual is repeated even for long term paired individuals. As darkness approached the male started hooting the common five note call of a Great Horned Owl. In between, the owl would "cluck" almost like a chicken but with a little more of a owlish hoot mixed in - cluck-cluck-cluck-cluck-cluck. A short while later the female gave the familiar five hoot call. The male hooted much more often, but whenever the female hooted the male would always join in to synchronize in the hoots. He went back to clucking in-between. After five to ten minutes of this the male flew from the nest over to a nearby wire and hooted from there. The female moved up to the top branch of the tree and hooted back. The male crapped to clear out his cloaca. In birds the fecal system, urinary system, and reproductive system all use a single opening called the cloaca. You can understand why that would need to be cleared out. Almost in unison, the female crapped to clear out her cloaca. Things are getting really sexy now! The male hooted once more and the female flew over and perched next to the male. The male mounted the female and they mated by touching their cloacae - the cloacal kiss. This process only takes a few seconds. The male flew off to his perch and hooted in celebration as he always does, likely bragging to any other male owls around that this is his territory and his mate. They both flew over and took up hunting perches overlooking a sagebrush field nearby. Darkness settled in leaving only their silhouettes visible in the moonlight. We wished them happy hunting and good night.

References:

Gill, F. 2007. Ornithology. W. H. Freeman and Company, New York, NY, USA.

Kenward, R. 2006. The goshawk. T & AD Poyser, London.