As I started my passage up Wildlife Drive....I got perhaps a few hundred yards into it when I saw a Northern Harrier slowly dipping and diving and flying low on the side of the drive making its way to me. I pulled over, took out my bins and just observed the harrier who was totally oblivious to my presence, for it kept coming right at me man. Having to refocus with each advance of it's wings, I could see the harrier crook its head from side to side, scanning for a small vole or mouse to eat. For a solid hundred yards or so, this pattern of dip....pull up to just no more than 10 feet above the grass line, dip again...pull up etc....continued. All the while it's head was jutting from side to side. A fabulous site and in 'full living color' ...better than an RCA.
This continued, up on the drivers side of the grass and when it came within a few yards of my car, it veered to the other side of the lane and had a gentle silent glide past my open window on the passenger side. I could have reached out and caught it with a net....or well....in theory I could have, but a sneaky inner perception of mine is telling me that I 'would haven missed' ....widely missed.
But that got me thinking. How many calories does a harrier burn and how many times in a day must it be a 'bit more successful' in its prey catch than what I saw here? When I returned home and did a bit of surfing, I found that according to Hawk Mountain Sanctuary a male harrier can weigh anywhere from 290-390 grams while a female harrier is from 390-600grams. Surprisingly the male is a better hunter, perhaps more aloof. Citing research by Craighead and Craighead (1956), the harrier diet consists mainly of small rodents (98%). Depending on the time of the year, it might consume 15-20% of it's body weight. Given that an average field mouse might weigh 20 grams or more, ...do the math. Taking an average weight of 425 grams for a harrier and 'x' it by 20% (since it is winter and it might need more food now), that would mean it would consume about 85 grams a day. Taking 85 grams and dividing it by 20 grams ( average weight of a field mouse) you get a bit over 4 mice a day in the wild. But a field vole might weigh up to 50 grams according to the University of Michigan so each portion will then becomes a bit more Americanized or Super-sized.
But does that make sense? Not sure....as research by Houston and Duke (1973) state, a red-tailed hawk might consume 75 grams a day. But a red-tailed hawk according to my Peterson Field guide is 2-3x larger in weight than a Northern Harrier. So why would they consumer an equal number of grams?
Now to translate that into caloric intake, field mice might roughly be 30-35 calories so if a harrier consumes 4 mice a day, that equates to only 140 calories...nah...that just can't be. Or is it too much? Or ...what? Confusing.
So the bottom line is that after spending a few hours surfing the web for this all important answer, I have one that is just convoluted at best. After all, has anyone really been crazy enough to have monitored a harrier in the wild and traced it's wing steps for an entire day to record food intake? Yeah, probably Tom Baxter.... I will have to ask him.....