Saturday, March 31, 2018

Will the Harris Hawks of Sandario Rd in Picture Rocks become homeless?

Watching a pair of Harris hawks at a tree that has hosted their family for years, maybe decades, made me wonder about the symbiosis between an endemic hawk and an imported tree.
Aleppo Pines were brought  to Arizona desert cities from Lebanon and Syria as landscaping trees. Most were planted 30 to 50 years ago. Although they were not really adapted to our typical rain pattern of monsoon showers in summer and soft. but productive rains in winter, they thrived for decades and outgrew in many cases their allotted space. Low slung ranch houses under towering, monstrous pine trees were a strange, but typical look for Tucson neighborhoods when I arrived here in the early nineteen nineties.

But nowadays, those trees are becoming rare. We lost a few on our own property: they became pale, nearly straw colored, produced a couple of panic crops of cones, and died. It made us sad and left us feeling guilty - thinking that we did not care for them properly.
Then there was a diagnosis: Aleppo Pine Blight.
 There was a lot of guessing as to the cause of this disease. Bark Beetles and 'nearly invisible' mites were blamed. I stripped the bark of our tree corpses: no tell-tale sign of bark beetle infestation at all. Anyway, those bugs like mountain air, not desert heat ... but still, bugs usually get the blame.

Bark beetle infestations leave typical tracks under the bark of trees killed by the beetles. They were not present in dying Aleppo pines.
By far not all plant diseases are caused by organisms (bugs). Disease may occur because moisture is not available to tissues; consequently, they malfunction or die as a result. Insect infestation may sometimes be exasperating the problems but they are usually secondary to climactic stresses. No insect infestation was found in most dead pine trees in Tucson. 
Excessive evapotranspiration occurs when soils are dry and sustained winds  blow at extremely low relative humidity. The high water loss from needles cannot be replaced by adequate water uptake from the soil. Climate change has increased these stresses over the last decade, so now even old, established trees are succumbing all over Tucson. Another factor: irrigation over decades might have leached nutrients from the desert soil and caused an increase in salts, while an ever increasing layer of caliche that further inhibits healthy root growth and water uptake. Temperature extremes are also increasing lately - maybe exceeding tolerance thresholds of the Mediterranean Pines.
But even if the huge pines were aesthetically uncool and also, while healthy, a burden on our limited water resources, they had a positive side: Harris Hawks loved them as nesting trees. 
Harris Hawks are superbly adapted to the Sonoran Desert in many ways. I am pretty certain that it was the pressure of this hard environment that made them evolve into the only social species of hawks: the resident, territorial pair allows several younger hawks to live close by. The hawks hunt as a group, share the kill, and the young 'satellites' help the main couple to raise its young. So an obvious advantage for the resident couple. From the viewpoint of the younger hawks, this altruism is a little hard to understand because genetic tests showed that the hawks are not usually related. So no kinship selection here. But, the entire group can take down larger prey than a single hawk could slay. This may be advantageous as so many small prey animals are night active here. Considering that about 50% of fledgling hawks usually die from starvation within their first year, communal living may give more time for the young guys to become strong adults.  Furthermore,  the younger birds are probably in an excellent position to take over territory if something happens to the owners.   
Free Flight Hawk Group of the Arizona Desert Museum
 Harris Hawks,  because they hunt cooperatively, can afford to be smaller than our other successfully free roosting desert birds of prey, Red-tails, Caracaras, and Great Horned Owls. (Kestrels breed in cavities, Gray, Black and Zonetail stay in riparian or mountain forests, Cooper's adapted well to human neighborhoods with trees).

Who knows who built this nest? GHO do not do it themselves
 But Harris Hawks share their habitat directly with their great arch enemy: the Great Horned Owl.  The owls hunt and slay birds up to the size of Harris Hawks, they rob hawk nestlings and they love to take over established nest sites like the classical Harris Hawk nest in the sturdy bowl of sheltering Saguaro arms. 
A different site, also now owned by the GH owls. Both Picture Rocks, west of the Tucson Mountains
 So Harris Hawks around Tucson may have really benefited from the prevalence of huge, dense Aleppo Pines - I knew at least half a dozen nests that successfully produced fledglings year after year.  I also know of successful owl attacks on some of these nests, but at least in one case, the hawks were back a year later. Maybe the dense pine branches give enough of an advantage to the smaller, maneuverable hawk over the large owl. 
Dead Pines along Sandario Road, still hosting an active nest
 Since we lived in Picture Rocks I saw Harris Hawks around, but although I found Red-tail, Screech and GH Owl, Cooper's, and Kestrel nests, I never found a Harris Hawk nest around here. But I always saw them flying, carrying branches or food,  into two huge pine trees at Sandario Rd. There had to be a nest! But it remained completely shrouded by those dark tree crowns. 
Sadly, this spring the needles are falling, the trees are nearly bare. There is a nest though, and the hawks have not quite given up on it. 
 Today I saw the female sitting on the rim of the nest. - freely silhouetted against the sky. Then she flew down, landed on a power line post, the male joined her and they mated. So the hawks are optimistic, life will go on, even in a dead tree. 

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