Friday, September 12, 2014

Hummingbird Predator


Not much eats a hummingbird between their speed and their aversion to sitting still.  Still every creature and plant has a place in the food web, including humans.  In the case below it was truly a food "web".

I was hiking along Bull Creek and noticed a few cardinal flowers down stream, a brilliant flash of bright red on a cloudy evening at sunset.  I went down to photograph them, and while trying to focus in the low light I saw something move off to the side.  It was a large yellow spider busily wrapping up its prey, suspended from a single silk "baseline" stretched between two branches of bushes.  It had the coloration of the common yellow garden spider,  Argiope aurantia,  discussed in a previous blog but was not quite a match.  This one was the banded Argiope, Argiope trifasciata.

As I was taking pictures, I suddenly focused on a long slender beak protruding from the silk wrapping.  On the other side I could just make out the iridescent green of a hummingbird's wing under the silk cloak.  There was no ruby throat so it must have been a female or juvenile. The web had been suspended in front of the cardinal flowers which just happened to appeal to a ruby-throated hummingbird.  It is possible however that the hummingbird wasn't an entirely innocent victim.
"Hummingbirds use spider webs as a source of spider's silk in nest construction, being necessary to bind the nest to the tree branch or other substrate and to hold the nest together. Even so, the hummingbird must be careful when removing the pieces of webbing, for it may become entangled and be trapped there. Spider's silk has a tensile strength comparable to steel on a weight basis. In one report, a ruby-throated hummingbird was caught in an active web, and quickly wrapped and encased by the spider, much as an insect might be." Hummingbirdsociety.org
Spider web construction varies with the species and many spiders don't even bother to make one.  The initial strand in web construction of Argiopes' is called the "baseline."  It then drops a single line in a "Y" and builds the radials before adding the sticky web.  Most lose stickiness after a day and are eaten by the spider who then, in the ultimate in recycling, uses the ingested silk to reconstruct the web.  Details and diagrams of their techniques are at spiderzrule.com.


When most prey hits the web, the spider rushes out and bites it, killing it before its thrashing allows it to escape.  With some venomous insects like wasps they just carefully wrap it in silk. Once our Argiope realized I was intruding it slowly made its way along the single strand of baseline to the anchoring branch.  The baseline is incredibly strong and it was stretched a foot as I pulled the mummified humming bird off the strand, before the baseline snapped back in place without damage.

Most things in nature have a purpose, even if we can't guess what that is.  A recent study on the Web Orientation of the Banded Garden Spider, Argiope Trifaciata found that they oriented their webs in an east-west direction, and then suspended their ventral (underside) surface to the south, picking up the maximum sun rays.  This surface is dark on A. trifaciata, which increases its heat absorption.
"The spiders grow throughout the summer, reaching full-size in late August and September. The males, which are much smaller than the females and do not produce webs, roam around the vegetation and mate the females in late summer. The female then lays one or more egg sacks, that appear somewhat like a small kettle drum with a tough papery cover and may contain 1000 eggs apiece. The spiders die after frost and there is only one generation produced each year."  Colostate.edu
 Stabilimentum-Wikimedia
Argiopes seem to be proud of their webs, creating a beautiful radial pattern and some species even construct a "stabilimentum" decoration along the vertical axis of the web.  There are many theories about the purpose of this structure which are discussed at this site.  Even before "some pig" appeared in Charlotte's Web, folk tales said that if you saw your name in the web, you were about to die.   I don't know how to spell "hummingbird" in bird-speak, but it may be a zig-zag.


And yes, the hummingbird had gone to that big sugar water feeder in the sky.

1 comment:

  1. What an amazing find, Bob! We have one of these beauties in our garden, but she must be directionally challenged, as her web faces East, running north & south - or maybe she just likes tomatoes!

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