Olive-backed Sunbird nesting: SPIDER ISSUES “STOP WORK” ORDER

posted in: Nesting-failed | 9

“Early in February, I noticed what looked like rubbish hanging on one of the hooks that I used to suspend orchid pots under the eves of my house. The mess turned out to be the beginnings of a nest being constructed by a pair of Olive-Backed Sunbirds (Nectarina jugularis) (below left)

“Work proceeded apace, with the female doing most of the construction (above right).

“A week later, the mess was beginning to take the shape of a typical sunbird nest, with trailing loose ends that disguise it as just another clump of dried vegetation (below left).

“Abruptly, work stopped. A spider had moved into the site, and had incorporated the incomplete nest into its web. The spider had tied up the trailing ends and bent them back so that the incomplete nest was now hook-shaped (above right).

“The bemused parents hung around for another two days, then abandoned the project. They have not returned for a fortnight (left).

“Though the spider is small, a long way from adult size, it appears to be one of the Saint Andrew’s Cross spiders (Argiope sp.) though I am not sure which one exactly.

“Why was such a small spider to chase away birds so much larger than itself?

“There are two interesting but unproven theories about this group of spiders, who get their name from their habit of sitting in the centre of their webs with their eight legs arranged into four pairs, like a huge letter X, or the cross of Saint Andrew. From the tips of their legs, they add highly-visible embellishments to the web.

“A theory has been put forward that though the spider webs are hard to see, the embellishments are very visible, and warn birds away so that they do not fly through accidentally and damage the spider’s work. Are birds scared of the webs of this type of spider?

“The other theory is that the embellishments reflect ultra-violet light, which attracts insects, and lures them into the web.

“Which do you think applies in this case?

“Forgive the slight fuzziness of the pictures, as they were all shot through the glass panes of my living room and my kitchen. Opening the windows would have frightened away the birds.”

Lee Chiu San
Singapore
2nd March 2014

9 Responses

  1. I am very interested in this spider mentioned. Why would the birds be afraid of it? Could it be because the sunbirds being small, could get trapped in their webs? I remember reading several articles about sunbirds getting tangled and stuck in spider webs.

    Also, I would be very grateful if anyone could help me identify this spider which I saw at the Botanic Gardens in February: http://heyammy.wordpress.com/2014/02/25/botanic-gardens/ (the first photo – can be clicked on for bigger size)

    Note that its web has those 4 zigzag reinforcements in an “X” shape. I find that very interesting as other spiders don’t seem to do this.

    • A St. Andrew’s Cross spider, Argiope sp.

      • Thank you! I forgot about this post until today. Do you think we can have an option to notify the original comment poster whenever they get a reply? Most websites have that. I know wordpress.com websites have this option.

  2. Lee Chiu San

    Your photo of the cross is clearer than mine. That’s the point I was referring to. Do St Andrew’s Cross spiders have these embellishments on their webs to warn birds away? And did the pair of sunbirds take heed of the warning and abandon their nest? It is now well and truly abandoned, but the spider is still there.

    • Such interesting questions – I want to know the answers too! Maybe we can have some entomologists come on board BES Group to write some articles as well?

    • A quick search found me this:

      “For many years, it was thought that the function of the zig-zag bands was to strengthen the web or to conceal the spider which often sits, head downward, at the centre of the stabilimentum. Some scientists in the United States established that the purpose of the stabilimentum is to advertise the presence of the web to birds. The birds will know to avoid the webs if they do not want to risk being entangled in the sticky silk. The spiders thus get to preserve their webs, which they will eat when worn out, as it is a precious source of protein supply for silk production.

      “However, recent research has uncovered a new explanation. The silk which makes up most of the web is a poor reflector of ultraviolet light, except the stabilimentum which reflects it very efficiently. In the same way flowers reflect ultraviolet light to attract pollinating insects, the stabilimentum is believed to attract insects to the web, by mimicking a flower.

      “We do not know for sure the reason for the zig-zag bands.”

      – from https://www.sbwr.org.sg/Wetlands/text/00-7-1-4.htm

      Also, what I find EVEN MORE interesting is that some spiders only make 2 zigzag lines, while others make 4! I have seen both kinds.

      Another local website also notes this:

      “Some spiders build a single vertical line, yet others a patch of zig zags in the centre of the web. No matter the design, the spider sits right smack in the middle. We do not know the purpose of these lines…”

      – from http://www.naturia.per.sg/buloh/inverts/argiope.htm

      If the purpose of the lines is to strengthen the web or warn off birds, then why do some spiders do 4 lines while others do only 2?

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