Nectar-feeding Rufous-backed Robins in western Mexico

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In earlier posts I highlighted two instances of nectar-feeding by birds in western Mexico, that of Tropical Parulas (Setophaga pitiayumi) and Streak-backed Orioles (Icterus pustulatus) on Spondias purpurea and various species of hummingbirds taking nectar from wild cotton (Gossypium aridum).

Recently, I had the opportunity to observe additional bird species collecting nectar from the African Tuliptree (Spathodea campanulata), a common ornamental species in the region. The bird species collecting nectar from this tree included Orchard Orioles (Icterus spurius),

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…Hooded Orioles (Icterus cucullatus),

 

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…and Yellow-winged Caciques (Cassiculus melanicterus).

 

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All of the above species are well-documented nectar collectors.1

The final bird I saw collecting nectar — the Rufous-backed Robin or Thrush (Turdus rufopalliatus) — was a considerably bigger surprise.

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While many of the True Thrushes, including Rufous-backed Robins, consume significant amounts of fruit, I could only find one record of them consuming nectar.  An unpublished source indicates that Asian species, the Grey-sided Thrush (Turdus feae), collects nectar from the flowers of the legume Acrocarpus fraxinifolius.2

So why would birds that eat sugar-rich fruit rarely if ever take the opportunity to collect similarly sugar-rich nectars?  Apparently, the answer to this question lies in the kinds of sugars found in fruit and nectar.3

The sugars in fruit are almost always glucose and fructose.  These sugars are simple and directly absorbed in the intestines of frugivores without need of further processing. Conversely, most floral nectars have sucrose as their predominant sugar.  Sucrose is a disaccharide formed from glucose and fructose and cannot be absorbed by the intestines until it is broken down into glucose and fructose by the enzyme sucrase.

Rufous-backed Robins in specific1 and, True Thrushes in general3, lack measurable quantities of sucrase. Therefore, they can’t digest sucrose-rich solutions. Some bird species lacking sucrase may suffer something called osmotic diarrhea when fed sucrose-rich solutions.  Not surprisingly, they tend to avoid consuming such solutions.

There are, however, some plants that produce floral nectars where glucose and fructose are the predominant sugars.3  Such plants are more common in the Old World than in the New.  This may be the result of a more equitable sharing of the avian nectar-feeding niche in the Old World between passerine species that can and can’t digest sucrose.  In the New World, sucrose-digesting hummingbirds dominate this niche possibly reducing the selection for plants that produce floral nectars rich in glucose and fructose rather than sucrose.

This then may explain why I was able to observe Rufous-backed Robins feeding on nectar.  The tree they were collecting nectar from, the African Tuliptree, is one of those Old World species whose nectar contains glucose and fructose instead of sucrose.4

As always, one question answered leads to others.  Is this a relatively new behavior for Rufous-backed Robins or have we managed not to see them collecting nectar from other native plant species whose nectars contain glucose and fructose instead of sucrose?  Also, the calyces of the African Tuliptree produce large quantities of water which are necessary for floral development and which then spills over into the opening flowers.  In the relatively xeric environment along this part of the west coast of Mexico, has this unusual feature played any role in the origin and maintenance of nectar feeding in this thrush?

References and Sources

1Martinez, del Rio, Carlos. (1990).  Dietary, phylogenetic, and ecological correlates of intestinal sucrase and maltase activity in birds.  Physiological Zoology 63(5): 987 – 1011.

2P. D. Round (in litt. 1998).

3Lotz, Chris N. and Jorge E. Schondube. (2006).  Sugar preferences in nectar- and fruit-eating birds: behavioral patterns and physiological causes. Biotropica 38(1): 1 – 13.

4Rangaiah, K., S. Purnachandra Rao, and A. J. Solomon Raju.  (2004). Bird-pollination and fruiting phenology in Spathodea campanulata Beauv. (Bignoniaceae). Beitrage Zur Biologie Der Pflanzen 73: 395 – 408.

Lambs and Wolves, Alcoholic Flux, Animals and Alcohol, Not Everything in the Garden was Rosy, and a Warning from Eubulus

In June of 2015, below the ostentatious facade of Casa Cuixmala and amidst the property’s exotic cast of zebras and impalas, my daughter and I were privileged to witness something truly elegant and special on an otherwise obscure liana (Paullinia fuscescens) growing along one of the trails:

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Neoponera ants and the vast majority of paper wasp species are powerful insect predators.[1],[2]  Such seemingly placid interactions between them and their potential prey might initially bring to mind the idyllic scene foretold by the Old Testament prophet Isiah :

The wolf and the lamb shall graze together; the lion shall eat straw like the ox…

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A Crab Spider and its Wasp

Doing a morning walk in Cuixmala on the trail along the Pistia-filled lake near Carretera 200 (across from the fundación), I came across something “floating” in the air in front of me.  Usually this is a caterpillar hanging by one of its silken threads.  As can be seen in these photos, that wasn’t the case here:

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Crab spiders (Family Thomisidae) are incredible in taking on larger and seemingly dangerous prey like this paper wasp (Polistes dorsalis) without the assistance of a web.  Instead, they hang out motionlessly and frequently camouflaged at flowers or on leaves with fallen flowers.  Here, they pounce on their prey and hold them with their first two pairs of legs while biting and injecting venom.  Crab spiders are not dangerous to humans but you have to think that their venom is pretty powerful.

What happened here?  While paper wasps don’t collect pollen to feed their offspring, they do occasionally visit flowers to feed on their nectar.  They also search plants for caterpillars and other insects that they feed their offspring.  For whatever reason, this one got too close to a crab spider.

While crab spiders don’t build webs, they still make and use silk.  Normally eating their prey where they catch them, this one shot a line of silk out and left its perch with the wasp in its chelicerae (jaws).  We can’t know exactly why it did this but perhaps a large ant or some other dangerous animal or potential predator got too close.

While the crab spider in the photos is holding the paper wasp by the base of its head, this doesn’t mean that’s where it delivered its deadly bite.  Crab spiders prefer to feed from the head where they inject digestive enzymes and then suck up the slurry.  What they suck up from the head must be very good.  One study of crab spiders feeding on fruit flies showed that they always started at the head and then switched to the abdomen to complete their meal.  If, however, a new fly appeared while a crab spider was feeding from the head of a captured fly, the spider would prefer trying to catch another fly rather than finish its meal at the other end of the captured one.

Anyway, shortly after the last picture the crab spider fell to the ground with its paper wasp where I presume it finished its meal.

References

Pollard, S.D. Oecologia (1989) 81: 392. https://doi.org/10.1007/BF00377089

Larceny of Wild Cotton Nectar by Jalisco Hummingbirds

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This spring I began to look at what pollinates a species of wild cotton (Gossypium aridum) found here in Jalisco. A chapter by Parra Tabla and Bullock in the Historia Natural de Chamela suggested that, given the floral characteristics of G. aridum, I was most likely to find that hummingbirds were the main pollinators.

It didn’t take much time to discover that hummingbirds did indeed visit G. aridum.  Initially these were mostly migrating female Black-chinned Hummingbirds (Archilochus alexandri) although males also occasionally visited. In all cases, though, these birds did not collect nectar through the floral opening but rather between the petals at the base of the flower.

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In the past, I would have called this nectar theft and as I thought of putting this post together, a number of witty titles came to mind.  However, in looking over some background for this post I ran across a note in the journal Ecology by David Inouye titled “The Terminology of Floral Larceny” where I discovered that using the term nectar theft for what I had seen would not be consistent with what the experts in the field would call it.

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A Mule and its Anis

 

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From southern Mexico to northern South America, one of the first birds a birder new to this part of the world will check off is almost invariably the Groove-billed Ani (Crotophaga sulcirostris).  Common in the kinds of disturbed environments nearly every visitor passes through and moving relatively slowly but noisily about in groups close to the ground, they’re hard to overlook.

Groove-billed anis have also been the focus of scientific research.  The famous Costa Rican naturalist and ornithologist Alexander Skutch first explored their natural history while groundbreaking behavioral biologist Sandra Lee Vehrencamp detailed their fascinating communal breeding system.  Others too have studied these birds.

The relative ease with which we can observe the birds and the fact that some very good researchers have done just that would suggest that we would have a solid handle on the basic natural history of these birds.  But there is one aspect of these birds’ behavior that’s been somewhat of a mystery.  Interestingly, it’s probably a mystery that countless campesinos could have answered for us had they wherewithal and any idea that anyone cared!

We have long known that Groove-billed Anis associate with cattle, horses, and mules. What hasn’t been clear are the forms this association takes.  We know that, like cattle egrets, Groove-billed anis occasionally follow large domestic mammals and catch the insects these animals scare out of hiding while wandering about.  Bent (1940) claimed, however, that Groove-billed Anis not only did this but also perched on cattle and even sometimes removed ticks.  Alexander Skutch, however, doubted that the birds did this. He suggested that people who thought they had seen this had confused anis with Giant Cowbirds, a similar-looking species well known for this kind of behavior.

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