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  An unpublished source indicates that Asian species, the Grey-sided Thrush (Turdus feae), collects nectar from the flowers of the legume Acrocarpus fraxinifolius.2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.

Food for Motmot Nestlings

One of the more lovelier birds in west Mexico is the west Mexican endemic, the Russet-crowned Motmot (Momotus mexicanus), shown below with a cockroach.

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It was my good fortune to watch a breeding pair provisioning their offspring from around the middle of June to the beginning of July.  I took pictures and made videos.  While doing this, I was reminded of something I read in John Terborgh’s book Diversity and the Tropical Rainforest.1  In his book Terborgh argues that one component of increased biological diversity in tropical rainforests as compared to elsewhere is a greater number of niches.  One such niche created by the presence of large tropical rainforest insects is the large tropical rainforest insect eating guild of birds, exemplified by the motmots.

While this wasn’t the rainforest and the insects and other things brought back to the nest weren’t all big, it was nevertheless an interesting assortment of food items. Because motmots nest in burrows and the chicks are hidden away, I couldn’t see them receiving their meals.  Suffice it to say, though, for a motmot chick a meal can be a real surprise!

Caterpillars are an important nestling food source for neotropical migrants.  For the photographed pair of Russet-crowned Motmots, they were important too.  At times it appeared that the pair identified a particular caterpillar host plant and returned to that plant repeatedly for the same species of caterpillar.  The first of three caterpillar photographs is also further evidence that the eversible osmeterium (the yellow “horn-like” projection arising from just behind the head) of swallowtail butterfly larvae, while secreting substances that deter ant and other invertebrate predators, is relatively ineffective against vertebrate predators.2

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The motmot pair also commonly fed their offspring adult cicadas and less commonly juvenile forms (the first photo).

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Beetles too were important components of the nestling’s diet. In the early part of my observations, elongate beetle larvae like the one shown in the last picture were frequently brought back to the nest.

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The adult pair also fed their nestlings an assortment of other animal foods including, among other things, ant alates, millipedes, crabs, snakes, frogs, and terrestrial slugs. The last photograph shows a bird with a vertebrate whose tail is missing although I can’t tell if it’s a lizard or young mammal.

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Finally, fruit – particularly guamuchiles (Pithecellobium dulce) shown in the first photograph – was also an important component of offspring diet.

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These photographs show that Russet-crowned Motmots feed their nestlings a variety of foods from locations as diverse as the crowns of trees to leaf litter on the ground.  They also provide additional evidence that the assumed chemical defenses of things like papilionid larvae2 and millipedes3 often do not deter bird predation.

 

References

1Terborgh, J. 1992. Diversity and the tropical rain forest. Scientific American Library, W. H. Freeman, New York.

2Leslie, A.J. and M.R. Berenbaum. (1990). Role of the osmeterial gland in swallowtail larvae (Papilionidae) in defense against an avian predator. Journal of the Lepidopterists’ Society 44(4): 245-251.

3Enghoff, H., N. Manno, S. Tchibozo, M. List, B. Schwarzinger, W. Schoefberger, C. Schwarzinger, and M. G. Paoletti. (2014). Millipedes as food for humans: their nutritional and possible antimalarial value—a first report.  Evidence-Based Complementary and Alternative Medicine (2014): 1-9.  https://doi.org/10.1155/2014/651768. (see references therein).

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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Nectar Feeding Tropical Parulas

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Once you know something about a group of animals, you come to expect that they will eat particular things.  When one thinks of the family Parulidae (New World Warblers) and their diet, the first thing that comes to mind are insects or other small invertebrates and most warblers have fine thin bills ideally suited for catching these things. Also, anyone who has watched these hyper-energetic little birds forage can’t but be impressed by how actively they search out their prey.

In terms of being energetic, the Parulidae are perhaps outdone only by the hummingbirds. Maybe it shouldn’t be surprising then that like the hummingbirds warblers might occasionally put their fine thin bills to use collecting floral nectar to satisfy their energetic needs. While the evidence that warblers do this is circumstantial — I know of no experiments or close observations ruling out the possibility that they are instead visiting flowers to catch small invertebrates — this evidence suggests that it is more likely that many are collecting nectar than not.

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