Monday, June 30, 2014

This one is a cliffhanger, part three

Once I focused on the fact that there were multiple swallow species present, the wide variation in the placement, structure, and stage of the nests began to make sense.

The cliff swallow nests looked the most precarious; they opened outwards and hung where the bridge beams met the supporting columns (see here and here).  These nests were also the furthest behind in development -- while the cliff swallows were still extending their nests and incubating their eggs, the other swallows were busy feeding their chicks.

The cave swallows had built their deep nests in the corners formed by perpendicular beams.  Although I could not see into the nests, the steady stream of feeding visits by the parents was a good indication that there were growing chicks hidden inside.

* To see this video in high definition (1080p), you may need to: 
(1) click "YouTube" to watch on the YouTube website
(2) change the settings at the bottom of the video screen

Meanwhile, the barn swallow chicks were quickly outgrowing their nests, which were shallow cups balanced on narrow ledges.

Four out of five barn swallow chicks agree: it is time for food!
These chicks eagerly watched each adult swallow that came nearby and opened their beaks wide for food, regardless of whether the adult was one of their parents or not.  Since there were many nests under the bridge, the chicks' gaping, yellow mouths were ignored more often than not.

* To see this video in high definition (1080p), you may need to: 
(1) click "YouTube" to watch on the YouTube website
(2) change the settings at the bottom of the video screen

Nevertheless, the chicks had soon gown enough to leave the nest and perch on a nearby beam.  There, they continued to watch the all hardworking adults and to wait for food to be delivered.

A few days later, the chicks were out of the nest -- but still waiting around to be fed.

Friday, June 27, 2014

This one is a cliffhanger, part two

I was not in any way surprised to see that the birds that returned to the colony under the bridge were swallows.  After all, the nests had looked exactly like swallow nests.  Yet, I did get a big surprise once I started to identify the swallow species.  Not only was the common Barn Swallow (Hirundo rustica) there...

A Barn Swallow (Hirundo rustica) perched under the bridge.
... but so too were the Cliff Swallow (Petrochelidon pyrrhonota)...

A Cliff Swallow (Petrochelidon pyrrhonota) sitting in its nest.
... and the relatively rare Cave Swallow (Petrochelidon fulva).

A Cave Swallow (Petrochelidon fulva) on its nest.
All three species had made their homes under the same bridge and were busy tending to their nests and nestlings.  To be continued...

Wednesday, June 18, 2014

This one is a cliffhanger

Last September, when I was exploring the park area around my new apartment, I found an abandoned colony underneath one of the bridges over the bayou.

The bridge over the bayou.
Many mud nests had been attached to the concrete columns and beams of the bridge.  However, all the nests appeared empty and some were falling apart.

A couple of empty nests underneath the bridge.
Would the colony residents return, and if so, when?  I doubted that there would be any new activity at the nests until the spring. Indeed, there was little to see under the bridge until April, when the colony members came swooping back.  To be continued...

Monday, June 16, 2014

Congratulations, you survived! (A Father's Day sentiment)

For crab spiders, becoming a father is a perilous endeavor that involves a lengthy search culminating in a cautious approach to an uncertain ending. While on a walk this Father's Day, I spotted a would-be father that had been insufficiently cautious.

A female Mecaphesa sp. crab spider eating a male.
Whether or not that male crab spider had succeeded in becoming a father, it had succeeded in becoming a meal and was indirectly nourishing a new generation of crab spiders.

A spiderling on another flower.
So, even if human fatherhood has its trials, be glad you aren't a crab spider.

Wednesday, June 11, 2014

Caterpillars on the grass, part four

By the time fungus-infected caterpillars crawl up to the tops of grass stems  (or to other elevated positions), they are almost certainly doomed.  Therefore, it might seem reasonable to assume that this 'height seeking' behavior is instigated by and for the benefit of the fungus.  Yet, there is another possibility.

Though it is too late for the dying caterpillars to save themselves, they may be acting to save their relatives.  Female moths and butterflies sometimes lay their eggs in clumps, with the consequence that sibling caterpillars develop in close proximity.  For these families, it would be advantageous if diseased individuals left the group to avoid infecting the others.  Furthermore, being stuck at the top of a grass stem may have significant drawbacks for the fungus if large herbivores or predators of the caterpillars are around.

Whether climbing up grasses was more beneficial for the caterpillars or the fungus, there were some caterpillars that avoided infection.  About the same time and in the same area that I found the sick, green caterpillar, I caught a much healthier version that had been crawling across the ground.

A healthy moth caterpillar.
I took the healthy caterpillar home to observe whether it too would fall victim to the (at the time) mysterious epidemic.  However, unlike the sick caterpillar, which was rapidly engulfed in fungus, the healthy caterpillar soon metamorphosed into a pupa...

A healthy moth pupa.
... and later emerged as a nondescript, grayish brown moth.

Monday, June 9, 2014

Caterpillars on the grass, part three

I took the sick caterpillars home, sealed within plastic bags to keep whatever might be inside the caterpillars from getting out into my apartment.  Within a few hours, my suspicions had been confirmed: the caterpillars were sprouting fungal fruiting bodies.  By the next morning, the caterpillars were completely covered in shrouds of fungus.

One of the sick caterpillars the next morning.
Once I had solved the central mystery of what was killing the caterpillars, it became possible to find explanations for the strange circumstances surrounding the caterpillars' deaths.

The first thing that had struck me as odd was that I found most of the dead caterpillars on the tops of grass stems.  Yet, the grasses -- unlike many nearby plants -- did not show any signs of being eaten by caterpillars.  Why were the caterpillars on the grasses?  The most likely answer is that the grasses were the tallest plants around.  Insects infected by some types of fungi exhibit "summit disease", in which they climb up to high points shortly before they die.  One hypothesis is that the insect host is being manipulated by the fungus into seeking out a spot that will maximize the transmission of the fungal spores to new hosts.

The second puzzling circumstance was that the caterpillars were firmly attached to the grass stems, despite sometimes being in awkward positions (such as the one pictured below).  Such attachment can be another result of manipulation by the fungus; however, in this case, it was more likely due to the fungus directly.  Fungal hyphae can grow through the insect and into the plant underneath, tightly anchoring the insect into a spot favorable for the transmission of fungal spores.

A dead caterpillar anchored to a plant stem.
The third unusual circumstance was that the only time I found dying caterpillars was in the evening after a rain storm.  Although possibly coincidental, these high humidity conditions are ideal for the production of spores and the infection of new hosts.  Indeed, some fungi have been shown to use the insects' own circadian rhythms to time the insects' deaths favorably (from the point of view of the fungi, of course).

Was the curious behavior of the caterpillars all for the benefit of the fungi?  To be continued...

Explore some more: BIZARRE INTERACTIONS AND ENDGAMES: Entomopathogenic Fungi and Their Arthropod Hosts

Friday, June 6, 2014

Caterpillars on the grass, part two

A few of the dead caterpillars had small amounts of fungus growing on them.  Was the fungus the killer or simply an opportunist benefiting from the work of another?

A dead caterpillar with some light gray fungus visible between it and the grass.
There seemed to be no way to tell.  However, when I returned to the meadow to investigate again a couple of days later, I made an exciting discovery -- in addition to several more dead caterpillars, there were two that weren't dead yet.

The first sick caterpillar, wet from a recent rain shower.
Although they appeared much less dead than the others, these caterpillars were still far from healthy.  When I clipped their grass stems and collected them for observation, the caterpillars did not react and just remained tightly attached to the grass.

Another sick caterpillar clinging onto grass.
There were no signs of external damage, making it unlikely that the caterpillars had been attacked by a predator.  Instead, whatever was ailing them was probably still inside them.

To be continued...

Wednesday, June 4, 2014

Caterpillars on the grass

Though the caterpillars feasting on bluebonnet seeds thrived, a prior cohort of caterpillars had not fared so well.  Initially, these earlier caterpillars must have been successful, since the ones that I found were quite large.  However, they were also quite dead.

A dessicated caterpillar stuck to a stem of grass.
I had been viewing the newly blooming bluebonnets when I noticed the first dead caterpillar on a nearby stem of grass.  Curious about whether it was an isolated case, I began searching the surrounding grasses.

Another dead caterpillar stuck to grass.
I soon found five more dead caterpillars.  They were all stuck to the tops of grass stems, just as the first one had been.  Meanwhile, there was no sign of any living caterpillars, either on the grasses themselves or on the neighboring plants.

Yet another dead caterpillar.
What was killing off all the caterpillars?  To be continued...

Monday, June 2, 2014

A caterpillar in the bonnet, continued

As I continued my search for the seed pod raiders, I soon came across more plausible suspects than the lady beetle. I found numerous caterpillars not only suspiciously near both old holes…

... and fresh holes...

… but also chewing the holes…

… and feeding from within the seed pods.

Because the seeds are provisioned with enough nutrients and energy to support the initial growth of a new plant, they make tempting meals -- even considering the effort required to tunnel through the protective pod.  Thus, by the time the seed pods finished maturing, a significant proportion appeared to have been plundered by the large population of hungry caterpillars.