August Warblers 2

August bought two additional early fall migrants to our banding station.  The first, a young Canada Warbler, is a subdued version of its bright, spring plumage.  Canada Warblers are not local breeders.  The second could be a local bird, and is definitely more of a challenge to identify.  It is a young Common Yellowthroat.  Field marks here include the ochraceous yellow on the throat and the brownish tinge to the fore-crown.

Shadow Darner

Sunday evening I rescued this Shadow Darner from my bird net. Shadow Darners are common across northern North America. Among the largest and fastest-flying dragonflies, Shadow Darners prefer shady areas, usually near water. They fly into the late evening and sometimes even at night (Montana Field Guide). They also fly late into the fall. Already our evenings and nights have been cool, with temperatures in the 60s F.


Shadow Darners can take advantage of these cool climates, habitats, and times because they can regulate their body temperatures. A number of factors contribute to this phenomenon. When I removed the darner from the net, the dragonfly acted lethargic (as you can see as it perched on my finger). Later I placed the darner on a lily leaf. Here the animal began to quiver its wings, a behavior called "wing-whirring." Like shivering, this action can elevate the body temperature.  Furthermore, a darner can control heat loss by altering the circulation between its thorax and abdomen. Finally, large dragonflies like darners more efficiently retain body heat than do smaller ones (May 1976). When I returned from closing my nets for the night, the Shadow Darner had flown.

Blue-fronted Dancer

Blue-fronted Dancers are damselflies named for their bouncy flight. This summer I found them to be common along many area waterways.  Males, like the one in the first photo, are fairly distinctive.  Field marks include their blue bodies (with very thin black lines separating the body parts) and the terminal azure abdomen segments.

I find dragonflies and damselflies to be difficult to identify. Immature males can resemble other damselflies. Female Blue-fronted Dancers complicate matters by coming in two forms, blue and brown. Blue females look like males, but lack blue-tipped abdomens. Brown females are similar to other female dancers, and I have relied on Scott King for my identification of the brown female below. (Kirk Mona recently blogged, "What if everyone could tell if you'd had sex? Ask a Dragonfly." I am tempted to title this post, "What if your prospective mates look like different species?")

August Warblers

Fall warbler migration in the Upper Midwest begins in the second half of August. This year I banded a Chestnut-sided Warbler on 21 August, an Ovenbird and a Mourning Warbler (both on 22 August), followed by a Black-and-White Warbler on 26 August. All were young and all but the Mourning were conceivably local birds.  I banded none of these species, however, earlier in the summer.

Last fall the migration appeared to began earlier. In 2010 I banded a Blackburnian and a Canada Warbler on 12 August; my first Black-and White Warbler was captured on 19 August (along with a second Blackburnian). The next day I banded three Chestnut-sided Warblers. I ringed over a dozen additional warblers as the month progressed.

Photos: Chestnut-sided Warbler (top); Ovenbird (second); Black-and-white Warbler (third); Mourning Warbler (bottom).

Least Skipper

Skippers are hard to identify. With a little bit of help from my friends, I labeled this one as a Least Skipper. Field marks include its small size, rounded wings, and dark forewings. The checkered antennae also help.

The Least Skipper inhabits the eastern United States and southern Canada. This butterfly is often abundant, but the flight can be short-lived.  Least Skippers fly about damp, tall grasses along roadsides, streams, and meadows.  I found it in unmowed grass along a small creek at the Dennison, Minnesota, sewage treatment ponds.

Duckweed

Duckweed is our smallest flowering plant. It floats on pond surfaces, often turning lakes green. The plant does not grow in moving water and favors nutrient-rich water. Duckweed reproduces both sexually (via a tiny flower) and asexually (by budding). A single duckweed produces a daughter bud about once a day. In about two weeks, this rate of reproduction results in about 17,500 plants! A pond covered with duckweed suffers from few underwater plants and usually low oxygen levels that cause fish to die. Koi, goldfish, and grass carp will eat duckweed, but prefer other plants. Ducks eat duckweed, and also disperse duckweed that stick to their feathers. When people complain about duckweed, however, I reply that I prefer a pond filled with a flowering plant rather than slimy algae. Furthermore, duckweed is a high-protein food source for waterfowl (and, potentially, for people). The plant, with its extraordinary reproductive ability, has the potential as a renewable resource for clean energy--an energy source that actually removes carbon-dioxide from the air.

Summer Azure

The Azures comprise almost a dozen forms that were once considered to be a single, variable, and abundant species across North America. Now several species are recognized, but their taxonomy is confusing, even for experts. Kaufman in his Field Guide to Butterflies of North America suggests that amateurs simply enjoy these bright blue-backed, white-undersided butterflies as Azures. Let the lepidopterists argue about species. In mid-summer in Minnesota, however, the choices are somewhat limited. I suspect the odds are excellent that this photograph is of a Summer Azure.

Many Azure caterpillars have a symbiotic relationship with ants. The caterpillars secrete a sugary substance that feed the ants, while the ants protect the caterpillars from predators. In other species the caterpillars feed the ants while on the caterpillar host plants, but then move into ant nests and begin feeding on ant larvae. Nevertheless, the ants continue to protect the butterfly pupae. The adults break out of the pupae case and must crawl out of the ant nest before the butterfly can take flight (Wikipedia).

American Dagger Moth Caterpillar

At Afton State Park, Erika and I happened upon this caterpillar of the American Dagger moth. Anything this bright must be an example of aposematic coloration--"be careful, I'm poisonous!" In this case, you'd be correct. Many people develop a nettle-like rash from handling this caterpillar. Seeing as how this critter was grazing on Poison Ivy, the habitat would get you if the caterpillar didn't! The moth is found east of the Rockies, primarily in the United States.

Slender Spreadwing

Slender Spreadwings are identified by their long, thin abdomens and by the whitish wing edges. Note that the wings narrow to stalks where they attach to the body. In July, Scott King found this individual at Lake Byllesby County Park in Dakota County. They favor ponds, wetlands, and streams.  Eastern Spreadwings are found acorss a wide area of eastern North America. I was tickled to learn from Scott that the Spanish for Odonata is Caballitos del Diablo--Devil's Ponies.

Henslow's Sparrow

On 19 August 2011, Erika and I strolled through the Afton State Park interpretive prairie loop.  Dragonflies were our main goal. Aware that Henslow's Sparrows were a possibility, I stopped short when I heard a distinctive, single, hiccup-like "tsi-lick" from within the grass. (In this link, used with permission of Thayer Birding Software, the song is repeated a bit more rapidly than what we heard.) Using the Sibley App on my smart phone, Erika and I quickly confirmed that this was a singing Henslow's Sparrow. The location of the call and the bird were difficult to ascertain. In the photo above, the sparrow is in the dead center of its preferred prairie habitat.

To our surprise and delight, the Henslow's Sparrow flew up and continued to sing from to the top of a nearby goldenrod. Visually this sparrow proved far more difficult to identify than was its song. The olive-green characteristic of this species in the spring was reduced to a patch on its nape and a very faint wash across the head. The underparts were also only very faintly buffy. Although the bird showed a few flank streaks, the breast was faintly, if at all, streaked. This relatively lack of color and streaking may be the result of the bird's being in a very worn late summer plumage.

On the other hand, the double "mustache" stripes are quite visible (specially in the photograph just above). These field marks are actually a subauricular line and lateral throat-stripe. Furthermore, the bill is large and conical and the top of the head profile is relatively flat. The rusty wing coverts and tertial feathers are also characteristic of a Henslow's Sparrow. The spots on the head are also good field marks.

Recently Henslow's Sparrow populations have declined at rates approaching 10% a year. This decline is the steepest of any North American grassland bird (Herkert et al. 2002). The main reason for this decline has been their shrinking grassland habitat.  Local population increases have been associated with the Conservation Reserve Program and with efforts at prairie restoration such as at Afton State Park. But these conservation efforts are probably not sufficient to buck this species' downward population trend.