Robust Conehead

by Carl Strang

One of the target species this year in my singing insects study was the robust conehead, a large katydid. It had proven to be absent from DuPage County, though the entire region is included in range maps for the species, and Richard Alexander had reported that he collected one in southern DuPage decades ago. Last year I concluded from my literature review that it primarily is a species of sandy soil areas, at least in the northeast Illinois-northwest Indiana region. As I drove the roads after dark in the Kankakee Sands bioblitz, I quickly found robust coneheads to be common.

Male robust conehead, wings elevated into singing position.

Their song is indeed as loud as the literature suggests. Like those of some other conehead species, it is a high-pitched continuous buzz, but it is so loud that it carries a long distance, and up close its volume is amazing. If a clincher is needed, the color of the cone provides it.

The underside of the cone at the tip of the head has no black area, though reportedly sometimes there is a narrow dark line at the end.

Robust coneheads generally occurred in tall dense herbaceous vegetation in open areas. On occasion this included narrow roadside strips backed by woodlands, though such sites were never far from more extensive openings.

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Spring Field Cricket Observations

by Carl Strang

There are indications that the season for spring field crickets is winding down. That is not surprising, given the early start that this year has given to insects, but it means suspending my driving survey until the fall field crickets are singing. I was able to cover a large part of western DuPage County in the time I had, though, and I was able to make some observations.

Only rarely were spring field crickets to be found away from areas dense with tall grasses.

Thus forest preserves, railroad corridors and some highway corridors were the most consistent places where I heard spring field crickets singing, and all high-density clusters of the insects were in such locations. That is not to say, however, that all fields with tall grasses had crickets.

One obvious example is the friary site at Mayslake Forest Preserve, which was bare soil until recently seeded with grasses.

History appears to be important here. Spring field crickets would seem to have limited dispersal ability, and local extinction is not readily followed by new invasion unless a source population is really close. This gives an inkling of what may differentiate the spring and fall field crickets. Spring field crickets overwinter as relatively vulnerable nymphs, and need more robust shelter from severe winter conditions. Fall field crickets overwinter as eggs, relatively safe as they are buried in the soil. This allows them to live in a wider variety of places, and makes them less susceptible to local extinction. At least now I have a hypothesis to work with.

Birds, Buckthorn and Oaks Final

by Carl Strang

As I mentioned in my previous post on this subject, the songbird migration was nearly done by mid-May. In fact I have only 3 observations to add to the ones from the early part of the season, two in the oaks and one in the buckthorn-dominated woodland. That mildly reinforces my earlier conclusion, that the only reason buckthorn woodlands appeal to birds early in the season in our area is that oaks, which dominate most of our woodlands, leaf out late. In this year when the oaks were in leaf throughout the migration season, songbird migrants nearly abandoned the insect-depauperate buckthorn at Mayslake Forest Preserve and spent their time in the oak woodlands.

South savanna at Mayslake, showing oaks well leafed.

So far all the analysis has been of the total species counts of neotropical migrants that do not nest at Mayslake. However, some of those species are brush specialists that might be expected to prefer the buckthorn over the more open oak woodlands regardless of what is happening in the canopy.

The Tennessee warbler is an example of a canopy species, on the other hand, which will not want to spend a lot of time foraging down in the buckthorn, though they might want to have it handy for resting and as a refuge from predators.

When I look at the two years’ data, sorting out brush from non-brush birds, the most curious observation is that hardly any brush-loving birds stopped by Mayslake this year. Only two birds out of the total sample of 30 were of brush species. Last year, 38 of 166 were brush birds. Looking at 2011, then, both brush and non-brush birds preferred the buckthorn woodland early in the season, though the preference was stronger for the brush species. Late last year, after the oaks were leafing out, non-brush species preferred oaks while the brush birds did not show a strong preference for either woodland type.

All of these data, coming as they do from one site over a short period of time, amount to a pilot study at best. Nevertheless, they support the notion that the apparent preference of migrant songbirds for woodlands dominated by invasive shrubs is an illusion. Birds avoid the buckthorn and prefer restored native woodlands when the latter are in leaf, capable of providing both food and essential shelter.

Birds, Buckthorn and Oaks

by Carl Strang

Now that the early part of the migration season is past, I can do a preliminary test of the Birds-Buckthorn-Oaks hypothesis. The idea is that migrating birds seem to prefer buckthorn infested woodlands early in the migration season because the buckthorn understory provides shelter unavailable in restored, oak-dominated woodlands because usually the oaks have not yet leafed out to provide food and shelter. Data from last year supported this idea, because late-season migrants showed a shift from the buckthorns to the newly leafing oaks. This year a further, better test was made possible by the warm early season stimulating oaks to break buds early, so that they were well in leaf for the early part of the migration season. So, what did the data show? I compiled the numbers of neotropical migrants that don’t nest on Mayslake Forest Preserve that I counted prior to May 19 (when oaks began leafing last year), comparing last year’s counts to this year’s. The results were stark: I counted only 5 of those birds in the buckthorn woodland, compared to 22 in the restored savannas (compared to 80 and 34, respectively, last year. Numbers were low this year, in part because my back strain limited my outings, but also a lot of migrants seemed to be bypassing Mayslake).

Things are moving along. This olive-sided flycatcher, a late season migrant, stopped by Mayslake on May 14, earlier than usual.

Using last year’s proportions to calculate expected values if there had been no difference between years, the values would have been 18.9 in the buckthorn-dominated woodland, 8.1 in the savanna. It should come as no surprise that the resulting chi-squared test statistic showed a highly significant difference (34.07, even with only 1 degree of freedom, is far above the threshold). With the migration progressing so rapidly, I don’t know if there will be enough observations in the late season to consider separately, but if so there will be an additional post. Either way, I am satisfied that the data support my point.

Another Year’s Squirrel Data

by Carl Strang

At the end of March I completed my third year of collecting data on habitat preferences of fox and gray squirrels at Mayslake Forest Preserve. The basic question is whether the usual preference of fox squirrels for savanna habitat and gray squirrels for forest will hold in an area where the savanna is high quality and the forest is low quality.

Fox squirrel

Over the first two years it was clear that both species preferred the savanna, but that preference has been stronger in the fox squirrel. This year the results were a little different. The area involved, 54.6 acres altogether, is 64% savanna and 36% forest. In the first year, 82% of fox squirrel observations were in savanna, 90% in the second year and 92% in this year just ended. The corresponding numbers for gray squirrels were 73%, 79% and 65%. That last number was perhaps the most remarkable, gray squirrels in the past year appearing in the exact proportions of the two habitat types on the preserve.

Gray squirrel

That is the only number in this entire study that shows no statistically significant difference from expected values (actually, the chi-squared test statistic is calculated from the numbers of observations rather than the percentage values; this year I had 157 observations of fox squirrels, 75 of gray squirrels). I continue to gather these data each year because the preserve continues to change, thanks to the efforts of the restoration team. High quality savanna is improving, and low quality forest is being cut back.

Testing the Birds and Buckthorn Hypothesis

by Carl Strang

A year ago I posted a hypothesis that this most unusual of seasons will allow me to test. To recap: When restored savannas are compared to woodlands with buckthorn and honeysuckle thickets in the understory in early May, when the bird migration is entering its peak, it seems that the birds prefer the invasive-degraded areas to the native savannas. Some birders take this as evidence that restoration is bad for birds. My hypothesis was that this observation is tied to the fact that oaks, the dominant trees in our savannas and woodlands, are among the latest trees to break bud and leaf out. Therefore they are not supporting leaf-eating insects, and also not providing the shelter that the birds need on their daytime migratory stops. As I pointed out a few weeks ago, like our flowering phenology, the oaks broke buds a few weeks early this year.

Remember this photo? I took it in early April, more than a month before oaks typically reach this point.

The stage thus is set. I have the records of where I saw the migrants last year, a late year when the oaks were not leafing out until the second half of May. I remarked in my notes that they were doing so around May 19, so I will take that as my dividing point. I can look at last year’s data, and this year’s, and see if the birds lose their apparent preference for the buckthorn woodlands now that they have leafy oaks as an alternative.

As a starting point I compared the bird counts from April 20 to May 18 last year to those from May 19 to May 31, when migration was essentially done. I considered only species that spend their winters south of the continental U.S. and that do not breed on the Mayslake preserve, to keep things as uniform and unbiased as possible. Even with those restrictions, I had 22 species to work with. For the moment ignoring species by species comparisons, here are the 2011 totals. In buckthorn woodlands before May 19, I made 80 observations of birds in the target group. The corresponding total for restored savanna areas in that same time period was 34 (these counts are not normalized for the relative areas of the two habitats, but the buckthorn area I used for this comparison is smaller, at 5.7 acres, than the savanna at 8.5). So the data support the notion that, at least early in the migration season before the oaks leafed out, the lower quality, buckthorn- and honeysuckle-dominated woodland harbored more birds. What about the latter part of the migration season, after May 18? Things had slowed down at that point, and the migrant species composition changed somewhat, but the totals last year were 24 observations in the buckthorn woodland and 28 in the savannas. Clearly the tide turned after the oaks began to leaf out (for the statistics cognoscenti, the chi-squared contingency table produced a test statistic value of 14.36 at 1 degree of freedom, highly significant). I will report on what happens this year, but if my hypothesis is correct, the oak savanna should prove more attractive to these migrants this year in both parts of the season.

Riding the Great Lacuna

by Carl Strang

One of the many little mysteries I puzzle over is the spotty local distribution of field crickets. We have two species, the spring field cricket and the fall field cricket, in northeast Illinois. They look alike, sound alike, and are active in different parts of the season. Though they prefer much the same habitat, and more often than not occur together, sometimes one of the two species (usually the fall field cricket) occurs alone.

Green circles mark DuPage County locations where I have found both spring and fall field crickets. Blue circles indicate where only spring field crickets occur, and yellow circles mark spots exclusive to fall field crickets. The red area and blue star are explained below.

You can see that in east central DuPage County I found only fall field crickets through 2010. The yellow circles mark York Woods Forest Preserve (the northernmost circle), and in a row from east to west: Fullersburg Woods, Mayslake, Lyman Woods and Hidden Lake Forest Preserves. Together they seem to define a space, or lacuna, where spring field crickets may be absent. On Monday I rode my bike between these locations, listening as I went for field cricket songs. Though much of this area is occupied by gated residential communities, the city of Oak Brook also maintains a fine system of bike paths which allowed me to zigzag through much of the zone marked in red on the map.

I found mainly mowed lawns, residences, woodlands and businesses (including the McDonald’s corporate headquarters and “Hamburger U”), but there were plenty of places just like ones in which I have heard numbers of spring field crickets elsewhere, with unmowed grasses or mixed grasses and forbs. In all that area, though, I heard only two crickets singing, close together in the location marked by the blue star on the map. That spot marks the northern extent of a zone I should investigate further, between the Fullersburg and Mayslake preserves.

Still, it seems the lacuna is indeed largely empty of spring field crickets. Next steps will be to find how far the boundaries of this area extend, and to look at old aerial photos for clues as to why this region might be different, keeping in mind that fall field crickets are present. I may repeat yesterday’s bike ride in the late summer or fall to see if fall field crickets occur in the spaces between those preserves.

Oak Leaves Expand

by Carl Strang

Last week I described the preference migrating songbirds were showing for an inferior woodland, rather than the high quality savannas at Mayslake Forest Preserve. I thought the security provided by the woodland’s buckthorn understory might be the significant factor. This week I found some support for that idea. The oaks have expanded their leaves.

Bur oak is the dominant tree in Mayslake’s savannas, followed by white oak, shagbark hickory, and Hill’s oak.

The migration is winding down, but there has been a clear shift of warblers, vireos and others into the savannas. The few remaining in the degraded woodland are species like yellow-bellied and alder flycatchers, which like low dense brush. Now that there are lots of hiding places in the oak canopy, it seems, that is the place to be.

Birds and Buckthorn

by Carl Strang

Birds eat bugs. That’s a 3-word description of the main action going on these days as migrant songbirds drop into our woodlands, refueling during the day before they continue on their way north at night.

Buckthorn provides no bugs, nor does honeysuckle. These Eurasian shrubs might as well be made of plastic, as far as our native insects are concerned, and so support none of the fuel needed by those feathered foragers.

So, why does it so often seem that more migrants can be found in woodlands with buckthorn and honeysuckle understories rather than restored woodlands with a diversity of herbaceous native plants beneath the trees?

This has been the hottest part of Mayslake Forest Preserve in recent days for diverse migrant songbirds. Its dominant understory plant is buckthorn. The nearby restored savanna, rich in native herbs, has some migrants, but not nearly so many species or individuals.

Some people who are excellent birders (but uninterested in ecology or any other aspect of natural history) have made similar observations, and so have a negative view of restoration.  It’s hard to blame them, given their data and focus.

So, what’s going on here? I have a hypothesis, but it needs testing (unless someone already has done so and I haven’t encountered the paper). I think the issue is security.

Here a Wilson’s warbler rests in the dense foliage of a buckthorn bush in the same area at Mayslake.

Both the savanna and the buckthorn-infested woodland have plenty of trees, and most of the insects these birds are after are feeding on the trees’ flowers and tender new leaves. The herbaceous plants in the savanna will provide abundant insects later in the season, but not much to speak of, this early. In other words, food availability probably is similar between the two kinds of places during this peak migration time (a test of my hypothesis would require measurements to confirm that statement, however).

I suspect that the buckthorn’s appeal is that it provides a ready hiding place for the migrants. They want a secure retreat if a hawk comes along, and the blanket of dense shrubbery beneath them has that quality. As dawn breaks, birds that have been flying all night are looking for a place to spend the day. Trees provide the food, and the dense patches of understory shrubs complete a clear target for weary, hungry, safety conscious birds.

Bird Habitat Preferences: Winter Residents

by Carl Strang

The dark-eyed junco may be our most familiar winter visitor among the birds. They nest in Canada and the northern states, so we see them here only from September to April. 

Juncos are gray and white sparrows, close enough relatives of white-throated sparrows that the two occasionally hybridize.

Their nesting habitat is forests, especially forest edges. What do they prefer in winter? Yesterday I showed how two local breeding birds prefer woodlands, to the near total avoidance of open areas. Such is not the case for juncos. At Mayslake Forest Preserve over two years I have a total of 36 observations in open habitats, 307 in savanna, and 61 in forest. Expectations based on the proportional acreage in those habitats are 125, 174 and 105, respectively. Though they spend some time in the open, and some time in forest, like Goldilocks they find the intermediate mix just right. Given that their winter diet is mainly seeds they pick up off the ground, this is not surprising as savannas provide both seeds and shelter, forests provide shelter but fewer seeds, and open areas have abundant seeds but limited shelter of the kind juncos like.

The other winter species for which I had enough data to look at habitat preference is the American tree sparrow.

The black spot on the chest, red cap, and 2-toned bill distinguish the American tree sparrow.

Here the observations number 62 in the open, 55 in savanna, and 37 in forest. Corresponding expectations based on habitat areas are 48, 66, and 40. It will come as no surprise to readers familiar with this species that the open appears to be preferred, but these observations are not far from the expectations and in fact the difference is not statistically significant. It appears that the tree sparrows are taking full advantage of the seed stocks available in the open as well as the shelter provided by wooded habitats. They nest north of where juncos spend the summer, in open tundra areas and zones of mixed tundra and scattered trees, so the differences between the two species on their wintering grounds are consistent.

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