Sunday, July 31, 2005

Midsummer

The weather has been perfect for birding the last couple of days--light breeze and cool mornings. And although this is a quieter time than the spring, it is also a time when there are both birds nesting, birds courting, birds molting, and birds back on the move.

Friday morning I saw and/or heard these birds as I stood in the yard: savannah sparrow, chipping sparrow, cardinal, wrens, chickadees, crow, blue jay, phoebe, chimney swift, cedar waxwings, goldfinches, flicker, rose-breasted grosbeak, and an unidentified singer. Out in the fields there was a goldfinch calling incessantly, a contact call I suspect, but one I didn't know. Five great blue herons flew overhead together, another followed a few minutes later--family group? A pair of towhees calling a singing. A field sparrow carrying food. And the female common yellowthroat was keeping watch at the willows by the cedar bush.

This morning was even more business than travel. Because it was so cool, and because the swamp is all mud, no water now, I was able to get right into the cedar bush for the first time in a long time.

This is a view across one of the swampy clearings in the bush.

And I finally caught the common yellowthroats carrying food. The first, by the willows on the outside of the bush, because I came along and caught her at it. The second, in a swampy clearing in the bush, because he didn't see me.

Joe Pye weed over my head.

I was hidden by the Joe Pye weed. This plant did pretty well last year for the first time since I've been walking here, but this year it is dominating the clearings in the cedar bush, and standing about 2 metres high, compared to just over a metre last year. Other years Virgin's bower clematis has dominated in here.

At the beginning of the breeding bird season this year when I looked over my data for the previous 4 years I was surprised to see that no one, not me, not the more expert birders who worked in my square last year, had been able to confirm the common yellowthroat as a breeder. To confirm a bird, you need to see it entering or leaving a nesting site or carrying food or a fecal sac, or you need to see new fledglings, a nest with eggs, or hear or see a nest with nestlings. The common yellowthroat was down as a "probable" breeder, on the basis that it had been observed to be on territory, i.e., had been heard singing in the same place over a period of time (two occasions a week apart is enough). The bird is so common here I was determined to upgrade its status. In a recent post I wonder why this bird is so successful compared to other warblers with similar habitat and food requirements. After spending the last little while concentrating on getting the breeding evidence I need I am beginning to develop some speculations about this.

The day that I saw the Brewster's and blue-winged warblers carrying food, and heard their young, the common yellowthroats were there too, but they spent the whole time that I was there watching me, chucking, etc. I knew that there was a nest or young in the area from their behaviour, but I didn't know where. I did know where the Brewster's young were, and had I had a taste for young warblers I could have swooped in and found them by following the parents and the noise of young being fed. Today, when I spied the male common yellowthroat from my hiding place in the Joe Pye weeds, I wondered why he wasn't "chucking." Then after a while, and after he'd captured a nice fat caterpillar and was carrying it away, suddenly he started to call. He'd seen me. Hypervigilance has its price. In a busy environment, and this is certainly that, I can count 7 nest predators just off the top of my head, the young of the hypervigilant get fed less often than those of the more casual parent. As I watched the day the Brewster's and friend were in the willows, I must have seen them carrying at least five caterpillars each to the brood, while the common yellowthroats got nothing. Feed them faster, they grow faster, and are less vulnerable sooner. On the other hand more vigilance, lose fewer nestings, more chance that the young that are raised make it to adulthood.

According to the Wildspaces life history pages on the common yellowthroat and the golden-winged warbler there are two interesting differences between these two that may be affecting their relative success. First, the common yellowthroat nest twice, while the golden-winged only nests once, though the clutch sizes are almost the same. And second, golden-winged nests are parasitized by brown-headed cowbirds at twice the rate of common yellowthroat nests. (Here's a link to the Wildspaces page on the blue-winged warbler--parasitism rate is somewhere in between.) Both birds are small, both nest low or on the ground. What accounts for the difference? Perhaps it is just that much more difficult to find a common yellowthroat nest.

I tried to find a control for the comparison, in case the parasitism rate is affected by the number of nestings. I wondered if this was important because I've noticed that the brown-headed cowbirds seem to finish up their breeding business by mid July. That is, the males stop singing and chasing females by then, and they seem to take off somewhere, perhaps to join the flocks of starlings and grackles that are forming around this time. But I couldn't find anything obvious quickly. American redstarts only nest once, have the same number of offspring per nesting as common yellowthroats, nest deeper in the woods and higher up, and have a similar parasitism rate to them (20% compared to 19%). The redstart also approaches the common yellowthroat as a vigilant parent. I don't have a reference for a rate of nestings lost to other causes (predation, insect infestation, weather, etc.) for any of these birds--but I'd be very surprised if the nest-hiding vigilance of the common yellowthroat did not keep its rate of nest predation down compared to other species.

Budding Joe Pye--This place will be spectacular when the blooms are at their peak.

When I'd had enough of this good father, I let him get back to feeding his young and made my way out of the cedar bush, flushing a grouse as I went.

Jewel weed--just past its prime.

A catbird was also relieved that I finally took myself off. As made my way to the western edge of the bush I heard a great crashing in another of the swampy clearings--the clearings are so choked with vegetation now that I couldn't see anything. Sounded like a deer's step--I haven't seen any bear sign this year, not since the cub's cries I heard from the edge of the far field earlier in the season. But the raspberries are ripening...

On the far side of the far field the agitation continued. An eastern towhee seemed not glad to see me, as was the case with the female American redstart and the female cardinal. I didn't see the Brewster's or the blue-winged warbler, nor any sign of the chestnut-sided warbler that sang so loudly and so often earlier in the season.

Heading back to the first field I noticed a kingbird on the old dead tree between the first and second fields. A second kingbird flew in, but I didn't see where it landed.

Keeping watch.


I suspect that they're nesting in this fencerow somewhere. After the two failures in the old apple tree in the far field (the second one almost certainly had eggs in it, as I observed a bird sitting on it for long periods on a couple of occasions before the nest disappeared), I'm glad that I can't see this nest--maybe the crows, or whoever took the other two, won't be able to see it either.



1 comment:

Trix said...

Hard to believe it is Midsummer when so many birds are on the move; we've seen lesser Yellowlegs and Solitary Sandpipers drop by, and tanagers losing their colours. It is a reminder only birders would notice. Sigh...