- 7 Jun: Daniel Froehlich (driver), Mike Walker, Don Norman
- 14 Jun: Daniel Froehlich (driver), Don Norman, Mary Huff, Burney Huff
The season started with two days of great banding weather: nights on the warm side with no dew, cloud cover without bright sun. Both days it started drizzling right after we closed the nets—what luck!
With the coldest April on record and the greatest number of days in row without breaking 70° in our region ever, this year’s witnessed delays in many seasonal markers. The Rufous hummers seem to be late too, with males still actively displaying and chasing females on the 14th. Often, the last males we see are during our first banding session in early June. On the 7th, net 19 kept catching the same female with a distinctive large patch of gorget feathers on the throat in the same spot, apparently in her flight path from stands of honeysuckle to her nest. She was still using the same flight path on the 14th, so her multiple encounters with our inconvenient net hasn’t disrupted her dogged activities.
Last summer I noted how tricky micro-ageing is when you try to jump into it cold. This year, I had another reminder. The first bird out of the net this season was an Orange-crowned Warbler male; the size of the crown patch at 15mm was in the overlap zone between SY and ASY birds, so I checked the wing molt criteria. Coverts and flight feathers showed strong wear and were clearly bleached brown, and the rectrix tips were heavily worn—tidy SY package was my conclusion. I discounted the creeping doubts I had about the clearly green-edged primary coverts. I was carried away by the heavy wear, classic symptoms of second-year birds who retain their juvenal flight feathers and primary coverts. Since they’ve been on the bird three months longer than flight feathers grown by an adult would be (grown in the fall molt) and since they contain less feather material (grown simultaneously with the body plumage in the nest), heavy wear and bleaching is a pretty reliable indicator for second-year birds, especially by comparison with adults wearing fresher definitive plumage. But ever since my banding work with Manomet Observatory in Belize in 1993, I’ve wondered about the impact of deforestation in Central America on feather wear and bleaching in migratory warblers. With an increasing proportion of habitat on their overwintering destinations converted from closed-canopy forests to heavily disturbed second-growth habitats, I’ve wondered whether more of the wintering birds find themselves making do in habitats where their exposure to sun and UV radiation is greater, generating more solar bleaching and more brittle feathers. For species for which such habitats are second-choice compared to the less-available but preferred closed-canopy forests, it’s likely to be individuals with limited social clout—typically SY birds and often the smaller females—coping with the resultant increased bleaching and wear.
Orange-crowned Warbler numbers are high this year. We may be experiencing a population boom; they’re singing all over the station. So when I recaught my worn individual #2650-04483 on the 14th, I had already seen several other OCWAs in hand this season. The wear was on the high side, true, but the bright green edging on the primary coverts was a dead give-away for definitive plumage: no SY showing that much sun-bleaching would still have clear green edging on the primary coverts. I’d been fooled by an adult male that apparently spent more of the winter exposed to strong sun than is typical for his demography!
Confusing birds like that make obvious molt limits all the more satisfying.
This Yellow Warbler’s molt limit in the greater coverts between the outer three, which are retained juvenal feathers, and the inner seven, which are probably formative feathers from last fall, is so obtrusive that observers are more likely to mistake them for different feather tracts than they are to miss the difference. The outer tertial has also been replaced. There are actually three feather generations represented: the innermost visible great covert (#9) and the median coverts seem to be alternate feathers replaced this spring.
Several individuals didn’t show any breeding condition (secondary sexual charactistics, specifically brood patch or cloacal protuberance). This is surprising since we’re well into the breeding season. For Swainson’s Thrush it’s actually not so unusual: we regularly catch a few birds during the first half of June that don’t show breeding condition—yet. It turns out that when we do recatch these individuals later in the season, they show full breeding condition, so they’re likely just late arrivals—after all they’re coming all the way from the Amazon basin—still undergoing their transformation from migratory to reproductive physiology.
But two Yellow Warblers and two Cassin’s Vireos without breeding condition were unexpected. Perhaps they’re a bit delayed because of the delayed season, but another Yellow Warbler singing in the bag while waiting for processing showed that at least his reproductive hormonal axis was already well-greased! The two Yellow Warblers lacking breeding condition sported female plumage, the sex migrating later in spring. But one of them surprised me by showing active molt characteristic of incipient prebasic molt.
Molt of a swatch of a few feathers somewhere on the back or side isn’t all that unusual among breeders: territoriality and mating are like Rumsfeld’s democracy: they’re messy and feathers will fly! But when just a few feathers are growing in the middle of both the breast and dorsal tracts, it looks suspiciously like the start of prebasic molt. A migrant starting its fall molt in mid-June, however, is forgoing an entire breeding season, a big evolutionary cost for a bird that can expect no more than five or six total breeding seasons over its life! I’ve observed “premature” fall molt in a few other species, including Carolina Chickadees molting in May in Virginia and Steller’s Jays.
It would be valuable to identify the triggers for skipping breeding. All the passerines I’ve handled favoring molt over breeding were SY birds, so they were skipping their first breeding season. This is a classic strategy for species with limited opportunities for success during their first breeding season, due to limited habitat or nest-site availability. Second-years of such species often forgo breeding in favor of serving as helpers-at-the-nest, usually of related individuals. But Carolina Chickadees or Steller’s Jays don’t serve as nest-helpers, nor do Yellow Warblers! The Yellow Warbler was shocking to me, since here’s a 10-gram bird that’s just flown several thousand kilometers—just to molt up here! What’s the trade-off for these individuals: Is suitable breeding habitat already saturated with breeders? Is their body or plumage condition too poor to begin with? Is breeding too great an additional strain? Evolutionarily speaking, one of these situations that guarantees breeding failure for first-time breeders is a precondition for evolving nest-helping. Does the Yellow Warbler gain beneficial experience or knowledge from being present during the breeding season even if she’s just molting, or are the multiple thousand kilometers round trip she’ll fly an evolutionary waste of energy?
As for losing feathers, we occasionally induce accidental feather loss among birds during handling, giving observers cause for concern. But we also regularly catch birds recovering from feather loss they incurred prior to meeting a human close-up.
Here, for example, is a Swainsons’ Thrush actively regrowing 11 of its 12 rectrices, lost recently perhaps in a territorial dispute or a narrow escape from a cat. It seemed none the worse for wear, though it was one of the Swainson’s Thurshes lacking breeding condition.
The first MacGillivray’s Warbler of the season had an unfortunate end: weakened during processing, it died shortly thereafter, a reminder of the risk netting imposes and the importance of aspiring to the safest handling possible. No other birds showed any cold symptoms, nor did he show any obvious injuries. To make matters worse, it turned out that this male was one of our elders, first captured in 2004 as AHY-M, so at least 8 years old! Further, the only MGWA song we heard on the 7th was at the net where he met his demise. We were relieved to catch and hear several others on the 14th.
Here’s what we caught 7 Jun:
- Rufous Hummingbird 8
- Bewick’s Wren 1
- American Robin 2
- Swainson’s Thrush 4
- Orange-crowned Warbler 2
- Yellow Warbler 3
- Common Yellowthroat 4
- MacGillivray’s Warbler 1
- Song Sparrow 2
- Purple Finch 4
- Total 32
And on 14 Jun:
- Rufous Hummingbird 8
- Willow Flycatcher 3
- Pacific-Slope Flycatcher 1
- Cassin’s Vireo 3
- American Robin 1
- Swainson’s Thrush 4
- Orange-crowned Warbler 6
- Common Yellowthroat 2
- MacGillivray’s Warbler 1
- Song Sparrow 4
- Oregon Junco 2
- Total 32