[TN-Bird] Your Migrants in Costa Rica (long)

Ola from Costa Rica! I would like to announce my intent this spring to file 
irregular reports on the passage of neotropical migrants as they leave or 
pass through Costa Rica.

SPRING MIGRATION

I have provided, at the end of this email, a list of the principal species 
that winter here in Costa Rica at our elevation (4000 feet) and position on 
the Pacific slope (Cordillera de Tilaran near Monteverde), and those that 
commonly migrate through here.  I have indicated the most common ones with 
asterisks (*).  Over the next two months or so, I will try to report when 
migrants are passing through (or appear to be leaving) and indicate whether 
numbers appear high (just a general impression) and when their numbers 
appear to have peaked.

MIGRANTS THIS PAST FALL AND WINTER

I have been very lax in reporting neotropical migrants here in our location 
in Costa Rica this fall and winter.  The chief reason has been the general 
lack of activity by most migrants in our area.

Aside from a few common regulars (Summer Tanager, Wilson?s Warbler, 
Black-and-white Warbler), numbers have been very low when compared to the 
previous winter.  Whereas last winter most insectivorous feeding flocks 
consisted of about 50% local species and 50% neotropical migrants, this year 
about 90% of the birds in these small flocks are local species (chiefly 
Golden-crowned Warblers, Common Bush-tanagers, Slate-throated Redstarts, 
Olivaceous Woodcreepers, etc.).  At times all of the usual neotropical 
species have been mixed into these flocks, but only in very low numbers and 
only occasionally (Yellow-throated Vireo, Golden-winged Warbler, Blue-winged 
Warbler, Tennessee Warbler, Black-throated Green Warbler, Townsend?s 
Warbler).

There are several hypotheses I propose for this rather drastic change 
between winter 2004-5 and this past winter:
1)      There are fewer migrants in Costa Rica this year for various reasons, 
possibly the extremely high number (and high intensity) of hurricances 
during fall migration?I personally do not think this is very plausible as I 
have seen good numbers of the same species at lower elevations.
2)      There was excessive rainfall here throughout September and October (42 
out of 46 days the sun did not come out at all and it rained pretty much 24 
hours a day?a result of the incredible hurricane activity in the southern 
Caribbean during those months).  This year was in stark contrast to the 
previous years? weather during these months, when rainfall was unusually low 
and most days were sunny.  This year?s conditions resulted in terrible 
feeding conditions for newly arriving insectivorous neotropical migrants, 
who therefore (according to this hypothesis) chose to set up there winter 
feeding territories elsewhere (probably at slightly lower elevations and/or 
on the Caribbean slope where there was less rain).
3)      The excessive rainfall in Sept. and Oct. resulted in very low 
reproductive success among many insect species, since many attempt to mate 
in the air and continuous rainfall day and night can make this very 
difficult.  This resulted in a very low supply of insect larvae and adults 
for months after the fall rains.  In fact, we have observed very few insects 
eating our orchard trees, native trees, or in our vegetables this winter 
compared to the year before.

I strongly believe that both hypotheses 2 and 3 are both together the causes 
of the change, though I have no hard data to back that up.  This, to me, 
demonstrates the tremendous potential impact that climate changes can have 
on bird behavior.  At the same time, it hints at the adaptability of most 
species to such changes when such adaptations are possible?for example, 
moving to a lower elevation or to a different slope position.  Of course, 
one should note the caveat of ?when such adaptations are possible.?
        Hypothesis 2 also indicates that once they chose a general wintering 
territory, many birds have a fair amount of faithfulness to these 
territories.
        Whether my speculations are correct or not, it certainly has been an 
interesting year here, though I prefer it when there are more species rather 
than less!

Here is my list of common neotropical migrants here, which I hope to report 
on:

WINTER RESIDENTS (and Passing Migrants) LEAVING:
Wood Thrush
*Yellow-throated Vireo
*Black-and-white Warbler
Worm-eating Warbler
*Golden-winged Warbler
Blue-winged Warbler
*Tennessee Warbler
Yellow Warbler
*Townsend?s Warbler
*Black-throated Green Warbler
Ovenbird
Kentucky Warbler
*Wilson?s Warbler
*Baltimore Oriole
Eastern Meadowlark
*Summer Tanager

MARCH MIGRANTS:
Broad-winged Hawk
*Ruby-throated Hummingbird
Barn Swallow
*Northern Rough-winged Swallow
*Swainson?s Thrush
La. Waterthrush

APRIL MIGRANTS:
Eastern Kingbird
Olive-sided FC
Eastern Wood-Pewee
Empidonax spp. (chiefly Alder, Willow, Yellow-bellied)
Philadelphia Vireo
*Red-eyed Vireo
*Blackburnian Warbler
Canada Warbler
Scarlet Tanager
*Rose-breasted Grosbeak
Indigo Bunting

Dev Joslin
Monteverde, Costa Rica


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