HOUSE WRENS IN MONTEVERDE: A POPULATION SINK?
Bruce E> Youngome habitats provide better conditions for sur-
vival and reproduction than others. Individu-
als that live in "source" habitats generally sur-
vive and reproduce well, with births outnumbering
deaths. In "sink" habitats, deaths outnumber births
and the local population is maintained by the immi-
gration of surplus individuals from source habitats
(Pulliam 1988). House Wrens living in pastures and
gardens in the upper Monteverde community appear
to be a species that occurs in a sink habitat maintained
by immigration.
In Monteverde, I individually marked all adult
House Wrens and their young in an area encompass-
ing 60 territories. During three years, I monitored the
survivorship of all individuals and the success of all
nesting attempts. I calculated X, the population multi-
plication rate (B. Young, unpubl. data); A, is less than 1
in declining populations, equal to 1 in stable popula-
tions, and greater than 1 in expanding populations. I
found that A varied from 0.79 ± 0.18 (95% confidence
interval) to 0.81 ± 0.19 in different years (B. Young,
unpubl. data), indicating that the population should be
declining. However, the number of occupied territories
on the study area never fluctuated by more than one
or two during the study. Instead, the population was
maintained by immigration from outside the study area.
At the onset of each breeding season, I found that about
60% of the marked adults from the previous season had
disappeared and been replaced primarily by unhanded
birds not born on the study site.
What caused the study area to be a sink? Based
on my observations of attacks, I suspect that Barred
Forest-Falcons in the woodlots surrounding pastures
took a heavy toll on fledgling and adult House Wrens.
Forest-falcons were common near my study area,
which was confirmed by Valburg's (1992) observa-
tions of their attacks on Common Bush-Tanagers.
The source habitat may be lower down on the Pacific
slope, where woodlots are smaller and forest-falcons
are less common (D. McDonald, pers. comm.). Two
female House Wrens that I banded as nestlings at a
lower site turned up as breeders in Monteverde, show-
ing that dispersal upslope occurs.
In the triage of conservation efforts, we should be
certain that the habitat we are protecting supports
source populations of the species we wish to protect
(Pulliam 1988). If the Bare-necked Umbrellabird oc-
curs across 40,000 ha of forest and we can only pro-
tect 15,000 ha, we need to choose the portions that
contain source populations. Unfortunately, we sel-
dom have time for the in-depth studies that pinpoint
where the source populations are.WINDBREAKS AS CORRIDORS FOR BIRDS
Karen Nielsen & Debra DeRosierragmentation of habitats can negatively affect
animal and plant populations by restricting
gene flow, immigration, emigration, or forag-
ing (Saunders and de Rebeira 1991). Deforested areas
between remnant forest habitats may impose barriers
that hinder movements of many forest-dwelling spe-
cies. The landscape surrounding the protected forests
of Monteverde has been fragmented into a patchwork
of large forest tracts, small remnant forest patches, and
open agricultural land. Farmers have planted wind-
breaks or maintained existing natural windbreaks to
protect their cattle and forage from the detrimental
effects of strong winds (Fig. 12.11). The planting ofthese windbreaks has been aided by the Monteverde
Conservation League, which has planted more than
1000 windbreaks on farms throughout the Monte-
verde zone in a program that has both economic and
ecological benefits (see Chap. 10, Conservation Insti-
tutions). We asked whether animals use these wind-
breaks as passageways, or "corridors," to move among
forest patches.
The idea of constructing a corridor between iso-
lated forest fragments makes intuitive sense. However,
since the inception of the corridor concept, there has
been much debate about the actual use and biologi-
cal importance of corridors (Noss 1987, Simberloff448 Conservation BiologySF