Jeffrey P. Hoover

Effects of forest patch size on nesting success of wood thrushes

-Declines of many forest-dwelling Neotropical migrants have been attributed, in part, to fragmentation of forest habitat on the breeding grounds in North America. During 1990-1991, we determined reproductive success of Wood Thrushes (Hylocichla mustelina) nesting within contiguous forest habitat (>10,000 ha) and in nine forest fragments ranging in size from 9.2 to 126.5 ha in Berks County, Pennsylvania. We located 171 Wood Thrush nests. Nesting success differed significantly among forest size categories, with 86% of the nests successful in contiguous forest, 72% successful in large fragments (>100 ha), and 43% successful in small fragments (<80 ha). The variable that best predicted nest survival was forest area (R2 = 0.86). Rates of predation differed significantly among forest size categories, and predation was the primary cause of nesting failure. We found 56% of the nests in small fragments were lost to predators as compared to 22% in large fragments and 10% within the contiguous forest. Visitation by mammalian predators to scent posts was significantly different between small and large forest sites (41 vs. 14%, respectively), and relative abundance of avian nest predators was significantly higher in small forest fragments than in the large forest sites (x = 1.04 vs. 0.41 birds per census point). Brown-headed Cowbirds (Molothrus ater) parasitized 9% of the nests. Rates of brood parasitism did not differ significantly among forest size categories and had little influence on nesting success. Our results suggest that reproductive success of Wood Thrushes nesting within contiguous forest is high and that severe reproductive dysfunction as a result of high rates of nest predation is an important consequence of forest fragmentation. Received 6 July 1993, accepted 21 November 1993. MANY FOREST-DWELLING NEOTROPICAL migrant songbirds have undergone population declines in eastern deciduous forests of North America (Robbins et al. 1989a, Askins et al. 1990). Most declining forest species are found less commonly than expected in small areas of forest than in large (area sensitive), and some of these songbirds have decreased in abundance or disappeared completely from small forest patches (Ambuel and Temple 1983, Blake and Karr 1984, Askins et al. 1987). Because forest fragmentation results in smaller forest patches or fragments separated from each other by nonforest habitat, these declines have been attributed, in part, to fragmentation of forest habitat in North America (Robbins 1979, Askins et al. 1990). Numerous hypotheses have been proposed to explain why forest-dwelling Neotropical migrants decrease in abundance or disappear from small forest patches (Askins et al. 1990). As a forest becomes fragmented, the amount of edge habitat increases and the amount of interior de3Present address: Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, Illinois, 61820, USA. creases. Mammalian nest predators such as eastern chipmunks (Tamias striatus) and raccoons (Procyon lotor; Bider 1968, Forsyth and Smith 1973) tend to be more abundant along the forest edge than in the forest interior. Whitcomb et al. (1981) suggested that avian nest predators, such as Blue Jays (Cyanocitta cristata) and American Crows (Corvus brachyrhynchos), show a similar pattern. The Brown-headed Cowbird (Molothus ater), a brood parasite, also is more abundant along edges (Brittingham and Temple 1983). In addition, rates of both nest depredation and brood parasitism are higher near forest edges than within the forest interior (Gates and Gysel 1978, Chasko and Gates 1982, Brittingham and Temple 1983, Temple and Cary 1988). Consequently, one hypothesis is that forest birds decline in number in small forest patches as a result of poor reproductive success due to high rates of predation on eggs and nestlings, and to brood parasitism (Brittingham and Temple 1983, Wilcove 1985, Askins et al. 1990). Experiments with artificial nests indicate that nest predation is more frequent in small forest patches than within extensive areas of forest, thus supporting this hypothesis (Wilcove 1985,

Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs

Why do many hosts accept costly avian brood parasitism even when parasitic eggs and nestlings differ dramatically in appearance from their own? Scientists argue that evolutionary lag or equilibrium can explain this evolutionary enigma. Few, however, consider the potential of parasitic birds to enforce acceptance by destroying eggs or nestlings of hosts that eject parasitic eggs and thereby reject parasitism. This retaliatory “mafia” behavior has been reported in one species of parasitic cuckoo but never in parasitic cowbirds. Here we present experimental evidence of mafia behavior in the brown-headed cowbird (Molothrus ater), a widely distributed North American brood parasite. We manipulated ejection of cowbird eggs and cowbird access to predator-proof nests in a common host to test experimentally for mafia behavior. When cowbird access was allowed, 56% of “ejector” nests were depredated compared with only 6% of “accepter” nests. No nests were destroyed when cowbird access was always denied or when access was denied after we removed cowbird eggs, indicating that cowbirds were responsible. Nonparasitized nests were depredated at an intermediate rate (20%) when cowbirds were allowed access, suggesting that cowbirds may occasionally “farm” hosts to create additional opportunities for parasitism. Cowbirds parasitized most (85%) renests of the hosts whose nests were depredated. Ejector nests produced 60% fewer host offspring than accepter nests because of the predatory behavior attributed to cowbirds. Widespread predatory behaviors in cowbirds could slow the evolution of rejection behaviors and further threaten populations of some of the >100 species of regular cowbird hosts.

Multiple effects of brood parasitism reduce the reproductive success of prothonotary warblers, Protonotaria citrea

Abstract Avian brood parasitism often has multiple negative effects on the reproductive success of the host. Most studies have focused on one or two of these effects, but rarely have they all been studied simultaneously for one species. I studied prothonotary warblers to quantify the effects of different intensities of (i.e. multiple) brood parasitism by brown-headed cowbirds, Molothrus ater, on the production of host and cowbird young and on the between-year returns of adult warblers. Host clutch size decreased with an increase in the number of cowbird eggs laid in nests. The hatching success of warbler and cowbird eggs decreased with increased cowbird eggs in nests, but was always higher for cowbird eggs than warbler eggs. The survival of warbler nestlings, but not cowbird nestlings, decreased with increased cowbird nestlings in the brood. An increase in the number of cowbird nestlings in broods resulted in a reduction in the average mass of warbler nestlings but not cowbird nestlings. The number of cowbird eggs or nestlings present did not affect nest predation, and the fledging of cowbirds did not influence the renesting interval of female warblers. In addition, the between-year returns of adult warblers were not negatively affected by brood parasitism. Decreased hatching success and nestling survival reduced the reproductive output of the warblers the most. These effects were substantial and appear to favour the evolution of behavioural responses that reduce the effects of brood parasitism on prothonotary warblers. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

Juvenile Survival in a Neotropical Migratory Songbird Is Lower than Expected

Attempts to estimate and identify factors influencing first-year survival in passerines, survival between fledging and the first reproductive attempt (i.e. juvenile survival), have largely been confounded by natal dispersal, particularly in long-distance migratory passerines. We studied Prothonotary Warblers (Protonotaria citrea) breeding in nest boxes to estimate first-year survival while accounting for biases related to dispersal that are common in mark-recapture studies. The natal dispersal distribution (median = 1420 m; n = 429) and a distance-dependent recruitment rate, which controls for effects of study site configuration, both indicated a pattern of short-distance natal dispersal. This pattern was consistent with results of a systematic survey for birds returning outside the nest box study sites (up to 30 km in all directions) within a majority (81%) of total available bottomland forest habitat, further suggesting that permanent emigration outside of the study system was rare. We used multistate mark-recapture modeling to estimate first-year survival and incorporated factors thought to influence survival while accounting for the potential confounding effects of dispersal on recapture probabilities for warblers that fledged during 2004–2009 (n = 6093). Overall, the average first-year survival for warblers reared without cowbird nestmates was 0.11 (95% CI = 0.09–0.13), decreased with fledging date (0.22 early to 0.03 late) and averaged 40% lower for warblers reared with a brood parasite nestmate. First-year survival was less than half of the rate thought to represent population replacement in migratory passerines (∼0.30). This very low rate suggests that surviving the first year of life for many Neotropical migratory species is even more difficult than previously thought, forcing us to rethink estimates used in population models.

Experiments and observations of prothonotary warblers indicate a lack of adaptive responses to brood parasitism

It has been suggested that prothonotary warblers, Protonotaria citrea, respond adaptively to brood parasitism by brown-headed cowbirds, Molothrus ater, even though they lack historical habitat and range overlap with cowbirds. I studied behaviours functioning as potential defences against brood parasitism in the prothonotary warbler, a cavity-nesting host species. Opening sizes preferred by prothonotary warblers were not small enough to exclude cowbirds, and warblers were parasitized heavily in nests with larger openings. Male and female prothonotary warblers were always away from their nests before sunrise when cowbirds laid eggs in their nests. Prothonotary warblers infrequently (6% of 560 nests) deserted nests that were parasitized during the egg-laying period, but frequently (56% of 151 nests) deserted nests that were parasitized before a female warbler laid her first egg. Prothonotary warblers also deserted 60–70% of nests where a cowbird egg, warbler egg or die were experimentally added before egg laying. However, the experimental addition of one of these three objects during the egg-laying period did not elicit desertion. The desertion of parasitized nests was not affected by nest site availability as has been reported elsewhere in the literature. This lack of a response to brood parasitism by prothonotary warblers may be an example of evolutionary lag, because it is likely that they have only recently been exposed to widespread parasitism, and they accept parasitism at a high cost to their own reproductive success.  2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Effects of hydrologic restoration on birds breeding in forested wetlands

Channelization of rivers and streams threatens bottomland forest bird communities because it can lead to the formation of lateral gullies that connect streams to adjacent wetlands and unnaturally accelerate the draining of wetlands, potentially exposing some birds to high rates of nest predation. I studied how the hydrologic restoration of off-channel wetlands (plugging gullies that drain off-channel wetlands) affects the diversity, abundance, and nesting success of birds breeding within forested wetlands within the Cache River watershed in Illinois. I compared surface area, water depth, bird diversity, bird densities, and nesting success between treatment (gully plugs added) and control (gully plugs not added) wetlands pre- and post-treatment. During the breeding season of birds, treatment wetlands retained more flooded area and greater depths of water compared to control wetlands. Bird diversity was unaffected by the installation of gully plugs. The density and nesting success of prothonotary warblers (Protonotaria citrea) was higher in treatment wetlands than in control wetlands. Documenting changes in the bird community in response to this conservation action provides a means to measure the success of restoration activities in the Cache River watershed and inform conservation plans and restoration efforts in other bottomland forest ecosystems.

Effectiveness of Small Nature Preserves for Breeding Birds

The ecological effects of habitat fragmentation pose problems for birds breeding in small nature preserves. Negative effects of habitat fragmentation have been well documented in breeding birds of midwestern forests and grasslands (Robinson 1988, Robinson and Wilcove 1994, Herkert 1994, Robinson et al. 1995). Area sensitivity, the absence of birds from small tracts even when suitable habitat is present, is pronounced in midwestern grasslands (Herkert 1991) and forests (Bond 1957, Kendeigh 1982, Ambuel and Temple 1982, Hayden et al. 1985, Blake and Karr 1984, 1987). Freemark and Collins (1992) found that area sensitivity was more extreme in the isolated tracts of the Midwest than in the more closely spaced tracts of forest of eastern North America. Although there are few long-term censuses from midwestern fragmented habitats, available evidence suggests long-term declines in populations of many species in fragmented grasslands (Herkert et al. 1992) and eastern forests (Hagan 1993, Wilcove and Robinson 1990).

A generalist brood parasite modifies use of a host in response to reproductive success.

Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds (Molothrus ater) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler (Protonotaria citrea), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.

Relative abundance, habitat use, and long-term population changes of wintering and resident landbirds on St. John, U.S. Virgin Islands

Abstract St. John, U.S. Virgin Islands, is one of the most forested islands in the West Indies and provides an opportunity to conserve both resident birds and wintering neotropical migrants. We conducted double-observer point counts of landbirds in December 2005 and 2006 in Forest Inventory and Analysis (FIA) plots and National Park Service (NPS) trails in Virgin Islands National Park (VINP) to assess population trends of birds in subtropical dry and moist forests. We recorded 2,270 individual birds representing 35 species at 150 point count stations in 2005, and 3,092 individuals of 32 species at 143 of these stations in 2006. The increase in birds per point from 2005 (15.1) to 2006 (21.6) was due to resident species, 17 of which were recorded more frequently in 2006. The 17 species of neotropical migrants composed 11.8% of all registrations in 2005 and 2006. Subtropical moist and dry forest habitats differed strongly in vegetation characteristics and plant species, but no species of birds exhibited a strong affiliation with either habitat type on FIA plots. Data from NPS trails showed that most migrant species were detected more often in moist, mature forest. The resident Bridled Quail-Dove (Geotrygon mystacea) also was correlated with mature forest. Plant and bird species co-occurrence with positive correlations that may carry a signal of preferred frugivory included Guettarda odorata (Rubiaceae) with Bridled Quail-Dove, and Myrciaria floribunda (Myrtaceae) with Pearly-eyed Thrasher (Margarops fuscatus). Migrant species did not exhibit strong long-term changes in relative abundance since founding of VINP in 1957, but four open-country resident species declined significantly between 1957 and 2006 as the forest matured. Forest maturation should continue on St. John, yielding a bright future for most of its landbirds barring catastrophic hurricanes, pathogens, or invasive plants.