NESTING PRODUCTIVITY OF WOOD THRUSHES ON THE NIAGARA ESCARPMENT

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1 NESTING PRODUCTIVITY OF WOOD THRUSHES ON THE NIAGARA ESCARPMENT Authors: LYLE E. FRIESEN AND LESLEY N. CAMPBELL Abstract: We monitored 222 Wood Thrush nests in one large forest (1,300 ha) and two small forests (20 ha each) along the Niagara Escarpment from 1997 to 2001 to determine patterns of breeding success, brood parasitism, and population viability. Across all sites, Wood Thrush nesting success was 51 percent with 41 percent of nests being parasitized. There was no significant difference in nesting success in the large forest and the small forests (44 percent vs 57 percent respectively) but there was a significant difference in the parasitism rate (8 percent vs 69 percent respectively). Nesting productivity was relatively high (1.42 female fledglings/female/year). Using a range of estimates for adult and juvenile survival, we estimated that populations on all the sites were sources in most years. Our findings suggest that the high nesting productivity in southern Ontario relative to that in the American midwest results principally from lower parasitism intensity in Ontario. Predation pressures on nests in both small woodlots and large woodlots on the were similar but nests in small woodlots experienced dramatically higher parasitism levels. Large forests in southern Ontario may be especially important to small-bodied songbird species such as Red-eyed Vireo (Vireo olivaceus), and Ovenbird (Seiurus aurocapillus) that are less able to cope with cowbirds than are the Wood Thrush. INTRODUCTION Recent concerns over the persistent, long-term declines of many species of migratory forest songbirds have focused attention on the importance of breeding habitat for sustaining populations. One hypothesis predicts that large, intact forests support highly productive source populations that produce surplus offspring that disperse and may help to bolster falling populations elsewhere (Robinson et al. 1995). Small, fragmented forests, on the other hand, are unproductive sinks that depend on immigrants from large forests to subsidize their songbird populations. Reduced breeding success in the small fragments might be the result of more intense edge effects because generalist mammalian and avian predators that prey on eggs and nestlings, and Brown-headed Cowbirds (Molothrus ater) that parasitize nests, occur in greater abundance in edge-dominated landscapes. Most of southern Ontario s forest cover has been severely fragmented, first by agriculture and now increasingly by urban/rural development. Nevertheless, pockets of significant forest cover persist along the running from the Niagara River to the tip of the Bruce Peninsula. The largest block of continuous forest that remains along the Escarpment south of the Bruce Peninsula is the Hilton Falls Conservation Area (~1,300 ha) in the Region of Halton (Figure 1). The relatively large size of this forest suggests that it may serve as an important regional source for forest songbirds. 1

2 Figure 1. study area: Wood Thrushes were studied in sites marked 1, 3, and 6 Since 1997, the Canadian Wildlife Service, in partnership with the, has conducted research into Wood Thrush (Hylocichla mustelina) nesting success in the Hilton Falls forest and nearby fragmented forests. The Wood Thrush is a relatively common forest bird throughout its breeding range, including southern Ontario. However, the species has experienced steep and widespread population declines and has been identified as a high-priority species for monitoring and management by a number of agencies (e.g., Partners in Flight 2001). 2

3 We monitored Wood Thrush nests from 1997 to 2001 and tracked colour-banded individuals over the last three years to: 1) measure the level of predation and parasitism on nests; 2) measure variation in nesting success between forests of different size and between years; and 3) determine the seasonal fecundity of Wood Thrushes in order to assess population viability. METHODS Study sites The study was conducted in the Halton Region of the, an area with 44 percent forest cover (Canada MAB 2000). Three study areas were monitored in the Hilton Falls Conservation Area (a large 1,300 ha site) and two small woodlots (20 ha each) located within 15 km of the large forest were monitored in their entirety. Primary habitat at all sites was upland deciduous forest dominated by sugar maple (Acer saccharum), American beech (Fagus grandifolia), and white ash (Fraxinus america) with smaller amounts of eastern hemlock (Tsuga canadensis) and eastern white cedar (Thuja occidentalis). Nest monitoring and banding We located Wood Thrush nests and monitored their status every two to three days using a mirror attached to a telescoping pole (see Friesen et al for details on nest searching and monitoring). We mist-netted and banded adults at their nests throughout the breeding season in order to determine incidences of double-brooding and renesting among females (see Friesen et al for details on capture and banding techniques). Estimating parasitism and nesting success Parasitism rates were calculated as the proportion of nests found during the incubation stage that contained cowbird eggs. Nesting success was estimated using the Mayfield (1975) method, based on a 25-day nesting cycle. A nest was considered to be successful if Wood Thrush or cowbird fledglings survived to at least 10 days of age, when fledging is known to occur (Roth et al. 1995). Estimating population viability Published estimates of annual adult survival of Wood Thrushes range from 0.58 to 0.70 (Roth and Johnson 1993; Roth et al. 1996); juvenile survival rates range from 0.29 to 0.35 (Anders et al. 1997; Trine 1998). We adopted conservative and optimistic survival rates for adult and juvenile Wood Thrushes to calculate source-sink thresholds, following the methods in Donovan et al. (1995): 0.86 female young were therefore required to maintain population stability if survival rates were at optimum while 1.45 female young were needed if survival rates were at the minimum level. Our fecundity model (see Donovan et al for details) used nesting patterns observed in our study population, i.e., all Wood Thrushes were double-brooded (Friesen et al. 2000) and 15 percent attempted three nests in a season (L.F., unpublished data). Statistical analyses The multiple comparison method (Sauer and Williams 1989) was used to test whether the Mayfield estimates of daily survival of Wood Thrushes differed among years and woodlot size classes. Standard chi-squared procedures (Sokal & Rohlf 1981) were used to examine differences in parasitism rates among the woodlot size classes and between years. We pooled data across years, unless noted otherwise, and considered results significant if P

4 RESULTS Two hundred and twenty-two Wood Thrush nests were monitored across the study sites from 1997 to 2001 (Table 1). Nesting success for all years was 51 percent. There was no significant difference (x 2 = 3.21, df = 1, P = 0.07) in nesting success between the large Hilton Falls forest (44%, n = 102) and the two small woodlots (57 %, n = 120). The overall parasitism level on the Escarpment was 41 percent, with no significant differences in the rate between years (x 2 = 0.70 df = 4, P >0.05). However, there was a significant difference in the overall rate of parasitism between the large forest and the small forests (8 percent vs 69 percent respectively; x 2 = 37.18, df = 1, P <0.001). Fecundity values suggest that populations in both the large and small forests were potential sources in all years assuming optimistic thresholds of adult and juvenile survival (Figure 2). Using the conservative survival estimates, populations in the large and small forests would have surpassed or approached source status in two and three years, respectively. Annual fledgling productivity in large and small forests Female offspring/pair/season all years Small Forests Large Forests Figure 2. Annual female fledgling productivity of Wood Thrush pairs on the. Upper solid line is the level of productivity at the conservative source-sink threshold (adult and juvenile survival rates of 58 percent and 29 percent respectively). The lower solid line is the level of productivity at the optimistic source-sink threshold (adult and juvenile survival rates of 70 percent and 35 percent respectively). 4

5 Table 1. Nesting success, parasitism, and productivity of Wood Thrushes. Site Number of nests Nesting success (%)* Nest parasitism (%) Fecundity** Small Large Combined * Calculated by the Mayfield (1975) method. ** Mean number of female fledglings produced per adult female per year. Table 2. Annual variation in nesting success and nest parasitism of Wood Thrushes on the. Location Year Number of Nests Nesting success (%)* Parasitism (%) Small sites Large site * Calculated by the Mayfield (1975) method 5

6 DISCUSSION Both the large Hilton Falls forest and the two small forests supported Wood Thrush populations that approached the more stringent source-sink threshold and that were well above the optimistic sustainable level (Figure 2). The relatively high levels of nesting success for the combined sites (51 percent, Table 1) and annual production of female young/breeding pair (1.42) suggest that the effects of fragmentation on Wood Thrush reproduction are less severe along the Niagara Escarpment than in some other regions. By way of contrast, annual productivity per breeding pair in three forest tracts (1,100 to 2,200 ha) in southern Illinois (Trine 1998), and in 14 forest fragments (7 to 500 ha) in northern Indiana (Fauth 2000), was about half that on the Niagara Escarpment. Wood Thrush nesting success in Indiana and Illinois is often as high as in southern Ontario (Trine 1998; Fauth 2000). What sets southern Ontario apart from those regions, however, is a much lower intensity of cowbird parasitism, a pattern which conforms to other studies which show that parasitism levels decrease with distance from the historic range of the cowbird (Hoover and Brittingham 1993). Parasitism rates of over 90 percent were reported in Illinois and Indiana, a level twice that of the Escarpment s. Cowbirds frequently remove host eggs during the early stages of incubation, and the fact that most nests are afflicted in the American midwest results in fewer host eggs and more cowbird eggs per nest than in Ontario (Trine 1998; Fauth 2000). Of parasitized nests (n = 61) in Illinois that ultimately fledged young, 31 percent produced only cowbirds (Trine 1998) compared to just 9 percent in our study sites (n = 54). In southern Ontario, the presence of cowbird eggs in a nest during the incubation stage was in itself no guarantee that cowbirds would fledge even if the nest was successful, as 17 percent of such nests produced only Wood Thrushes. In almost all of these instances of cowbird failure, the cowbird eggs failed to hatch (as opposed to cowbird fledglings dying in the nest). Nesting success varied significantly between years at our large study site (32 to 63 percent) and the small sites (31 to 74 percent; Table 2). Annual variability has been reported elsewhere (Trine 1998; Simons et al. 2000; Fauth 2000) and underlines the importance of long term studies for determining the source-sink status of habitats. Taken in isolation, results from a single year could result in markedly different interpretations of the viability of a population. One of the surprising aspects of our study was the high level of nesting success in the small forests relative to the much larger Hilton Falls forest. However, high levels of Wood Thrush nesting success have been reported from other small fragments (Roth and Johnson 1993; Friesen et al. 1999). It may be that conventional fragmentation theory, which holds that nests in small forests invariably experience higher predation rates than those in contiguous forests is overly simplistic or varies by region. For example, nesting success of Wood Thrushes in the Great Smoky Mountains National Park, the largest continuous block of unfragmented forest (205,665 ha) in the eastern United States, was only 35 percent (Simons et al. 2000). This unexpected low result prompted researchers to conclude that large forests may support a more diverse and abundant predator community than is found in forest fragments. 6

7 The true value to forest birds of large habitat blocks in southern Ontario, if not protection from generalist nest predators, may be the refuge they offer from cowbird parasitism. Wood Thrushes are a relatively large forest passerine species with a short incubation period that typically raises all or most of their own young that hatch in a parasitized nest (Rothstein and Robinson 1998, L.F, personal observation). Of greater concern are smaller hosts such as vireos and Empidonax flycatchers that nearly always experience total losses of host young when parasitized (Trine et al. 1998). The fact that only 8 percent of Wood Thrush nests in Hilton Falls were parasitized (Table 1), that parasitism was absent in three years, and that none of the nests in the study plot located deepest in the forest were ever victimized, suggests that in southern Ontario forests of this size may be critical to the viability of smaller species such as Least Flycatcher (Empidonax minimus), Red-eyed Vireo (Vireo olivaceus) and Ovenbird (Seiurus aurocapillus). ACKNOWLEDGEMENTS We are grateful to the landowners that permitted us access to their lands and the many field assistants who participated in the project over the years. This project was supported in part by the Ontario Region of Environment Canada s Science Horizons and the Canadian Wildlife Service, the, Human Resources Development Canada and the Ontario Heritage Foundation. LITERATURE CITED Canada MAB Landscape changes at Canada s biosphere reserves. Toronto: Environment Canada. Donovan, T. M., F. R. Thompson, J. Faaborg and J. R. Probst Reproductive success of migratory birds in habitat sources and sinks. Conservation Biology 9: Fauth, P.T Reproductive success of Wood thrushes in forest fragments in northern Indiana. Auk 117: Friesen, L.E., M.D. Cadman and R.J. MacKay Nesting success of Neotropical migrant songbirds in a highly fragmented landscape. Conservation Biology 13: Friesen, L.E., V.E. Wyatt, M.D. Cadman, R.J. MacKay, E.D. Cheskey, M. Allen and D. Ramsay Extent of double-brooding and seasonal movement of nesting females in a northern population of Wood Thrushes. Wilson Bulletin 112: Hoover, J.P. and M.C. Brittingham Regional variation in cowbird parasitism of Wood Thrushes. Wilson Bulletin 105: Mayfield, H Suggestions for calculating nest success. Wilson Bulletin 87:

8 Partners in Flight Watch list [online]. Rothstein, S.I. and S.K. Robinson The evolution and ecology of avian brood parasitism. In Avian brood parasitism, ed. S.I. Rothstein and S.K. Robinson, Oxford: Oxford University Press, Great Britain. Roth, R. R. and R. K. Johnson Long-term dynamics of a Wood Thrush population breeding in a forest fragment. Auk 110: Roth, R. R., M. S. Johnson and T. J. Underwood Wood Thrush (Hylocichla mustelina). In The birds of North America, no. 246, ed. A. Poole and F. Gill. Philadelphia, Pennsylvania: The Academy of Natural Sciences; Washington: The American Ornithologists Union. Sauer, J.R. and BlK. Williams Generalized procedures for testing hypothesis about survival or recovery rates. Journal of Wildlife Management 53: Simons, T.R., G.L. Farnsworth and S.A. Shriner Evaluating Great Smoky Mountains National Park as a population source for the Wood Thrush. Conservation Biology 14: Sokal, R.R. and J.J. Rohlf Biometry. 2nd Ed. New York: W.H. Freeman and Company. Trine, C.L Wood Thrush population sinks and implications for the scale of regional conservation strategies. Conservation Biology 12: Trine, C.L., W. D. Robinson and S.K. Robinson Consequences of Brown-headed Cowbird brood parasistism for host population dynamics. In Avian brood parasitism, ed. S.I. Rothstein and S.K. Robinson, Oxford: Oxford University Press, Great Britain. CONTACTS Lyle E. Friesen Canadian Wildlife Service Ontario Region Lesley N. Campbell Canadian Wildlife Service Ontario Region 8