Our Li\ini^ Rfsoiirit-s — Biiil.s 



25 



the East hut giKui in the West. These results sug- 

 gest that produclivily in a given year may inllu- 

 ence population sizes and population dynamics 

 in subsequent years for many species over a 

 large area. 



BBIRD data likewise suggest that weather 

 may substantially alTect nesting pn)ducti\ily. 

 Unusually wet weather conditions were report- 

 ed at 6 of 14 BBIRD sites in 1992 when nest 

 success of several species, including wood 

 thrush (Hylocichhi iniistelina) and red-eyed 

 vireo {Vireo olivaceiis). was lower in 1992 than 

 in 1993 (Table I ). These same two species also 

 had reduced breeding productivity based on 

 MAPS data. They produced fewer young per 

 successful nest in 1992 than in 1993, a fact 

 which also may be related to weather; some 

 research suggests that clutch size as well as 

 fledging success can be affected by weather 

 conditions and may even provide a particularly 

 sensitive measure of a species' tolerance to 

 changing climatic conditions (e.g., Rotenherry 

 and Wiens 1989). Further research may show 

 that climatic variability is an important influ- 

 ence on the population trends of species. 



Table 1. Wood tlimsli and red-eyed vireo nest Mieeess 

 based on Mayficid ( 1%I. 1975) estimates at midwestem 

 BBIRD sites during 1492 and 1993 (numhcrs of nests are 

 in parentheses). 



Habitat-specific Differences 



Forest fragmentation, where large forest 

 blocks are cut and interspersed with open habi- 

 tat, is believed to be particularly detrimental for 

 breeding nongame birds. For example, BBIRD 

 data show that fragmentation was associated 

 with lower nest success in several species at 

 midwestern BBIRD sites. Ovenbirds {Sciunis 

 aurocapilliis) were particularly sensitive to 

 fragmentation effects; their reduced nest suc- 

 cess resulted primarily from increased preda- 

 tion, although the parasitism rates of brown- 

 headed cowbird (Molothnis titer) were also 

 higher in fragments. No clear effect of fragmen- 

 tation was noted for red-eyed vireos. although 

 nest success differed substantially among 

 unfragmented sites, potentially reflecting more 

 subtle differences in habitat suitability or land- 

 scape-level effects (Table 2). 



Adult Survival in Two Eastern Thrushes 



Analysis of 3 years ( 1 990-92 ) of MAPS data 

 for veery (Cathariis fiisce.scens) and wood 

 thrush indicated low and substantially different 



iP < 0.06) adult survival probabilities from 

 IWO to 1991. According to Breeding Bird 

 Survey data, veery populations declined by 

 l.O^f per year between 1966 and 1991. while 

 wood thrush populations showed a statistically 

 greater decline of 2.0% per year (Peteijohn and 

 Sauer 1993). This difference in population 

 declines is mirrored by survival indices; MAPS 

 estimates of wood thrush survival are half that 

 of the veery. possibly because of differences in 

 adult survival over winter. This possibility is 

 especially interesting because wood thrushes 

 winter in Mexico and Central America where a 

 greater proportion of the tropical forests have 

 been cleared than in South America where 

 veeries winter. Differences in estimated survival 

 of the two species, however, could simply 

 reflect different life-history traits (e.g., wood 

 thrushes having lower adult survival associated 

 with higher fertility; Martin in press). Estimated 

 survival differences could also result from dif- 

 ferences in breeding-site fidelity, which is relat- 

 ed to nest success; a variety of evidence shows 

 that birds disperse more in breeding seasons 

 that follow nesting failure, potentially biasing 

 survival estimates. Further nest-monitoring data 

 frotn North America and survivorship data from 

 both North America and the Neotropics are 

 needed to identify causes of population declines 

 in these and other Neotropical migratory land- 

 birds. 



Trends 



Preliminary results from the MAPS and 

 BBIRD programs suggest that population 

 trends of nongame landbirds are influenced by 



Table 2. Ovenbird and red-eyed 

 vireo nest success based on 

 Maytleld (1961. 197.5) estimates at 

 fragmented and unfragmented 

 midwestcni BBIRD sites during 

 1942 and 1993 (numbers of nests 

 are in parentheses). 



IVlonitonng of nests, such as this 

 one belonging to a red-faced war- 

 bler (CardeUina ruhrifnms). pro- 

 vides information on breeding pro- 

 ductivity. 



