David F. DeSante¹

INTRODUCTION

Recent analyses of data from the North American Breeding Bird Survey (BBS) and other large-scale, long-term monitoring programs suggest that many species of landbirds, including a number of Nearctic-Neotropical migratory species, have undergone pronounced population declines over the past three decades, and that these declines may be accelerating, at least in the eastern and central parts of the continent (Robbins et al. 1989, Terborgh 1989, Peterjohn et al. 1995, 1996). Indeed, these analyses have provided much of the impetus for the establishment and growth of the Neotropical Migratory Bird Conservation Initiative, "Partners in Flight."

Despite the general success of large-scale monitoring programs, such as the BBS, in describing geographic and temporal patterns of avian population trends and identifying potentially declining species, such monitoring programs provide little information as to factors responsible for population declines and even less direction as to appropriate management actions to reverse declines (Peterjohn et al. 1995, DeSante and Rosenberg 1998). This is because they provide no information about the stages of the life cycle that control the population changes (Temple and Wiens 1989), and thus fail to distinguish problems caused by productivity factors that operate on the breeding grounds from those caused by mortality factors that may operate primarily on the wintering grounds and migration routes (DeSante 1992, Sherry and Holmes 1995). Clearly, critical data on primary demographic parameters (productivity and survivorship) are needed to determine the factors responsible for the population declines in Neotropical migratory birds and to identify conservation and management actions to reverse the declines (DeSante 1995).

The Monitoring Avian Productivity and Survivorship (MAPS) Program was established by The Institute for Bird Populations to provide these critical data (DeSante et al. 1993, 1995). MAPS uses standardized, constant-effort mist netting and banding during the breeding season at a continentwide network of stations, and was patterned after the British Constant Effort Sites Scheme (Baillie et al. 1986, Peach et al. 1996, Peach and Baillie in press) that has been in operation since 1981.

The specific objectives of MAPS are to provide, for a suite of target landbird species (including both Neotropical- and temperate-wintering species) and at multiple spatial scales: (1) annual indices of adult population size and post-fledging productivity from data on the numbers and proportions of young and adult birds captured; and (2) annual estimates of adult survivorship, adult population size, and recruitment into the adult population from mark-recapture data on the adult birds captured. These demographic indices and estimates are to be used to describe temporal and spatial patterns in the demographic parameters of the target species, and to model interrelationships between demographic parameters and environmental variables including weather and landscape-level habitat characteristics (particularly those resulting from management actions).

The major long-term goal of MAPS is to use the demographic information generated on target species to aid in: (1) identifying proximate demographic causes of population changes in these species; (2) identifying conservation strategies and management actions to reverse population declines; and (3) evaluating the effectiveness of the conservation strategies and management actions implemented. Indeed, monitoring primary demographic parameters may be the most judicious way to determine whether management actions are working effectively (DeSante and Rosenberg 1998). This is because management actions affect primary demographic parameters directly, and these effects can potentially be observed over a short time period (Temple and Wiens 1989). Because of buffering effects of floater individuals and density-dependent responses of populations, substantial time lags may occur between changes in primary parameters and resulting changes in population size or density (DeSante and George 1994). Moreover, because of the vagility of most bird species, local variations in population size may be masked by recruitment from a wider area (George et al. 1992) or accentuated by lack of recruitment from a larger area (DeSante 1990).

Additional long-term goals of MAPS are to: (1) forge cooperative partnerships among federal and state agencies, non-governmental organizations, researchers, and independent banders to use public lands for long-term avian monitoring efforts; and (2) provide a means for direct public participation in landbird conservation efforts by encouraging banders to collect rigorous mark-recapture data. These two objectives are addressed elsewhere in this volume (see Burton and DeSante this volume). Whether MAPS can achieve its major long-term goal depends upon whether temporal and spatial patterns in demographic parameters of target species generated from MAPS data reflect actual population processes at the scale of interest. Criticism of MAPS has primarily been twofold: (1) because of constraints on locations where long-term mist netting is practical and permissible, stations are not sited by a probability-based sampling strategy—thus, inferences regarding productivity indices and survival-rate estimates cannot be made beyond the sample of stations; and (2) with regard to productivity indices, the populations sampled at MAPS stations are not clearly identified due to limitations on our knowledge of bird dispersal patterns—thus, the adequacy of productivity indices based on the proportion of young captured is not well known.

This paper documents the growth of the MAPS Program from 1989 through 1996, and describes, at two geographic scales (a large semi-continental scale--North America east of the Rocky Mountains and north of Mexico; and a smaller regional scale--the Sierra Nevada physiographic province as defined by the BBS), patterns of productivity with respect to nest location and migration strategy and patterns of survivorship with respect to migration strategy. I show that patterns generated by these data are temporally and spatially consistent both with patterns expected from ecological theory and with patterns generated from other empirical data.

METHODS

Data Collection
The overall design of the MAPS Program and the methods used to collect data were outlined in DeSante et al. (1993, 1995) and were described in detail in DeSante and Burton (1997). Briefly, MAPS stations were established in 20 ha study areas where long-term mist netting was practical and permissible. Typically, about 10 permanent net sites were distributed opportunistically (at locations where birds can be captured efficiently) but rather uniformly throughout the central 8 ha of the study area, and one 12 m, 30 mm-mesh mist net was erected at each net site.

Stations were operated in a standardized manner, typically for six hours per day, beginning at sunrise, for one day per 10-day period, and for 8-12 consecutive 10-day periods, depending on latitude, from May 1 to August 28 (MAPS protocol was changed in 1997 to minimize captures of fall migrant individuals: the last two 10-day periods, August 9-28, were eliminated). To minimize captures of spring migrant individuals and subsequent net avoidance by permanent resident and early arriving summer resident individuals, starting dates for MAPS stations were delayed until the time when most spring migrant individuals of the target species have moved through the study area; thus, starting dates were later at more northern latitudes.

Each bird captured was marked with a uniquely numbered aluminum leg band and all birds captured (including recaptures) were identified to species, age, and (if possible) sex. The times of opening and closing each net and beginning each net run were recorded each day so that effort could be calculated for each 10-day period and standardized between years. A separate record was kept of all species seen or heard within the boundaries of each station on each day of operation, to determine, for each year, whether the species was a summer resident and probable breeder at the station.

Data Analysis
Methods of data analysis were described in detail in DeSante et al. (1993) and DeSante and Burton (1994). Very briefly, after computer entry and proofing, capture data were run through verification programs that: (1) checked the validity and ranges of all coded data; (2) compared the species, age, and sex determinations against the aging and sexing criteria used, and flagged discrepancies or suspicious data; (3) screened data for unusual band numbers or band sizes; and (4) screened all records of each band number for inconsistent species, age, or sex determinations. All inconsistencies, discrepancies, or suspicious data were examined in detail and corrected if necessary.

Following procedures pioneered by the British Trust for Ornithology in its CES Scheme (Baillie et al. 1986, Peach et al. 1996), I used the proportion of young (hatching-year) birds in the catch [# young/(# young + # adults)] during the entire MAPS season (May 1 to August 28) each year as the annual index of post-fledging productivity. For productivity (or survivorship) analyses in any given year, I included only stations that were operated for at least five periods, at least three of which occurred during the earlier portion of the season (when adults predominate in the catch) and at least two of which occurred during the later portion of the season (when young birds predominate in the catch).

I investigated patterns of productivity from eastern North America over the four years, 1992-1995, by two approaches: (A) the constant-stations approach, in which I pooled data from the 61 stations east of the Rocky Mountains that lay within the breeding range of the species and that fulfilled the above criteria during every one of the four years; and (B) the variable-stations approach, in which I pooled data from all stations east of the Rocky Mountains that lay within the breeding range of the species and that fulfilled the above criteria in any of the four years. The numbers of stations from which data were pooled using this latter approach were 81 in 1992, 109 in 1993, 169 in 1994, and 203 in 1995.

I included species in productivity analyses from eastern North America for which at least 50 aged individuals were captured during each of the four years at all stations that were pooled each year. I classified each species by nest location (cavity nester, open-cup tree nester, open-cup shrub nester, ground nester) using information from Ehrlich et al. (1988), and by migration strategy (permanent resident, temperate-wintering migrant, Neotropical-wintering migrant) using information from AOU (1983) and DeSante and Pyle (1986). I excluded Brown-headed Cowbird (Molothrus ater) from these analyses because it is a brood parasite, and an appropriate nest location classification could not be assigned. I also excluded Cedar Waxwing (Bombycilla cedrorum) and American Goldfinch (Carduelis tristis), because adults of these species begin nesting in eastern North America substantially later than other species and, unlike all species that were included, pre-breeding adults often were captured in sizeable flocks during the MAPS data-collection period. For this reason, productivity indices for these two species likely would be biased low compared to all other species.

For the Sierra Nevada analyses, I used the variable stations approach; that is, I pooled data for each species each year from all stations within the Sierra Nevada physiographic province that lay within the breeding range of the species. I included species in this analysis for which a total of at least 100 aged individuals were captured during the four years at all stations that were pooled each year. I classified species by nest location and migration strategy as described above. I excluded House Wren (Troglodytes aedon), Orange-crowned (Vermivora celata) and Nashville (V. ruficapilla) warblers, and Lesser Goldfinch (Carduelis psaltria) from these analyses because large numbers of individuals of these species, especially juveniles, undergo up-mountain drift after the breeding season. Therefore, productivity indices for these species likely would be biased high compared to all other species.

I calculated maximum-likelihood estimates of annual adult survival rates from four years (1992-1995) of mark-recapture data pooled from stations that were operated in every one of the four years. I used a modified time-constant Cormack-Jolly-Seber analysis (Pollock et al. 1990) that incorporated the transient model described by Pradel et al. (1997) into the computer program SURVIV (White 1983). The transient model allows estimation of the time-constant proportion of resident birds among newly captured individuals, as well as estimation of the time-constant recapture probability. In order for the estimate of proportion of residents to be biologically meaningful, I pooled only data from stations where the species was known to be a regular breeder, that is, where evidence existed that the species was a summer resident in at least three of the four years, 1992-1995. In order to make meaningful comparisons of productivity and survivorship, I re-calculated productivity indices for both eastern North America and the Sierra Nevada using the same stations from which survival rates were estimated, that is, stations that were operated during each of the four years 1992-1995 and at which the species was a regular breeder. For analyses from eastern North America, I included species for which a total of at least 200 aged individuals were captured over the four years; for the Sierra Nevada analyses, I included species for which a total of at least 60 aged individuals were captured over the four years. I inferred the statistical significance of differences in productivity indices or survival-rate estimates between groups of species having different migration strategies from non-overlapping 95% confidence intervals.

RESULTS

Growth of the MAPS Program
MAPS grew from 17 stations in 1989 to 38 in 1990, 66 in 1991, 178 in 1992, 236 in 1993, 326 in 1994, 391 in 1995, and 413 stations in 1996 (Figure 1). The substantial growth of the program in and subsequent to 1992 and the increase in the percentage of stations supported at least partially by federal funds was caused by the increased involvement of various federal agencies in PIF, particularly the USDA Forest Service; the USDI Fish and Wildlife Service, National Park Service, and National Biological Service (now Biological Resources Division of USGS); and the USDoD Departments of the Navy and Army.  The distribution of the 413 stations operated in 1996 is shown in Figure 2.

Figure 1. Growth of the Monitoring Avian Productivity and Survivorship (MAPS) program from 1989 to 1996 and percentage of stations for which at least some federal support was received.

Patterns of Productivity from Eastern North America 

Annual (1992-1995) productivity indices (proportion of young in the catch) and nest-location and migratory-status classifications are presented for each of the 43 species included in this analysis in Appendix 1. In terms of nest type, they included 8 cavity nesters, 12 open-cup tree nesters (hereafter, tree nesters), 14 open-cup shrub nesters (hereafter, shrub nesters), and 9 ground nesters. In terms of migratory status they included 7 permanent residents, 13 temperate-wintering migrants (hereafter, temperate migrants), and 23 Neotropical-wintering migrants (hereafter, Neotropical migrants). For 36 of these 43 species, a total of 50 or more aged individuals were captured each year at the 61 stations that were operated during each of the four years.

Figure 2. Map of North America showing the distribution of the 413 MAPS stations operated in 1996 (as determined by the 10-minute block in which the station was located). Thus, an individual square mark can include multiple stations. Clusters of stations (indicated by clusters of square marks), in fact, can include up to 14 stations.

Click on the map above to view a larger version of it.

The relationship between productivity and nest-location is shown for each of the four years in Figure 3. In general, for both the constant- and variable-stations approach, cavity nesters showed the highest productivity indices, followed by ground, tree, and shrub nesters in that order. This pattern was found in 7 of 8 year-approach cases; in the only exception (1994, constant-station), the order was cavity, tree, ground, shrub nesters. Indeed, cavity nesters showed significantly higher productivity than shrub nesters in all 8 year-approach cases, and significantly higher productivity than tree nesters in 5 of the 8 year-approach cases, including all four years using the variable-stations approach.

Figure 3. Productivity indices for 1992 through 1995 for species in eastern North America (all stations pooled east of the Rocky Mountains lying within the breeding range of the species) as a function of nest location from: (A) constant-stations and (B) variable-stations analyses (see text). Shown are the means and 95% confidence intervals. The number of species included and the number of stations from which data were pooled are shown for each year and nest-location group.

The relationship between productivity and migration strategy is shown for each of the four years in Figure 4. Permanent resident species showed the highest productivity indices and Neotropical migrants the lowest, while temperate migrants were intermediate. This pattern was consistent in all 8 year-approach cases. Indeed, permanent resident species showed significantly higher productivity indices than Neotropical migrants in all 8 year-approach cases.

Figure 4. Productivity indices for 1992 through 1995 for species in eastern North America (all stations pooled east of the Rocky Mountains lying within the breeding range of the species) as a function of migration strategy from: (A) constant-stations and (B) variable-stations analyses (see text). Shown are the means and 95% confidence intervals. The number of species included and the number of stations from which data were pooled are shown for each year and migration-strategy group.

In addition, for most groups, using either the constant- or variable-station approach, productivity increased from 1992 to 1993, increased again from 1993 to 1994, and decreased from 1994 to 1995. This exact pattern was found in 5 of 8 nest location-approach cases (Fig. 3) and in 4 of 6 migration strategy-approach cases (Fig. 4). In 4 of the other 5 cases, only 1 of the 4 years was different than the above-mentioned sequence. 

Patterns of Productivity from the Sierra Nevada 

Annual (1992-1995) productivity indices for species groups in the Sierra Nevada physiographic province (Appendix 2; Fig. 5) showed the same patterns with respect to nest location and migration strategy as did productivity indices for analogous species groups from the eastern United States. That is, cavity and ground nesters tended to show higher productivity indices (with the 4-year mean for cavity nesters being slightly higher than that for ground nesters), while tree and shrub nesters tended to show lower productivity indices (with the 4-year mean for shrub nesters being lower than that for tree nesters, Fig. 5A); and permanent residents tended to show the highest productivity indices, followed by temperate migrants and then Neotropical migrants (Fig. 5B). In fact, 4-year mean productivity indices for permanent resident species were significantly higher than those for Neotropical migrants.

Figure 5. Productivity indices for 1992 through 1995 for species in the Sierra Nevada (all stations pooled in the Sierra Nevada physiographic province lying within the breeding range of the species) using the variable stations approach as a function of (A) nest location and (B) migration strategy. Shown are the means and 95% confidence intervals. The number of species included and the number of stations from which data were pooled are shown for each year and nest-location or migration-strategy group.

These patterns also tended to be consistent over time, despite the fact that the number of stations increased from 9 to 14 during the four years. Indeed, productivity indices decreased from 1992 to 1993, increased from 1993 to 1994, and decreased from 1994 to 1995 for all 7 species groups. The pattern of permanent residents higher than temperate migrants higher than Neotropical migrants held for all years except 1993, when the pattern was permanent residents higher than Neotropical migrants higher than temperate migrants. Similarly, both cavity and ground nesters showed higher productivity indices than both tree and shrub nesters during all four years.

The Relationship between Patterns of Productivity and Survivorship

In marked contrast to mean annual productivity indices (Fig. 6A), annual time-constant adult survival-rate estimates tended to be higher for Neotropical migrants than for any other migration-strategy species group (Fig. 6B; Appendix 3). This was true both for eastern North America and for the Sierra Nevada. Indeed, the similarities between the two areas in both productivity indices and adult survival-rate estimates as a function of migration strategy are striking.

Figure 6. (A) mean (1992-1995) productivity indices and (B) time-constant (1992-1995) annual adult survival-rate estimates from MAPS for species in eastern North America (E) from data pooled from 61 stations operated during each of the four years; and the Sierra Nevada physiographic province (S) from data pooled from 7 stations operated during each of the four years as a function of migration strategy. Shown are the means and 95% confidence intervals. The number of species are shown for each migration-strategy/geographical-area group.

DISCUSSION

Patterns of MAPS productivity indices from eastern North America indicate that cavity-nesting species tended to have higher productivity indices than any other nest-location group, and that tree and, especially, shrub nesters had the lowest productivity indices. These patterns also were reflected in MAPS data from the Sierra Nevada where cavity and ground nesters also tended to have higher productivity indices than tree and shrub nesters. These patterns agree well with analogous results on the percentage of nests lost to predators and the percentage of nests fledging at least one young as determined by direct nest monitoring (Martin 1995). Cavity nesters, for example, generally experience low rates of both nest predation and cowbird parasitism; moreover, they often have larger clutch sizes than the other nest-location groups. Ground nesters often experience relatively low rates of cowbird parasitism but rather high rates of nest predation. Tree and shrub nesters generally experience high rates of both cowbird parasitism and nest predation. Shrub nesters, in particular, experience high rates of nest predation from both terrestrial and arboreal nest predators.

Also interesting was the relationship in both eastern North America and the Sierra Nevada between productivity indices and migration strategy: permanent resident species showed the highest productivity indices while Neotropical migrants showed the lowest. Greenberg (1980) previously provided both empirical and theoretical evidence for this pattern. The reason for it may lie, at least partially, with the length of breeding season available to each of the three groups of species. Permanent resident species often begin nesting earlier in the spring than the other groups and, because they are not constrained by the need to molt, store fat, and migrate before the summer ends, can extend breeding later into the summer than the other groups. As a result, permanent resident species can more easily produce multiple broods and can have greater opportunities to make multiple re-nesting attempts (Whitcomb et al. 1981; but see also Martin 1995). In contrast, Neotropical migrant species tend to arrive late in spring and leave early in fall and, thus, have a relatively short time available to produce multiple broods or make multiple re-nesting attempts (Whitcomb et al. 1981, Martin 1995).

It also is important to note that the patterns of MAPS productivity indices presented here as a function of nest location and migration strategy were robust over time; that is, the same patterns tended to occur in each of the four years, 1992-1995, although the temporal patterns differed between eastern North America and the Sierra Nevada. Moreover, for eastern North America at least, the same patterns occurred each year regardless of whether data were included from only the 61 stations that were operated during every one of the four years, or from all of the stations that were operated each year, which nearly tripled from 81 stations in 1992 to 203 in 1995, indicating that these patterns were robust over space as well as time. This important result suggests that, while MAPS productivity indices may be biased, the biases tend to remain consistent over time and space. This is a fundamental requirement for any large-scale, long-term monitoring program.

In addition, the fact that patterns of productivity indices from MAPS for various nest-location and migration-strategy groups reflected patterns obtained from direct nest-monitoring as well as patterns predicted from theoretical considerations, suggests that biases in MAPS productivity indices also may remain fairly consistent among species. Indeed, the general agreement between patterns of MAPS productivity indices and patterns of reproductive success estimated from direct nest monitoring or predicted from theoretical considerations suggests that MAPS-generated patterns of productivity may well reflect real ecological processes operating at large but variable geographic scales. The fact that Neotropical migrants tended to have lower productivity indices than the other three groups is interesting in view of the tendency for many Neotropical migrants to be experiencing greater population declines than many species of the other three groups (Peterjohn et al. 1995).

These results should not be construed to suggest that low productivity is necessarily the cause of the negative population trends for many Neotropical migrants. It is important to note that population trends result from the interaction between productivity and survivorship. Species having low productivity (such as, perhaps, many Neotropical migrants) can have stable or positive population trends if their low productivity is at least balanced by a high survival rate. Thus, simultaneous survivorship information is needed along with productivity information to provide real insight into the causes of population trends and to identify appropriate management actions to reverse population declines. This is exactly what MAPS purports to do.

It is of considerable interest, therefore, that MAPS data suggests that Neotropical migrants in both eastern North America and the Sierra Nevada tend to have higher survival rates than permanent residents and temperate migrants. This also agrees with limited empirical data and theoretical considerations suggested by Greenberg (1980). Indeed, the apparent trade-off between productivity and survivorship, as illustrated by MAPS data, is fundamental to much of modern life-history theory (Martin 1995).

The two major criticism of MAPS have been that: (1) large-scale inferences regarding productivity indices and survival-rate estimates cannot be made beyond the sample of stations, and may not be representative of the region as a whole, because constraints on locations where long-term mist netting is practical and permissible preclude the use of a probability-based sampling strategy for siting stations; and (2) the adequacy of productivity indices based on the proportion of young captured is not well known, because limitations on our knowledge of bird dispersal patterns preclude the clear identification of the populations sampled at MAPS stations. However, the fact that patterns of productivity with respect to nest-location, migration strategy, and time remained consistent over a large geographical area, even when the number of stations under consideration tripled, suggests that patterns from the samples of stations actually may be representative of the pattern from the region as a whole. Moreover, the fact that patterns of productivity from MAPS generally agree well with productivity patterns from direct nest monitoring data and current life-history theory, suggests that productivity indices based on the proportion of young in the catch actually may be adequate to describe ecological processes at large spatial scales.

In summary, patterns of productivity and survivorship from MAPS, when examined at two large but different spatial scales, suggest that the MAPS program has the potential to provide useful demographic information on target species that can aid efforts to guide management strategies for them. This may especially be true when patterns of productivity and survivorship from MAPS are integrated with population trend data from the BBS and other long-term avian monitoring programs and with research results from direct nest-monitoring and habitat-utilization studies. I suggest that the MAPS Program is well-suited to serve as one important piece of an integrated monitoring program for North American birds, and that it should continue to serve as an integral part of an effective North American Bird Conservation Program.

ACKNOWLEDGEMENTS

I thank W. L. Kendall. J. D. Nichols, B. R. Noon, D. K. Rosenberg, and J. R. Sauer for much statistical guidance and many helpful discussions regarding the MAPS program. I thank M. J. Conroy and D. Petit for many helpful suggestions and astute comments that greatly improved an earlier version of this paper. I thank the many individuals and organizations that have contributed data to the MAPS Program; E. E. Feuss, D. Froehlich, E. D. Ruhlen, H. Smith, P. Velez, and B. L. Walker for preparing the data; and D. R. O'Grady and K. M. Burton for help with analyses and preparation of the figures. Financial support for the MAPS Program has been provided by the National Fish and Wildlife Foundation, USDI Fish and Wildlife Service and National Biological Service (now the Biological Resources Division of the USGS), Regions 1 and 6 of the USDA Forest Service, Flathead National Forest, Department of Defense, Department of the Navy, Texas Army National Guard, Denali and Shenandoah National Parks, Yosemite Association, Sequoia Natural History Association, and the Confederated Salish and Kootenai Tribes. I thank them all for their support. This is Contribution Number 56 of The Institute for Bird Populations.

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Appendix 1

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Appendix 2

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Appendix 3

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