Population status and the extent of declines
The substantial breeding population declines of tricolored blackbirds that we found from 1935 to 1980 (Fig. 1A) are consistent in magnitude with earlier reports, but much variation remains unexplained. We found a 63% decline in breeding abundance (mean colony size) from 1935 to 1975, whereas Beedy et al. (1991 – from data in their Table 1) amassed data showing a 76% decline in colony size between the “1930s and 1970s” (which we took as 1935 to 1975 seeking equivalence). We did not find evidence for a decline in average colony size since the 1970s despite having good sample sizes and reasonable statistical power. This is contrary to Beedy et al. (1991), whose data (their Table 1) show a 62% decline between the 1970s and 1980s. Kyle and Kelsey (2011) also reported a 34% decline in breeding bird numbers in 2011 compared with 2008, despite the 2011 survey including 72% more sites than the 2008 survey (Kelsey 2008). However, it is hard to interpret such short-term trends because the survey data show high interannual variability (“Population variability” section in Appendix). Ultimately, more years of surveys with a similar sampling effort to the statewide breeding surveys are needed. Appropriately, Kyle and Kelsey (2011) recommended a triennial range-wide survey and an annual survey in three especially important counties (Merced, Kern, and Tulare), all of which are within the San Joaquin Valley.
Unlike average colony size, total (summed) population size across all breeding sites was (not surprisingly) strongly related to the total number of sites sampled. Consistent with this problem of sample size dependency, Beedy et al. (1991) reported an 89% decline in total breeding populations from the 1930s to the 1980s, whereas we found a 69% decline in this time period in total breeding populations. Because of the sensitivity of total population size to sampling effort we do not recommend using total population size as a metric of population status for this species.
Habitat loss is stated as the reason for decline in breeding populations (Beedy et al. 1991). However, the direct loss of breeding sites cannot explain why colony size declined within existing marshes (Table 2; Fig. 4), many of which are protected (e.g., National Wildlife Refuges) and are the same localities since the 1930s. Wetland loss has also slowed in recent years because of protection and mitigation resulting from the 1977 amendment of the Clean Water Act and other measures (e.g., Dahl 2006). Changes in habitat quality are generally harder to evaluate. Likely quality changes have occurred in foraging habitats through agricultural intensification leading to disturbance from harvesting and increased pesticide usage, which diminishes insect populations required for breeding (Beedy et al. 1991; Beedy and Hamilton 1997; Benton et al. 2002). Meese (2013) also found that over a 6-year period (2006–2011) over 40 colonies had chronically low reproductive success, and reproductive success was correlated with insect abundance in foraging grounds; the determinants of insect abundance are largely unknown. There are also specific incidences of quality change that are clear, for instance the draining of marshes and senescence of marsh vegetation (Meese 2008). Historically, almost all wetlands in the Central Valley were managed for wintering migratory birds, with little attention to or capacity for managing spring wetlands when tricolored blackbirds would use these wetlands. In recent years some sites in National Wildlife Refuges (e.g., Kern, Merced) and some marshes owned by duck clubs have been managed specifically for tricolored blackbird breeding, however, these represent very few sites relative to the habitat requirements for this species.
Regional declines and habitat types
We found substantial changes in breeding populations in different regions and breeding habitat types (Figs. 1 and 4). These regional declines corresponded to trends in different breeding habitats, with four caveats. First, the total amount of variation in breeding bird abundances explained by habitat, region and time variables was only 14.5% to 20.8%, and some variation was shared by model terms (see h2-values in Tables 1 and 2); hence while there is an effect it is not strong. Second, we cannot, based on correlational data alone, distinguish whether habitats drive regional differences or vice versa (or whether an unrelated factor drives both). Third, our statistical analyses could not fully include triticale since it is represented by only a small number of records, so the effect is quantified and discussed but cannot be directly compared with the results in Tables 1 and 2. Fourth, the data analyzed are for the presence of colonies: we do not have data from consistent monitoring or habitat areas regardless of occupancy by breeding tricolored blackbirds.
Prior to 1980 the Sacramento Valley held the largest number of birds, whereas from 1980 onwards the San Joaquin Valley supported the largest total breeding populations of tricolored blackbirds. We believe two factors are involved in the slow decline in average colony size in the San Joaquin Valley and growth of total breeding populations (Table 1, Figs. 1G and 2). First, colonies in triticale were all within the San Joaquin Valley (or Sacramento County), all during the last 20 years, and they were >40× larger than colonies in other habitats during this period. Second, cattail sites and blackberry sites were uncommon in the San Joaquin Valley. Central coast colony-size declines resulted from four early records (Fig. 1F), and three of these came from cattails in which declines were rapid (Table 2, Fig. 4B). The decline of Sacramento Valley breeding populations consisted of both a reduction in average colony size (Fig. 2A) and the total breeding population (Fig. 2B), and hence the number of sites occupied. Given that many of the marsh (cattail and bulrush/tule) sites in this region are in wildlife refuges it is surprising that such colonies declined in average size. However, increased management for wintering waterfowl may have altered the marshes from their historical conditions. Possibly the observed declines indicate that something other than breeding habitat per se affects breeding populations, and this might be something such as insects in foraging habitat (e.g., Meese 2013), or be associated with climate change.
Overall the observed trends in breeding populations in different habitats are consistent with regional changes we observed (albeit subject to the caveats listed above). Our observed changes in populations in different regions and habitats are consistent with Beedy et al. (1991) and Cook and Toft (2005). Himalayan blackberry sites showed slower declines in average colony size than other habitats, and cattail sites declined in average colony size more rapidly than other habitats. Similarly, the high proportion of cattail sites in the Sacramento Valley was coincident with more rapid declines in this area than the statewide average. Differences among habitats clearly contributed to a change in net geographic distribution, as well as altering overall temporal trends. In our cases we do not directly know what aspect of habitat alters the demography or movements of tricolored blackbirds, whether it is breeding habitat or foraging habitat for instance (Meese 2013). A few other studies have related bird population trends to habitat types. Virkkala (1991) tied regional population trends in Finnish birds to habitat types, and found that habitats that experienced the greatest loss or fragmentation showed the largest population declines. Seoane and Carrascal (2008) found that trends in Spanish birds varied among habitat types, and Wretenberg et al. (2007) showed that Swedish birds occupying agricultural habitats were most likely to show population declines.
The long-term changes in the proportions of birds in different habitats (Fig. 2) could result either from birds moving among habitats (within or across years), or from the long-term differences in fitness playing out. Itinerancy likely contributes to change in the types of habitats used. Hamilton et al. (1995) reported that site occupancy was short-lived, 15% of sites being occupied for 2–3 years, and 26% being occupied for at least 4 years (the number of 1 year colonies was unclear). Meese (2011) also reported 84 new colony locations discovered from 2005 to 2011. These figures indicate some selection of new breeding colony locations on a yearly basis. Additionally, birds may breed at several different sites within a year (Beedy and Hamilton 1999) but the majority of database records represent the first spring breeding.
An important piece of biology is that we do not know whether tricolored blackbirds are philopatric to more permanent habitats, whether the same individuals regularly move among habitats, or whether there is more permanent emigration to different habitat types. In the closely related red-winged blackbird, source-sink dynamics were demonstrated with exchange of individuals between rural source (low predation) and urban sink (high-predation) habitats (Vierling 2000). For tricolored blackbirds it is unclear whether exchanges represent source-sink dynamics (Howe et al. 1991), ecological traps (Robertson and Hutto 2006), habitat selection, or buffer populations. In buffer populations, individuals in low-quality habitats represent individuals excluded by territoriality (density dependence) from high-quality habitats, but such individuals would move to higher quality habitats if populations in them declined, thereby buffering such populations from decline (Rodenhouse et al. 1997). While these details remain obscure, the ability of exchanges among habitats to modify range-wide and regional population trends is clearer.
The variety of breeding habitats used by tricolored blackbirds and the ephemerality of breeding site occupancy in many habitats makes it difficult to disentangle the factors behind overall population trends. Himalayan blackberry and thistles also represent nonnative invasive species, so we are left with a conundrum of needing to protect areas of an invasive species to protect tricolored blackbird colonies (Cook and Toft 2005). Furthermore, over 50% of the breeding population in any given year was within relatively few triticale farm field colonies, requiring protection of these in at least the short term for conservation of this species. Itinerant breeding and the potential for movement of birds among habitats lead to several recommendations: (1) Monitor breeding, protect colonies, and analyze population trends in a full range of habitats; even sink habitats may contribute to reproductive output (Howe et al. 1991). (2) Institute and reinforce conservation measures that allow colonies to regularly occur in new areas and successfully complete breeding, including in annual crops such as triticale (discussed further below). (3) Work with private landowners where agricultural field colonies occur to create alternative, sustainable natural habitats outside of grain fields. (4) To conduct further studies of habitat quality and breeding success (e.g., Meese 2013; K. Weintraub, unpubl. data) to ascertain whether there are long-term trends in these characteristics and quantify long-term habitat-specific demography in relation to both breeding habitats and surrounding foraging habitats. The effects of landscape context of breeding colonies also need further study (Meese 2013), to determine the extent to which the habitat used for nesting versus the foraging habitat influences breeding success. (5) Given the ephemerality of some colonies, construct a stochastic metapopulation model and obtain empirical data on colony longevity and productivity to evaluate the long-term contribution to persistence and total population size in different habitats.
Only one of 13 colony locations in triticale was recorded as lasting for >1 year, compared with 28% of colonies in other habitats. However, we have observed that tricolored blackbirds may move to adjacent habitat areas when an originally occupied area was unavailable (e.g., due to crop rotation). We know that in the case of triticale there is more frequent reuse of sites when the habitat was replanted (from records after data were extracted in 2009), but replanting was infrequent. Because of the large size of colonies, triticale is an important habitat for tricolored blackbirds but is vulnerable to harvesting of the crop prior to young birds fledging (e.g., Kyle and Kelsey 2011). Although protected by the Migratory Bird Treaty Act (Federal Register 2006), the conservation of colonies in ephemeral habitats is entirely voluntary, and some colonies are conserved while others are lost each year (Meese 2008).