Great blue heron (fannini subspecies) COSEWIC assessement and update status report: chapter 9

Population sizes and trends

Search effort

Surveys of the Pacific Great Blue Heron have concentrated on nesting colonies. Many published and unpublished papers have been produced based on these surveys (e.g., Norman et al. 1989; Butler et al. 1995; Butler 1997; Vennesland 2000; Vennesland and Butler 2004).

Search effort at Pacific Great Blue Heron nesting colonies on the coast of British Columbia has a long and variable history. The database held by the British Columbia Conservation Data Centre (http://www.env.gov.bc.ca/cdc/) has records of nesting colonies going back to 1920. Nesting sites have been documented from across the range of the Pacific Great Blue Heron, though surveys have concentrated on the core of the range (the Strait of Georgia). Survey effort prior to 1970 was minimal (the CDC database holds 77 colony-level observations over 50 years). From 1970 to the mid-1980s survey effort was increased with the implementation of specific research projects targeted at this species (182 colony level observations from 1970 to 1986). However, data collection over this period (e.g., Forbes et al. 1985a) concentrated on successful nests and commonly ignored failed nesting attempts (an important source of variation in nesting productivity; Butler et al. 1995; Vennesland 2000; Gebauer and Moul 2001). Some database entries include measures of productivity from successful and failed attempts, but sample sizes are small. Survey effort increased again after 1986 (882 colony-level observations from 1987 to 2005), with annual surveys following consistent methodology covering most of the Georgia Basin in most years during this period (2005 is the last year with data entered as of October 2007). However, due to a generally increasing search effort through this period, absolute population trends based on counts of herons at nesting colonies are difficult to ascertain.

Volunteer-based surveys have also been conducted for many years on the coast of British Columbia, including the wintertime Christmas Bird Count (CBC), wintertime Coastal Waterbird Survey (CWS) and summertime Breeding Bird Survey (BBS). Some caution must, however, be exercised when interpreting trend data from CBC surveys. This caution relates to increased participation in Christmas Bird Counts and the detectability of Great Blue Herons. Specifically, the number of volunteers participating in Christmas Bird Counts has increased significantly over the past 20 years in the lower mainland of British Columbia (D. Fraser pers. comm.), but because herons are easily sighted, this increasing effort does not necessarily result in an increase in the number of birds detected. Given that the trend is calculated as “the number of birds per party hour”, there will be a negative bias introduced as the number of party hours increases without a corresponding increase in the likelihood of detecting additional herons. Thus, declines may partially be a function of an increasing number of observers.

Coastal Waterbird Surveys should be considered to have the most robust data for several primary reasons: participants are relatively well trained, survey locations are geo-referenced and visited multiple times per year and results have been analyzed through well-designed statistical procedures (Badzinski et al. 2005). However, results from CWS have been analyzed for only one, five-year period (1999/2000 to 2003/04), which limits their utility compared to CBC and BBS data for which longer data sets are available.

Abundance

Population size is difficult to estimate for the Pacific Great Blue Heron because colonies are not stable and are difficult to track in a standardized fashion, and most coastal areas, especially outside the Strait of Georgia, have not been systematically surveyed (Butler 1997; Vennesland 2000; Gebauer and Moul 2001).

The weakest information on the Pacific Great Blue Heron is from outside the Strait of Georgia, where few projects have been undertaken. A colony of 9 pairs found near Tahsis in 1989 was the first colony reported from the west coast of Vancouver Island. Since then, 1-3 heron nests were located near Bamfield on the west coast of Vancouver Island and at Rose Harbour, and Ramsay and Murchison Islands in the Queen Charlotte Islands (P. Clarkson & B. Johnston, pers. comm.). Campbell et al. (1990) reported small numbers of herons nesting near Prince Rupert. Nevertheless, herons are seen, usually alone, along much of the coastline feeding from floating kelp, on wharves and in shallows, although the numbers are low. Since 1990, much of the coast has been visited during the nesting season by one of us (RWB). Individual adults were sparsely distributed and no concentrations were located. An extensive search by Parks Canada contractors located a few individuals and three nests in Gwaii Haanas National Park Reserve in 2006 (P. Dyment, pers. comm.). This project searched for herons along 1180 km of shoreline on Graham Island and the north end of Moresby Island and resulted in the observation of 19 adults and 6 immature herons. If all 19 adults represented nesting pairs and one of each pair was on a nest, then the number of adults would be 38 and the total number of herons would be 44. At a similar density over the entire 4660 km of coastline in the islands would result in about 174 herons. Engelstoft and Sopuck (2005) surveyed 500 km of shoreline in Gwaii Haanas during the nesting season and counted 8 herons. If those also represented half of the mated pairs, then there might have been 16 herons present (0.032 herons/km). The densities of both surveys are similar (0.037 vs 0.032). We do not have any estimates for other parts of the coast but our observations along much of the central and north coast indicate a low density. The approximately 25 000 km of shoreline outside the Strait of Georgia might support about 875 herons, at a similar density to the Queen Charlotte Islands, but there is no way to know the accuracy of this estimate. Given that not all areas may be suitable, it is likely a liberal estimate. No nests have been located in Pacific Rim National Park Reserve, though a maximum of 6 birds were observed in Grice Bay during surveys in the summer of 2007, some of which were flying inland between foraging bouts suggesting they were nesting nearby (Vennesland, unpubl. data). Although herons are scarce there during the nesting season, up to 100 birds use mudflats near Tofino in August and September after the nesting season (P. Clarkson, pers. comm.).

The most recent published estimate of population size for the Pacific Great Blue Heron in British Columbia is about 3600 nesting adults, of which 3300 were thought to occur in the Strait of Georgia and 300 elsewhere on the coast (Gebauer and Moul 2001, based on data in Butler 1997). Based on 2005 data for the Georgia Basin (1833 active nests estimated in 46 colonies), recent work on the Queen Charlotte Islands (c. 200 herons, P Dyment, pers. comm.), and our estimate of 900 herons outside the Strait of Georgia on the mainland coast, we believe a better estimate of population size to be about 4000-5000 nesting adults in Canada.

Censuses of Pacific Great Blue Herons in neighbouring Washington State have not been as thorough as in British Columbia, but recent attempts to find herons there estimate the population at about 5500 nesting adults (Eissinger 2007). No information is available from Alaska, though populations are likely small as on British Columbia’s north coast. In total, the global population of Pacific Great Blue Heron is likely between 9 500 and 11 000 nesting adults.

Fluctuations and trends

The trend in Pacific Great Blue Heron populations has been assessed in this report from surveys at nesting colonies (B.C. Conservation Data Centre 2007), Christmas Bird Counts (CBC), Coastal Waterbird Surveys (CWS), Breeding Bird Surveys (BBS) and population modelling. Results are variable, with some measures showing declines (nesting productivity from colony surveys, CBC, and demographic modelling), others apparent stability (nest counts from colony surveys, BBS) and one an increase (CWS), though see further for discussion.


Colony surveys – trends from colony size information

Counts of pairs at colonies give a distorted view of trends because survey effort has been variable through time (generally increasing), and herons move between colonies and regions within and between years (Simpson et al. 1987). However, if many colonies are included, the overall trend in numbers might capture inter-colony movement and reflect local changes in abundance. The method here is to sum the annual increases and decreases at colonies (e.g., Σ[Colony X Year 2 – Colony X Year 1; Colony Y Year 2 – Colony Y Year 1; etc] across all colonies).


Figure 4. Annual sums of increases and decreases in the number of nesting pairs at Pacific Great Blue Heron colonies in south-coastal British Columbia from 1986 to 2005.

Annual sums of increases and decreases in the number of nesting pairs (see long description below).

Annual colony sample sizes are provided in brackets (B.C. Conservation Data Centre 2007).

Description of Figure 4

Sums of increases and decreases in the number of nesting pairs at colonies from 1986 to 2005 showed that the number of nesting pairs leaving and entering colonies from year to year appears to have remained relatively stable over the period 1986 to 2005. A bias of this method is that it draws samples only from colonies that have multiple observations in consecutive years. Thus, this method gives a snapshot of a relatively small sample of regularly visited and stable colonies and may not illustrate exact trends in colony occupation. Nevertheless, drastic changes in the nesting population would be evident through this analysis.

The large negative sum in 1997 (-569) occurred mostly because of large declines at the two largest colonies in the Strait of Georgia (Point Roberts and Pacific Spirit Park). Both colonies recovered in numbers in subsequent years, although Pacific Spirit Park was abandoned completely in 2004. Point Roberts was abandoned completely in 2003 and a new location was colonized in 2004 – this event is illustrated in Figure 4 by the large negative value in 2003 and the large positive value in 2004 when these birds colonized the new location.


Colony surveys – trends in nesting success and productivity

As previously outlined, a particular challenge with this dataset is that survey effort has increased markedly through time. Thus, it should be noted that some analyses are anecdotal in nature and the time periods included vary.

Nesting success (the proportion of nesting pairs that successfully raise at least one fledgling) for herons on the coast of British Columbia currently is much lower than in the past. Forbes et al. (1985a) estimated that about 92% of nesting pairs were successful during the period 1977 to 1981 and a literature review by Forbes et al. (1985a) at the time showed a continent-wide success rate of 80%. Vennesland (2000) estimated that less than half of all nesting attempts were successful in 1998 to 1999 and this trend has been documented in future years as well (Vennesland 2003; McClaren 2005; Chatwin et al. 2006).

Mean nesting productivity (fledglings per active nesting attempt) on the coast of British Columbia in recent years has been the lowest of any studies in North America or British Columbia (Vennesland and Butler 2004). In 1971-1986, mean nesting productivity was 1.7 fledglings per active nesting attempt and 2.55 fledglings per successful nesting attempt (R. Vennesland, unpubl. data.). These values are roughly typical for North America at that time (Vennesland 2000). Reported productivity values for south-coastal British Columbia over recent years are 0.82 fledglings per active nesting attempt, and 1.98 fledglings per successful nesting attempt in 1999 (Vennesland and Butler 2004), 0.82 fledglings per active nesting attempt and 1.84 fledglings per successful nesting attempt in 2002 (Vennesland 2003), and 1.3 fledglings per active nesting attempt and 1.7 fledglings per successful nesting attempt in 2004 (McClaren 2005). Nesting productivity has therefore reduced to nearly half of historic levels (Figure 5).


Figure 5. Mean productivity for all active nesting attempts for Pacific Great Blue Heron colonies in south-coastal British Columbia from 1971 to 2005.

Mean productivity for all active nesting attempts (see long description below).

Productivity per active nesting attempt is the mean number of fledglings produced in all active nesting attempts. Sample sizes of colony-level observations in each period are shown in brackets. No observations were available in 1996. Error bars represent one standard error of the mean (B.C. Conservation Data Centre 2007).

Description of Figure 5

Figure 5 presents a summary of trends in nesting productivity per active nesting attempt from 1971 to 2005. Before 1987, most studies ignored nesting failure and only documented the number of fledglings from successful nesting attempts. As many studies in recent years have shown, nesting failure has an important influence on overall nesting productivity (e.g., Butler et al. 1995; Vennesland and Butler 2004). Due to this oversight, few observations before 1987 are available for analysis. Consequently, our analysis here groups colony-level observations into three time periods: 1971 to 1986 (relatively low annual effort – 19 observations total), 1987 to 1995 (increased annual effort – 125 observations total), and 1997 to 2005 (maximum annual effort – 251 observations total). One year (1996) was not included because no data on productivity were collected. The data show that nesting productivity has declined significantly across the three time periods.


Figure 6. Mean annual nesting productivity for all successful nesting attempts at Pacific Great Blue Heron colonies in south-coastal British Columbia from 1977 to 2005.

Mean annual nesting productivity (see long description below).

Productivity per successful nesting attempt is the mean number of fledglings produced in all nests that fledged one or more young. Data from 1982-1987, 1994-1996 and 2000 were excluded due to samples sizes under n = 5. The annual number of colonies analyzed is provided in brackets. Error bars represent one standard error of the mean (B.C. Conservation Data Centre 2007).

Description of Figure 6

Figure 6 presents a summary of trends in nesting productivity per successful nesting attempt from 1977 to 2005. As this dataset has been collected more consistently than productivity per active nesting attempt, an analysis was possible with annual colony-level observations. However, for clarity data are grouped into years. The analysis uses a poisson generalized linear model (Agresti 2002), adjusting for overdispersion, a likely consequence of having some colonies repeatedly measured over time (Agresti 1996). Nesting productivity per successful nesting attempt declined significantly over this period.


Colony surveys – trends in the effects of colony size

A 1999 study showed that nesting productivity per active nesting attempt increased significantly with colony size, presumably due to higher rates of nest failure at small colonies, but that productivity per successful nesting attempt did not relate to colony size (Vennesland and Butler 2004). Here we look at the effect of colony size on productivity over time. Data were grouped into colony size categories and the analyses were conducted on colony-level observations using a logistic generalized linear model (Agresti 2002), adjusting for overdispersion, a likely consequence of having some colonies repeatedly measured over time (Agresti 1996).

From 1987 to 2001, productivity per active nesting attempt increased significantly with colony size (Figure 7), and from 1977 to 2005, productivity per successful nesting attempt decreased significantly with colony size (Figure 8). Herons in large colonies therefore were more successful in their nesting attempts over all (due to lower levels of nest failure). However, when excluding nesting failure (i.e., successful nests) herons in large colonies raised fewer offspring per nesting attempt than herons in smaller colonies.


Figure 7. Mean productivity per active nesting attempt for colony size categories of Pacific Great Blue Herons in south-coastal British Columbia from 1987 to 2001.

Mean productivity per active nesting attempt for colony size categories of Pacific Great Blue Herons in south-coastal British Columbia from 1987 to 2001.

Productivity per active nesting attempt is the mean number of fledglings produced in all initiated nesting attempts. Data from 1994 and 1995 were excluded due to samples sizes under n = 5. Error bars represent one standard error of the mean (B.C. Conservation Data Centre 2007).


Figure 8. Mean productivity per successful nesting attempt for colony size categories of Pacific Great Blue Herons in south-coastal British Columbia from 1977 to 2001.

Mean productivity per successful nesting attempt for colony size categories of Pacific Great Blue Herons in south-coastal British Columbiafrom 1977 to 2001.

Productivity per successful nesting attempt is the mean number of fledglings produced in all nests that fledged one or more young. Data from 1982-1987, 1994-1996 and 2000 were excluded due to samples sizes under n = 5. Error bars represent one standard error of the mean (B.C. Conservation Data Centre 2007).


The relatively high productivity per active nesting attempt at larger colonies (Figure 7) implies that nest failure is relatively unimportant at these colonies (Vennesland and Butler 2004). However, previous studies have identified high levels of nest failure on the coast of British Columbia as an important reason for low levels of nesting productivity (Gebauer and Moul 2001; Vennesland and Butler 2004). More importantly, the data show that the loss of young from successful nesting attempts is another reason for low levels of productivity observed in the Strait of Georgia (as per the significant decline in Figure 6). This is particularly a concern for larger colonies that have lower productivity per successful nesting attempt than at smaller colonies (Figure 8).

Furthermore, most nesting pairs breed in large colonies, which are localized in a small part of the subspecies’ range (the lower Strait of Georgia). In 2005, 68% of 1833 nesting pairs (n = 46 colonies) were concentrated at six colonies of more than 100 nesting pairs each. Four of these six colonies were located in the lower Fraser Valley, with the other two on southern Vancouver Island and the southern Gulf Islands.

Butler and Vennesland (2000) hypothesized that herons may leave large colonies due to increased disturbance and further disperse in their nesting as they try and find sites relatively free from eagle and human disturbances. However, the opposite pattern has been recently documented in Washington State. Eissinger et al. (2007) showed that the proportion of nesting pairs in small colonies is declining while the number in large colonies is increasing, perhaps to dilute predation risk. If this pattern also occurs in Canada and if productivity continues to decline, herons may either suffer high levels of nest failure at small colonies or low numbers of fledglings at large colonies.


Colony surveys – effective range size

Although Pacific Great Blue Heron productivity has declined significantly (by both active nesting attempts and successful nesting attempts) since the 1970s, colony occupancy appears to be generally stable (Figure 4). Some population retraction and severe nesting productivity issues have been observed along the margins of the Strait of Georgia.The number of herons observed nesting on the Sunshine Coast dropped from 97 in 1978 (Forbes et al. 1985b) to 4 in 2004 (the last year there are records for that area). About 90 pairs of herons nested in Pender Harbour and 6 small colonies were located along the Sunshine Coast in the 1970s (Simpson 1984). A few colonies were still present there in the 1990s (Butler 1997), but recently a few herons have been observed breeding there (Vennesland 2000). Furthermore, recent surveys on north-eastern Vancouver Island also are troubling. Chatwin et al. (2006) report that all colonies north of Nanoose Bay failed to raise any young in 2005. The significance of the population retraction on the Sunshine Coast and the absence of productivity on northern Vancouver Island in 2005 to the overall Pacific Great Blue Heron population is not known.

Although the Pacific Great Blue Heron occurs across the coast, the majority of herons nest in the southern Strait of Georgia and northern Puget Sound where the largest human and significant Bald Eagle threats occur, and this is the only area of the coast where significant successful reproduction occurs.

Trends from volunteer-based population surveys

Christmas bird counts

Gebauer and Moul (2001) reported that CBC surveys showed populations to be either declining modestly (Lower Fraser Valley) or sharply (Sunshine Coast). We conducted an analysis of CBC data using data from all coastal count circles (i.e., locations specific to Pacific Great Blue Herons). We limited this analysis to the past three generations to provide relevance to the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) listing criteria. To determine generation time (defined here as the average age of breeding individuals), a population matrix model (as per Caswell 2001) was computed (M. Drever, unpubl. data) assuming survival rates through three life stages (first winter survival rate of 0.273, second year survival rate of 0.5 and annual adult survival rate of 0.727; Butler 1995), an annual nesting productivity of 1.12 fledglings per active nesting attempt (R. Vennesland, unpubl. data for 1986 to 2005) and a maximum life span in the wild of 24 years (Butler 1992). The resulting average age for a breeding Pacific Great Blue Heron in British Columbia was 5.6 years. Given this average age, we have looked at CBC survey data over two periods: from 1991/92 to 2006/07 (assuming a 5-year generation time) and from 1988/89 to 2006/07 (assuming a 6-year generation time).


Figure 9. Christmas Bird Count (CBC) data analysis for three generations of the Pacific Great Blue Heron in Canada.

Christmas Bird Count (CBC) data analysis (see long description below).
Description of Figure 9

Data are shown from the 1988/1989 winter (1988 on figure) to the 2006/2007 winter (2006 on figure). Generation time for the species is calculated at 5.6 years, so trend lines are shown for both 5 years (16-year period – dashed line) and 6 years (19-year period – solid line). Equations are provided for trend lines. The circled data points correspond to the years of CBC data that the Coastal Waterbird Survey analysis (Badzinski et al. 2005)also covers.

Since 1991/92, the relationship between the mean number of herons observed per party hour and year shows a significant 19% decline (t = -2.18; P < 0.05; n = 16; r2 = 0.20). Since 1988/89, the relationship between the mean number of herons observed per party hour and year shows a significant 26% decline (t = -3.18; P < 0.01; n = 19; r2 = 0.34). As mentioned earlier, these results should be treated with some caution because of the increase in search effort over time with this highly detectable bird (see Search Effort section).


Using CBC data from across south-coastal British Columbia (assuming a 5-year generation time – a conservative estimate given our result of a 5.6-year average age for breeding adults), we found the lower Fraser Valley showed a nearly significant increase in mean herons observed per party hour (t = 1.98; P = 0.07; n = 16; r2 = 0.16), Vancouver Island showed a significant decline in mean herons observed per party hour (t = -2.36; P < 0.05; n = 16; r2 = 0.23) and the Sunshine Coast showed a significant decline in mean herons observed per party hour (t = -4.08; P < 0.01; n = 16; r2 = 0.51). These results are consistent with other survey data (see previous) and our demographic analyses (see further) that the lower Fraser Valley is the only region of British Columbia that has effective reproduction and that a range contraction may be occurring in northern parts of the Strait of Georgia.


Coastal waterbird survey

CWS data show a significant increase in Pacific Great Blue Herons in winter on the coast of British Columbia from 1999/2000 to 2003/04 (6.9% increase per winter; P < 0.05 ; Badzinski et al. 2005). The CWS generally has a more robust survey methodology than both CBC and BBS. Both CWS and CBC are conducted during winter, so the results of these surveys as reported here present an apparent contradiction. However, the increasing CWS trend is from a five-year period, as opposed to the much longer period analyzed for CBC data. The period of the CWS analysis corresponds to a period of increased observations in CBC data (though not a significant increase; t = 1.69; P = 0.19; n = 5; r2 = 0.32; Figure 9). This suggests that the time period in the CWS analysis may be too small to accurately reflect long-term trends.


Breeding bird survey

BBS data indicated a significant decline in herons on the British Columbia Coast of 5.7% from 1966 to 1994 (Downes and Collins 1996). However, closer inspection of the BBS data apparently has revealed that one census route was driving the analysis down, and when it was removed the downward population trend was no longer significant (B. Smith, unpubl. data).

Summary of trends in nesting productivity and population status

Counts from colony surveys show relatively stable levels of colony occupancy, but these surveys are of limited use for determining population status. Measures of nesting productivity show significant declines, with the number of fledglings per active nest falling by nearly half since the 1970s. Declines in productivity per successful nest may disproportionately affect large colonies in a localized area (about 5000 km2 in extent) where most of the breeding for the subspecies occurs. Range contraction may be occurring in one region (the Sunshine Coast), and another region (northern Vancouver Island) has shown an absence of productivity in at least one year.

Christmas Bird Count data show a significant 19-26% decline over three generations, Coastal Waterbird Survey data show a significant increase, but these surveys cover only a five-year period, and Breeding Bird Survey data show stable trends.

Trends from demographic analyses

The demographic model for Pacific Great Blue Herons assumes a first winter survival rate of 0.273, second-year survival at 0.5 and a subsequent survival of 0.727 (Butler 1995). We started with 1000 nesting females and mean annual nesting success sampled from colonies on Vancouver Island and the lower Fraser Valley between 1988 and 2003. Nesting productivity was significantly higher in the lower Fraser Valley than on Vancouver Island (Figure 10; F = 15.3, df = 1, P < 0.01), as has been previously reported (Vennesland 2000; Vennesland 2003; McClaren 2005, Chatwin et al.2006).

To maintain the population requires herons to successfully raise fledglings in about 63% of all attempts. On Vancouver Island, the mean nesting success never reached this minimum threshold, whereas colonies in the lower Fraser Valley exceeded this threshold on five of the seven years with available data. In this analysis, the lower Fraser Valley is a source of recruits for Vancouver Island. Using the same analysis, the lower Fraser Valley produces about 66 young for every 1000 females whereas Vancouver Island colonies have a shortfall of 230 young for every 1000 females. Together with our analysis of CBC data (see previous), these preliminary results suggest that over the past three generations the lower Fraser Valley has not been able to compensate for the shortfall in production of juveniles on Vancouver Island (only the lower Fraser Valley has positive population indices).

With the lower Fraser Valley as the source of recruits, this increases the importance of maintaining productivity in this relatively small area (the lower Fraser Valley area is only about 5000 km2 in extent). It also suggests that the vulnerability of the Pacific Great Blue Heron is greatest where the highest human population resides – in the lower Fraser Valley. Although the range of the Pacific Great Blue Heron is relatively large, its effective range size may be limited to a small and heavily populated region of the province (i.e., the lower Fraser Valley).


Figure 10. Mean productivity per active nesting attempt for Pacific Great Blue Herons in south-coastal British Columbia from 1992 to 2003 comparing Vancouver Island to the lower Fraser Valley on the mainland (including years from 1987 that have at least 5 colony level observations for each region in each year).

Mean productivity per active nesting attempt for Pacific Great Blue Herons in south-coastal British Columbia

Productivity per active nesting attempt is the mean number of fledglings produced in all initiated nesting attempts (B.C. Conservation Data Centre 2007).

Rescue effect

Rescue effect from the south (i.e., from Washington State) is theoretically high due to the contiguous nature of Puget Sound and Georgia Strait, and the roughly equally sized heron populations between the regions. However, threats to heron populations and habitat in the U.S. are similar to those in Canada, perhaps with even higher threats and impacts due to the larger, more established human populations there (Georgia Basin Ecosystem Initiative 2002).

Detailed colony surveys for nesting productivity or population trends are not available from Washington State. However, CBC data from count circles close to the Canada/U.S. border (those within about 100 km of the Strait of Georgia or Juan de Fuca Strait) show a significant population decline of 34% (t = -3.02; P < 0.01; n = 16; r2 = 0.35) over the last 15 years or three generations (assuming a five-year generation time). Therefore, Pacific Great Blue Heron populations near to Canada in Washington State appear to have a higher rate of decline than Canada and so are likely to provide limited rescue.

Rescue effect from the north is likely low due to the small populations of herons that occur there. In addition, productivity of northern areas is unknown.

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