HERON AND EGRET MONITORING RESULTS AT WEST MARIN ISLAND: 2003 NESTING SEASON

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1 HERON AND EGRET MONITORING RESULTS AT WEST MARIN ISLAND: 2003 NESTING SEASON A Report to the San Pablo Bay National Wildlife Refuge John P. Kelly a and Binny Fischer Cypress Grove Research Center, Audubon Canyon Ranch P. O. Box 808, Marshall, CA a kelly@svn.net ACR Technical Report March 2004, Audubon Canyon Ranch 4900 Shoreline Highway, Stinson Beach, CA 94970

2 2003 Heron and Egret Monitoring at West Marin Island 1 HERON AND EGRET MONITORING RESULTS AT WEST MARIN ISLAND: 2003 NESTING SEASON A Report to the San Pablo Bay National Wildlife Refuge INTRODUCTION Audubon Canyon Ranch (ACR) has been monitoring the number of nesting herons and egrets on West Marin Island since 1979 and the annual reproductive success of Great Egrets and Great Blue Herons since 1993 (Kelly et al , Kelly and Fischer ). Nests are monitored during repeated visits each year, from viewing positions on East Marin Island and by boat. This work is part of a regional study of heron and egret colonies in the northern San Francisco Bay area (Kelly et al. 1993). METHODS Methods for estimating reproductive success are described in detail in Kelly et al. (1996). In 2003, we mapped the locations of 69 focal Great Egret nests and 10 Great Blue Heron nests on panoramic photographs of the nesting colony. We used telescopes to monitor the nest survivorship, seasonal timing, and prefledging brood size of the numbered nests during four visits to East Marin Island (10 March, 15 April, 12 May, and 10 June). A subset of 13 focal Great Egret nests was monitored more frequently, on 45 observation days from the mainland. On 10 June, we counted nests of all bird species nesting on all sides of West Marin Island. As in other years, we counted nests on the west side of the Island from an 18-foot Boston Whaler, drifting slowly with the motor off. We counted nests on the south and northeast sides from anchored positions. We then cruised slowly along the northeast side to check for nests that were not seen from our anchored position. Finally, we landed on East Marin Island and rechecked the initial estimates for number of Great Blue Heron nests. Viewing conditions were very good, with overcast skies and calm water. We also conducted partial earlier counts of active Great Egret nests on 15 April and 12 May, from a viewing position on the wharf at East Marin Island.

3 2003 Heron and Egret Monitoring at West Marin Island 2 RESULTS AND DISCUSSION Great Egret On 10 June 2003, we counted 81 active Great Egret nests, suggesting a decline in colony size from 121 active nests in June Earlier counts conducted from the wharf on East Marin Island, indicating 80 (15 April) and 94 (10 May) active nests, further suggested that the number of active nests had declined below abundances observed over the last decade (Table 1). However, focal nest survivorship (percent fledging at least one young) in 2003 was 83% (n = 69 focal nests) and close to the survivorship of nests in 2002 (84%), suggesting that the lower nesting abundance this year did not result from a higher rate of nest failure. Although we found evidence of significant predation by Common Ravens (see below), nest predation did not appear to strongly reduce overall nest survivorship (Figure 1) Nest survivorship Figure 1. Annual survivorship of Great Egret Nests at West Marin Island. Numbers indicate sample size. Mean prefledging brood size was 1.78 young per successful nest (SE = 0.08, n = 42), which was similar to productivity in 2002 (1.82 young per successful nest, SE = 0.08, n = 39; Figure 2). Because nest survivorship and productivity of successful nests were similar to 2002, overall productivity in 2003 (1.47 young per nesting attempt (SE = 0.12) was similar to 2002 (1.54, SE = 0.12). Great Egrets were slightly more productive overall during 2002 and 2003 than in the previous few years (Figure 3). These estimates of overall success were obtained by adjusting the productivity of successful focal nests (prefledging brood size) to reflect all nest fates, based on focal nest survivorship. These productivity estimates should be interpreted with caution because they were adjusted

4 2003 Heron and Egret Monitoring at West Marin Island 3 using a single focal estimate of nest survivorship each year and therefore may underestimate sampling error. Great Egret colony size estimates in April and May, noted above, also suggested that overall nest survivorship was fairly high. 3 Prefledging brood size Figure 2. Mean annual prefledging brood size of successful Great Egret Nests at West Marin Island. Error bars = standard errors. 3 Young per nest attempt Figure 3. Mean number of young produced per Great Egret nest at West Marin Island, , based on prefledging brood size of successful nests adjusted for overall nest survivorship. Error bars = standard errors.

5 2003 Heron and Egret Monitoring at West Marin Island 4 Table 1. Number of active heron and egret nests on West Marin Island, based on counts conducted by boat and from East Marin Island. Black-crowned Great Blue Year Great Egret Snowy Egret Night-Heron Heron ` a b c d a 115 Black-crowned Night-Herons were present on adjacent mudflats on 17 April b Number includes one nest on East Marin Island. c Number of active nests during the standard early-june census window, on 5 June A count on 10 May indicated an earlier peak number of 161 active Great Egret nests. d Number of nesting pairs in 2001 was 8.

6 2003 Heron and Egret Monitoring at West Marin Island 5 Snowy Egret We counted only 103 Snowy Egret nests on West Marin Island in 2003, indicating a decline from 204 nests in 2002 and 217 nests in However, lower nest densities occurred in the mid-late 1990s (Table 1). Although Snowy nest numbers declined substantially at West Marin Island, they increased at several other sites, and regional abundance apparently declined only slightly (Table 2). The Snowy Egret colony at Napa State Hospital continued to increase in size, smaller colonies occurred at five previously active colony sites, and three new Snowy Egret colony sites were established in 2003 (Table 2). This was the third year in which Snowies did not nest at nearby Red Rock or Brooks Island. We emphasize that our results reflect only the number of active nests on observation days and do not directly reflect the number of breeding pairs. Apparent changes in the breeding population could be influenced considerably by differences in the rates or timing of nest failure among colony sites or years. We were unable to monitor nest survivorship or productivity on West Marin Island because Snowy Egrets generally conceal their nests. 16 Adult Snowy Egrets Figure 4. Number of adult Snowy Egrets recovered in raven prey remains found on East Marin Island (P = 0.13). As in other years, Snowy Egrets concentrated their nests in the area of low shrubs on the northeast side of the Island (Table 3). Prey remains near the Common Raven nest site on East Marin Island indicated that ravens consumed at least 8 adult Snowy Egrets in Observations since 1999 suggest an increasing trend in raven predation on adult Snowies, although additional data are needed to determine if this pattern is significant (Figure 4; P = 0.13).

7 2003 Heron and Egret Monitoring at West Marin Island 6 Table 2. Number of active Snowy Egret nests in the northern San Francisco Bay area from 1991 to Colony Site Brooks Island Napa State Hospital a Picher Canyon, ACR Penngrove Santa Rosa Creek Gold Hill, Suisun Marsh Red Rock West Marin Island Alcatraz Island Bodega Harbor Petaluma Wastewater Shollenberger Park Campbell Ranch Las Gallinas West Total a a Estimate only; nests not individually counted at the Napa State Hospital in Great Blue Heron The peak number of 10 active Great Blue Heron nests in 2003 was the highest on record for West Marin Island, indicating a continuing increase since Great Blue Herons colonized the Island in 1990 (Table 1). No herons or egrets nested on East Marin Island. We determined the productivity of only half of the eight successful Great Blue Heron nests. Two successful broods of two young and two successful broods of three young resulted in an estimate of average prefledging brood size of 2.5 young per nest (SE =

8 2003 Heron and Egret Monitoring at West Marin Island , n = 4). Estimated productivity over all nesting attempts, accounting for the failure of two nests, was 2.00 young per nest (SE = 0.52), which was the same as in Table 3. Number of active nests observed on West Marin Island on 10 June Number of occupied nests West side South side Northeast side Total nests Great Egret Snowy Egret Black-crowned Night-Heron Great Blue Heron Western Gull Black Oystercatcher 1 a 0 a 0 1 a On 10 June, we observed a pair of oystercatchers on the west side with two fledged chicks but no nest; on 12 May, we observed an adult oystercatcher on a nest on the South Side of the Island on 12 May. Black-crowned Night-Heron The estimated number of active Black-crowned Night-Heron nest sites on 10 June was slightly lower than the 64 counted in 2002, but it was comparable to other recent estimates (Table 1). These estimates have shown considerable variation over the course of this study (Table 1). This variation probably reflects sampling error associated with conducting the counts from remote positions by boat. Because night-herons often conceal their nests in dense vegetation, our estimates provide only a rough index of trends in colony size. The estimated number of night-heron nest sites on nearby Red Rock (36 nests) revealed a continuing decline from 63 nest sites in 2001 and 48 in Night-herons continued to nest at Skaggs Island in the Napa Marsh; by clapping cautiously, we flushed 31 adults and 2 juveniles from the nesting area in a cattail pond. At nearby Alcatraz Island, the peak number of active nests increased slightly from 69 in 2002 to 75 in Such changes in peak numbers of nests at other colonies are consistent with annual shifts in the distribution of nests among colony sites. We emphasize, however, that differences in peak number of active nests may not closely reflect differences in the actual number of breeding pairs. This is because the peak number of active nests is influenced by the extent of synchrony of nest initiations, which may vary among colony sites and years.

9 2003 Heron and Egret Monitoring at West Marin Island 8 Common Raven The level of raven nest predatory activity at West Marin Island was similar to recent years. Ravens nested successfully on East Marin Island, frequently spent time in the heron and egret colony on West Marin Island, and rarely if ever left the vicinity of Marin Islands. On our last visit to the Islands, the resident ravens had successfully fledged four young, but the young ravens remained in the vicinity of East Marin Island. As in previous years, the young ravens may have subsequently expanded their range to West Marin Island and spent extended periods of time in the heronry. As noted above, prey remains found near the Common Raven nest site indicated that ravens ate at least 8 adult Snowy Egrets in 2003, and observations since 1999 suggest an increasing trend in raven predation on adult Snowies (Figure 4). In addition, results from West Marin Island and other sites (Kelly et al. 2004) suggest that raven nest predatory behavior may increase annually, although predation rates may vary more strongly with raven productivity (food demand). Shell fragments found near the raven nest further indicated predation of at least 250 heron and egret eggs in Recovered eggs identified to species suggested that most eggs taken by ravens were from Black-crowned Night-Herons (54% in 2001, n = 70; 70% in 2003, n = 20). Analysis of prey remains and nest predation rates further suggested that the resident ravens at West Marin Island obtain most or all of their energy requirements from the heronry (Kelly et al. in review). On 10 June, both adult ravens harassed a Red-tailed Hawk, which had landed in the heronry, until it left the Island. The Red-tailed Hawk had, in turn, been harassing the nesting birds, causing frequent fly-ups of Snowy Egrets and Great Egrets. Thus, the presence of resident ravens might indirectly reduce the threat of predation by other nest predators. Dramatic increases in the numbers of Common Ravens in the San Francisco Bay area (Kelly et al. 2002) have focused concerns over possible increases in nest predation by Common Ravens in heronries (Kelly and Roth 2001). We will continue to monitor raven activity on the Marin Islands. Other Species We counted 48 Western Gull nest sites on West Marin Island on 10 June. On 12 May we observed an active Black Oystercatcher nest on the south side of West Marin Island, and on 10 June, a pair of oystercatchers with two chicks were seen in the intertidal area on the west side.

10 2003 Heron and Egret Monitoring at West Marin Island 9 ACKNOWLEDGEMENTS We thank Jean Starkweather for contributing to the census on 10 June. Giselle Downard of the San Pablo Bay National Wildlife Refuge was extremely helpful in coordinating access to the Marin Islands, in monitoring raven prey remains, and in measuring the dimensions of eggs recovered from raven cache areas on East Marin Island. The Loch Lomond Marina in San Rafael generously provided use of their boat launching facility. REFERENCES CITED Kelly, J. P., K. L. Etienne, and J. E. Roth Factors affecting nest predatory behaviors of Common Ravens in heronries. Submitted to Condor (in review). Kelly, J. P., K. L. Etienne, and J. E. Roth Abundance and distribution of the Common Raven and American Crow in the3 San Francisco Bay area, California. Western Birds 33: Kelly, J. P., and B. Fischer heron and egret monitoring results at West Marin Island. Annual Reports to the San Pablo Bay National Wildlife Refuge. Kelly, J. P., B. Fischer, and H. M. Pratt heron and egret monitoring results at West Marin Island. Annual reports to the San Francisco Bay National Wildlife Refuge. Kelly, J. P., H. M. Pratt, and P. L. Greene The distribution, reproductive success, and habitat characteristics of heron and egret breeding colonies in the San Francisco Bay area. Colonial Waterbirds 16(1): Kelly, J. P., and J. E. Roth Audubon Canyon Ranch raven project progress report, December Audubon Canyon Ranch Tech. Rpt