Reproductive Success of Black-crowned

Transcription

1 Prepared for: The National Park Service, Golden Gate National Recreation Area Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 29 By Roger L. Hothem and Brianne E. Brussee Data Summary 29 U.S. Department of the Interior U.S. Geological Survey i

2 U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia McNutt, Director U.S. Geological Survey, Reston, Virginia 21 For product and ordering information: World Wide Web: Telephone: ASK-USGS For more information on the USGS the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: Telephone: ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. ii

3 Contents Abstract... 2 Introduction... 3 Methods... 3 Study Area... 3 Nest Monitoring... 3 Estimates of Reproductive Success... 4 Results and Discussion... 5 Black-crowned Night-Herons... 5 Nest Monitoring... 5 Area Trends... 5 Nesting Habitat... 6 Reproductive Success... 7 Snowy Egrets...1 Nest Monitoring...1 Nesting Habitat...1 Reproductive Success...11 San Francisco Bay Trends...13 Conclusions...14 Acknowledgements...16 References Cited...17 iii

4 Figures Figure 1. Black-crowned night-heron nesting subcolonies on Alcatraz Island, San Francisco Bay, California, during ; Snowy egrets nested only in the Greenhouse subcolony in Figure 2. Comparison of black-crowned night-heron and snowy egret nest initiations at Alcatraz Island in 26, 27, 28, and Figure 3. Weekly nest initiation dates by year for black-crowned night-heron nests found at Alcatraz Island, during weeks beginning March 11 July 22, Figure 4. Range of initiation dates, median initiation dates, and numbers of nests monitored for blackcrowned night-herons at Alcatraz Island, Figure 5. Total black-crowned night-heron and snowy egret nests monitored at Alcatraz Island during Totals include estimated 34 nests of black-crowned night-herons in 1991 and Figure 6. Total black-crowned night-heron nests (circles; right y-axis) and spatial trends (bars; left y- axis) at Alcatraz Island, : North Coast Area = Foghorn and Power Plant; Central Area = Wall, Shower, Greenhouse, Recreation Yard, and Warden s House; South Coast Area = Auxiliary dock, Dock, Bench, Rubble West and Tunnel Bush (See Fig. 1) Figure 7. Nesting habitat used by black-crowned night-herons at Alcatraz Island, Figure 8. Mean incubation-period success and 95% confidence interval for Black-crowned Night- Herons at Alcatraz Island, Figure 9. Mean nestling-period success and 95% confidence interval for Black-crowned Night-Herons at Alcatraz Island, Figure 1. Black-crowned night-heron nests: Initiations, hatching and predation, Alcatraz Island, 2938 iv

5 Figure 11. Trends of gull (Western and California Gulls) (Acosta et al. 26 and S. Acosta, pers. comm.) and black-crowned night-heron nests on Alcatraz Island, Gull totals include 5 (24), 14 (25), 21 (26), 23 (27), 21 (28) and 14 (29) California gull nests Tables Table 1. Black-crowned night-heron reproductive chronology, Alcatraz Island, Table 2. Number and percentage of black-crowned night-heron nests per subcolony, Alcatraz Island, Table 3. Estimates of clutch size and percentage nesting success (and 95% CI) of the black-crowned night-heron at Alcatraz Island, Table 4. Egg and chick survival of the black-crowned night-heron at Alcatraz Island, Table 5. Fates of black-crowned night-herons at Alcatraz Island, Table 6. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, Table 7. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, Table 8. Estimates of clutch size and percentage nesting success (and 95% CI) of the snowy egret at Alcatraz Island, Table 9. Fates of snowy egret nests at Alcatraz Island, Table 1. Egg and chick survival of the snowy egret at Alcatraz Island, v

6 Abstract As part of a monitoring program initiated in 199, we documented nesting chronology, habitat use, subcolony use, and hatchability for black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) nesting at Alcatraz Island, San Francisco Bay, California. In 29, we monitored 143 night-heron nests, a 42% increase from the 11 nests found in 28. We monitored 85 snowy egret nests, a number similar to the 82 nests found in 28. As in the previous 5 years, all snowy egret nests were located in the Greenhouse subcolony. Blackcrowned night-heron incubation-period success at Alcatraz Island in 29 was 24%, nestlingperiod success was 8%, and overall reproductive success was 19%, all decreases from 28 and all lower than the 2-year averages. Incubation-period success and overall success were the lowest observed during the 2-year study. A primary cause of this poor reproductive success was the elevated rate of nest predation (46%), more than two times higher than the 2-year average (2.4%), and the highest rate of predation in the 2-year study. Incubation-period success (75%) and overall success (75%) for snowy egrets were lower than rates observed in the previous four years; nestling success (1%) was similar to previous years. Predation on snowy egret nests was low (7%). The most common habitats for night-heron nests in 29 were century plant (Agave americana) (28%) and English ivy (Hedera helix) (28%), followed by rubble (concrete, reinforcing rods, and other remnants of demolished buildings) (24%). Snowy egrets nested primarily in plume albizia (Paraserianthes lophantha) (33%) and fig (Ficus carica) (36%). The numbers of breeding gulls on Alcatraz decreased for the first time since 1998, with 888 Western Gull (Larus occidentalis) and 14 California Gull (Larus californicus) nests counted. As found in previous years, interspecific competition with gulls was the most likely factor limiting reproductive success of the night-herons on Alcatraz Island in 29. Common ravens (Corvus corax) again nested on Alcatraz and likely contributed to predation on an unknown number of night-heron nests. The total number of night-heron nests found on Alcatraz Island in 29 exceeded the total found in South San Francisco Bay and comprised about 1% of the total North San Francisco Bay population. The total number of snowy egret nests at Alcatraz was similar to the total found in the South San Francisco Bay and comprised about 21% of the North Bay snowy egret nests. 2

7 Introduction We monitored reproduction by black-crowned night-herons and snowy egrets on Alcatraz Island for the 2 th consecutive year in 29. The primary objectives of this study were to: 1) conduct baseline monitoring to describe and estimate the distribution and abundance of blackcrowned night-herons and snowy egrets nesting on Alcatraz Island; 2) evaluate reproductive parameters, including nesting chronology, clutch size, incubation-period success, nestling-period success, and overall reproductive success; 3) evaluate the distribution, abundance, productivity, and effects of disturbance on reproduction; and 4) compare annual and long-term variation and trends in the nesting populations. Methods Study Area Alcatraz Island (37 o 49 N, 122 o 25 W), a 9.1-ha island and our study area in the San Francisco Bay, is managed as a National Historic Landmark by the National Park Service since 1973 (Howell and Pollak 1991) (Fig. 1). Nest Monitoring Nest searching and monitoring techniques followed the methods used in previous years (Hothem and Hatch 24). We searched each previously occupied nesting subcolony (Fig. 1) and other potential sites for active night-heron and snowy egret nests. Subcolony visits were conducted from 27 April to August 2 (Table 1). The Shower and Wall subcolonies, previously used for nesting by night-herons, were not checked in 29 (Table 2) because habitat at these two sites is now unsuitable for nesting by wading birds. As in all previous years, except 199 3

8 and 1993, monitoring was terminated early in certain subcolonies to prevent observer-induced adverse interactions between gulls and the wading birds (Table 1). Except for one night-heron nest located near the cellhouse, the Greenhouse colony (with 85 snowy egret nests) was last visited on 18 May. Four subcolonies (Rubble, Bench, Rubble West, Tunnel, and Dock) with large numbers of nearby gull nests were visited through 22 June. The last visit to the Warden s House and the Foghorn was on 7 July, to the Auxiliary Dock was 27 July, and to the Power Plant was on 3 August. Although early termination of nest monitoring has prevented us from compiling a total census of the numbers of nests on the island, the use of standardized monitoring procedures has produced data on nest numbers and reproductive success that are comparable among years. Estimates of Reproductive Success Reproductive success for 29 was calculated similar to previous years (Hothem and Hatch 24). Since we delayed our first visits to the subcolonies until many of the birds had begun incubation, and since we did not visit subcolonies daily, it is likely that some nests were initiated and lost between visits. We used the Mayfield method (Mayfield 1961, 1975), a less biased estimator of reproductive success, to calculate incubation-period and nestling-period success. We also calculated apparent rates of predation and other causes of nest failure. Mean clutch size was calculated based on the nests considered to have completed clutches (i.e., no increase in eggs between successive visits to an active nest). Nests that failed before a full clutch was achieved and those nests first found more than 7 days after hatching were not included in analysis of clutch size. Hatchability (egg success) was calculated by dividing the total number of eggs that hatched by the total number of eggs that were monitored (to hatch or failure). We also calculated apparent egg and chick survival. 4

9 Results and Discussion Black-crowned Night-Herons Nest Monitoring In 29, night-herons initiated nests (first egg laid) over an 87-day period from 7 April to 3 July, 21 days longer than in 28 (Fig. 2; Table 1). Based on an analysis of weekly initiation dates, nesting by night-herons at Alcatraz Island ranged from a minimum of 1 weeks in 1992, to a maximum of 19 weeks in 1993 (Fig. 2-4). The median initiation date in 29 (May 18) was 12 days later than the 19-year average. The earliest documented nest initiation in 29 occurred 13 days later than the overall average, and the latest initiation was 5 days later, resulting in a breeding period 8 days shorter than the long-term (199-29) average (Table 1). The number of nests monitored on the island each year has ranged from a low of 68 in 21 to a high of 341 in 1996 (Table 2; Fig. 5). The total number of night-heron nests found in 29 was 42% higher than the total found in 28, but it was 18 nests fewer than the 2-year average of Area Trends We considered each nesting subcolony to belong to one of three areas: the South Coast, the Central Island, or the North Coast (Fig. 6; Table 2). With 43 more nests in the South Coast subcolonies in 29 compared to 28, the percentage of the total found in the South Coast area of the island rose from 83% in 28 to 88% in 29 (Fig. 6; Table 2). In 29, the Bench subcolony had 3.4 times the number of nests compared to 28, and twice as many nests were found in the Rubble colony in 29 compared to 28. Only two nests were located in the Auxiliary Dock, an increase from the zero nests found in 28, but less than the 2-year average of 1 nests. We detected no apparent reason for the desertion of the Dock and Auxiliary Dock 5

10 subcolonies in 28 and 29 after the high numbers observed in 27 (23 and 8 nests, respectively). Of the nests found in the Bench subcolony in 29, 37 of the 47 were in an area densely populated by Agave sp., an area that is difficult to assess because of the hazard posed by the Agave s spines. Only three night-heron nests were found in the subcolonies of the Central Island area (2% of the island total), the same number as was found in 28, but the lowest percentage of all the years of the study (Fig. 6; Table 2). Most of this was related to the decline in numbers of nightheron nests in the Greenhouse subcolony, from 3 nests in 27 to none in 28 and 29. The Central Island area also comprised a low percentage (5 1%) of the overall island night-heron breeding population during , with the numbers of night-heron nests in the Greenhouse declining from 13 in 1997 to in 21. The Recreation Yard had no nests in 29, and night-herons nesting at the Warden s House declined from eight nests in 27 to only one in 28, and two in 29, less than the average of 5. nests per year for the previous 8 years. Thirteen night-heron nests (9% of the island total) were found in the North Coast area in 29 (Fig. 6; Table 2), below the average for the previous 19 years of 11.%. Nine nests were found in the Power Plant subcolony, similar to the ten nests found in 28, and higher than the 19-year average (7.5 nests). Four nests were found in the Foghorn subcolony, an area that has been sparsely used for nesting during An average of 15.6 nests was found during the first 1 years of the study, while the average per year in the past 1 years has been only 2.3 nests. Nesting Habitat In 29, century plant (Agave americana) (28%) and English ivy (Hedera helix) were the most common nesting substrates for the black-crowned night-herons, both with 28.% of the 6

11 total nests. Rubble, which consisted primarily of the remains of demolished buildings (i.e., concrete, reinforcing rods, metal, scrap wood, and miscellaneous debris), was the third most common nesting substrate for night-herons (23.8%), down from 36.6% in 28 (Fig. 7). Despite being the dominant habitat for nesting by night-herons in the 199s (Fig. 7), only 6.3% of the nests were found in mirrorbush (Coprosma baueri) in 29. Plants that have replaced mirrorbush as important nesting habitats include English ivy (28.%), rose (Rosa spp.) (7.%), and agave (28.%). The availability of nesting habitat and its change over the years have not been measured, but it appears that the area of mirrorbush has declined over the course of the study, while the area of ivy has increased. The areas with figs, roses, plume albizia, and rubble appear to have changed little over the course of the study. Reproductive Success We estimated that the mean clutch size for black-crowned night-herons in 29 was 2.8 eggs per nesting attempt, similar to the 2-year mean (2.87) (Table 3). The estimated incubationperiod success (nests that hatched at least one egg) for night-herons during 29, as measured by the Mayfield (1961, 1975) method, was 24% (95% confidence interval = 17-34%), which was lower than the study average (55.7%) and the lowest observed during the study (Table 3, Fig 8). The 2-year average, however, was similar to that estimated by Kelly et al. (26) for nightherons throughout the San Francisco Bay area during (62%). The estimated nestlingperiod success (the percentage of successful nests in which at least one chick survived to 15 days of age) for night-herons during 29 was 8% (95% confidence interval = 67-95%). a value lower than in the previous 1 years, and lower than the average nestling-period success for the 2-year study (87%) (Fig. 9). The overall success (incubation period and nestling period success 7

12 combined) of nesting night-herons on Alcatraz in 29 was 19%, the lowest value recorded in this study (Table 3). Failed eggs have been documented throughout the course of this study. In 29, we found that only two of the 92 eggs (2.2%) in successful nests failed to hatch (Table 4). Egg hatchability averaged 94.4% during the study, ranging from 89.1% in 22 to 98.6% in 26. Of the 9 eggs that hatched in 29, we observed that 39 chicks (43.3%) in 21 nests survived to at least 15 days; three chicks were found dead. Production averaged.27 fledglings per monitored nest or.95 chicks fledged per hatched nest (Table 4). Both values were lower than the 2-year means for fledglings per monitored nest (.67) and per hatched nest (1.18). Bay-wide, the average number of young fledged per nest attempt for (1.15) was about four times higher than at Alcatraz (.27) in 29, and the number of chicks fledged per successful nest was about two times higher in San Francisco Bay (1.82) (Kelly et al. 26) than at Alcatraz in 29 (.95). As found in other night-heron nesting studies (Wolford and Boag 1971; Tremblay and Ellison 198; Henny et al. 1984; Blus et al. 1997), predation was a primary limiting factor at Alcatraz in 29 (Table 5; Fig. 1). Based on apparent nest success, we estimated that eggs in 46% of the monitored nests were destroyed by predators before hatching (Table 5), higher than any other year of the study, and more than twice as many as the average for the previous 19 years (19.%). Renesting, often a response to pre-hatching losses to predators, was presumed to have occurred at 17 night-heron nests that had been destroyed by predators in 29. A major source of the elevated predation rate in 29 was the Rubble subcolony. Of the subcolonies assessed in 29, the Rubble contained 18% of the monitored nests, but 23 (88.5%) of those nests were destroyed by predators, the highest percentage of all subcolonies with > 2 nests assessed in 29 (Table 6). Of the 63 nests destroyed by predators prior to hatching, 36.5% 8

13 were in the Rubble subcolony. In the previous 19 years, 19.3% of the monitored nests in the Rubble were destroyed by predators, a rate similar to that observed for the total island during the 2-year study. In previous years, we have attributed much of the predation at Alcatraz to nesting Western gulls. In 29, however, although the predation rate increased, the numbers of breeding nesting Western and California gulls both declined. A total of 888 Western gull nests was found, a decrease of 16% over the 1,61 found the previous year. The number of nests in 29 was still almost twice as high as the average for (453 nests) (Fig. 11) (S. Acosta, pers. commun.). Fourteen California gull (Larus californicus) nests were observed near the Rubble West subcolony in 29, seven fewer than were found in 28, a decline of 33%. As in previous years, one pair of common ravens (Corvus corax) was observed nesting on Alcatraz, and it is likely that the ravens preyed on the night-heron nests. The presumed predation rate for hatched nests in 29 was 12%, more than twice as high as the average for the previous 19 years (5.3%). The pre-hatching nest abandonment rate in 29 was 5%, which is similar to the 19-year average of 3.6%. Where termination of subcolony visits was consistently earlier (i.e., Greenhouse, Bench, Rubble, and Rubble-West subcolonies) the percentage of the monitored nests that were classified as unknown was generally higher. In 29, the Bench, Rubble-West, and Tunnel subcolonies had high percentages of unknown fates, similar to the 2-year means (Tables 6 and 7). The exceptions were the Rubble, where 88% of the nests were observed to have been depredated, leaving few to be unknown, and the Greenhouse, which had no nesting by night-herons in 29. Unknown fates included those that were either lost or which were not rechecked for any reason (Table 5). 9

14 Snowy Egrets Nest Monitoring Beginning with three nests in 1997, we have monitored a total of 398 snowy egret nests on Alcatraz Island, with annual totals ranging from none in 22 to 85 in 29. Since 24, all snowy egret nests on Alcatraz have been located in the Greenhouse subcolony. In 29, snowy egret nests were initiated over a 54-day period between 22 March and 14 May, 1 days longer than the initiation period observed in 28 (Fig.2). Based on linear regression, during the period 23-29, the numbers of snowy egret nests increased significantly in the Greenhouse (r 2 =.894; P =.1), while the numbers of night-heron nests in the same subcolony decreased significantly (r 2 =.844; P =.3). As it was observed in 28, snowy egret nesting in 29 was earlier and more synchronous than the night-herons on Alcatraz, with 85% of the egret nests initiated between April 1 and 21. The median initiation date for egrets in 29 was 14 April, 7 days earlier than we observed in 28. The egret median initiation date was 34 days earlier than that observed for the night-herons in 29 and 35 days earlier than the night-herons in 28. The earlier nesting by the egrets in 28 and 29 (Fig. 2) may have been a major factor in the failure of night-herons to nest in the Greenhouse subcolony for the second straight year. By the time 55 egret nests (65% of the total) had been initiated in the Greenhouse in 29, only 11 night-heron nests had been initiated on the remainder of the island. Nesting Habitat Of the 85 snowy egret nests monitored in 29, 36% were found in fig trees (Ficus carica), and 33% were found in Albizia. Himalayan blackberry (Rubus discolor) (25%), Viburnum (5%), and mirrorbush (1%) comprised the other egret nest substrates in 29. Based 1

15 on linear regression, the number of egret nests found in the figs increased significantly (r 2 =.772; P =.9) during 23-29, from 2 nests in 23 to 31 in 29. Conversely, the numbers of night-heron nests found in the figs has steadily declined during the same time period, from 18 nests in 23 to none in 28 and 29 (r 2 =.757; P =.11). In 199, 94.7% of the night-heron nests (36 of 38 nests) were found in figs, the most found in this habitat in the 2-year study. That number is similar to the average number of snowy egret nests (34.5) found in figs in 28-29, suggesting that the fig area may be approaching its carrying capacity. Reproductive Success The mean clutch size for snowy egrets in 29 was 3.71 eggs per nest (Table 8), higher than each of the means for the previous 5 years, significantly higher than the mean clutch sizes in 24 and 27, and similar to the clutch sizes found in studies cited by Parsons and Master (2). The overall mean clutch size for snowy egrets (24-29) was 3.4 eggs per nest, higher than the 2-year mean for night-herons (2.87) (Tables 3 and 8). During 24-29, the median clutch size for night-herons was 3 eggs. The median for snowy egrets during all but 29 was also 3 eggs; in 29, however, the median was 4. Snowy egret clutch sizes for each of the years were significantly higher than those for night-herons for the same years (Kruskal-Wallis One Way Analysis of Variance on Ranks, P <.1). As with the night-herons, we used the Mayfield method to calculate incubation-period, nestling-period, and overall success. Incubation-period success was 75% (95% confidence interval:59-95), nestling-period success was 1%, and overall nest success was 75% (Table 8). These values were lower than those calculated for the previous three breeding seasons (Table 8), but were far greater than the values found for the night-herons (Table 3). Kelly et al. (26) reported nest survivorship during to be 66% for snowy egrets throughout the North 11

16 San Francisco Bay area, lower than that observed at Alcatraz in 25-29, but similar to that observed in 24 (Table 8). Although there were no night-heron nests monitored in the Greenhouse in 28 and 29, as there were in previous years, the density of the 85 snowy egret nests made unobtrusive monitoring difficult because nests were so close to one another. To prevent observer-caused abandonment and chick mortality, monitoring was terminated early (May 17) in the Greenhouse subcolony, before the fates of all the nests could be determined. Thus, 36% of the incubationperiod fates and 52% of the nestling-period fates were unknown (Table 9). Eggs hatched in 48 of the 85 egret nests (56%) in 29 (Table 9). Predators destroyed six nests (7%), and no nest was abandoned. Of the 48 nests that hatched, none was destroyed by a predator or abandoned, and 23 (48%) were observed to fledge young to 1 days. As in previous years, Western gulls and common ravens were present in 29 and were potential predators of snowy egret eggs and chicks, but we only observed predation on six nests (7%) before hatching and none after hatching. Compared with the 143 night-heron nests found on the island in 29, the incubation-period predation rate for egrets was about 15% that of the night-herons. These data indicate that the Greenhouse subcolony is a relatively safe nesting site for snowy egrets. Hatchability (97.3%) of snowy egret eggs was similar to that observed in 28 (98.%), but higher than (Table 1). Four of the 147 eggs observed to hatching date failed. Egret egg hatchability in 29 was similar to night-heron hatchability (97.8%), but higher than the average night-heron hatchability for the 2-year study (94.3%) (Table 4). Of the 143 egret eggs that hatched, we observed that 69 chicks (48%) survived to at least 1 days (1.44 chicks fledged per successful nest;.81 chicks fledged per monitored nest), and one chick was found 12

17 dead (Table 1). The number of fledged chicks per successful nest in 29 was similar to that observed in 28 (1.45), but, because there were about 5% more nests that had unknown fledging fates in 29, the number of chicks fledged per monitored nest was higher in 28 (1.1) (Table 9). Fledging rates were much better in (Table 1). Snowy egret nestling-period success was lower than that reported by Kelly et al. (26) for the entire San Francisco Bay area (2.5 chicks per successful nest and 1.8 chicks per monitored nest), even though our nestling-period success was based on survival to 1 days of age while Kelly et al. used 14 days. San Francisco Bay Trends The total numbers of night-heron nests (absolute numbers) at Alcatraz were 42% higher in 29 than 28, and, based on data from Audubon Canyon Ranch (E. Condeso, pers. commun.), the total peak number of active nests (including Alcatraz and an estimate from a boat survey for West Marin; See Kelly et al. 26) at all North San Francisco Bay colonies increased by about 33%. Peak number of night-heron nests on Alcatraz was 9.7% of the North Bay total in 29, compared with 12.4% in 28 and 16.4% in 27. The percentage of night-heron nests found within the Central Bay area (West Marin, Yerba Buena, Red Rock, Brooks, and Alcatraz islands) was 17.4% in 29, slightly lower than 28 (19.9%), and much lower than 27 (39.2%). This decline in percentage of nests at Alcatraz is mostly related to the increase in the estimated number of nests at West Marin, which increased from 113 in 27 to 216 in 28 and 34 in 29. An estimated 79 night-heron nests were counted in South San Francisco Bay in 29 (Robinson-Nilsen et al., 29), similar to the 74 in 28, but fewer than the 138 in 27. Redwood Shores was the colony with the most night-heron nests in both 28 and 29. Counts 13

18 by the San Francisco Bay Bird Observatory were likely conservative because they report the peak number of nests and only estimate night-heron nest numbers at colonies that also have other species of interest (i.e., great blue herons (Ardea Herodias), great egrets (Ardea alba), or snowy egrets). However, it appears that fewer numbers of night-herons are breeding in recent years in the South San Francisco Bay area than in the central and northern parts of the Bay. The total numbers of snowy egret nests (absolute numbers) at Alcatraz remained relatively stable during 27-29, increasing from 79 to 85. Based on data from Audubon Canyon Ranch (E. Condeso, pers. commun.), the total peak number of active nests (including peak numbers at Alcatraz) at all North San Francisco Bay colonies increased during by about 61.9%. Nests on Alcatraz comprised about 27.% of the North Bay population in 27, but declined to 21.% in 29. However, snowy egret nests as a percentage of the nests found within the Central Bay area (West Marin, Yerba Buena, Red Rock, Brooks, and Alcatraz islands) decreased from 41.3% in 27 to 3.6% in 29, largely as a result of a 4-fold increase in snowy egret nests from 43 in 27 to 175 in 29 at West Marin Island. An estimated 87 snowy egret nests were counted in South San Francisco Bay in 29 (Robinson-Nilsen et al., 29), a decline from the 138 nests in 27 and 16 in 28. The largest snowy egret colony in the South Bay, as for night-herons, was at Redwood Shores. The total number of snowy egret nests recorded in the South Bay in 29 was similar to the total from Alcatraz that year. Conclusions During 29, we documented reproduction by black-crowned night-herons and snowy egrets at Alcatraz Island for the twentieth consecutive year. In 29, 143 night-heron nests were monitored, an increase from the 11 nests monitored in 28. Snowy egret nesting in 29 was similar to 28, reaching a study high of 85 nests. As in 28, incubation-period and overall 14

19 success for night-herons were less than the overall averages for the study, but in 29 the nestling-period success was also lower than the overall average. All categories of reproductive success were higher for snowy egrets than the night-herons, but snowy egret success rates were lower than those observed in the previous 3 years. Predation appears to be the most limiting factor for night-herons nesting on Alcatraz Island, but predation on snowy egrets has not been significant. Predation on snowy egret nests was low in 29 (7%), and the average for was 4.8 %, far lower than the night-heron predation average for the same period (32%). In 29, there were 26 night-heron nests located in the Rubble subcolony, a 1% increase over the total in 28. However, 23 of the nests (88.5%) were destroyed by predators. We previously hypothesized that the density of the vegetation, primarily ivy, in the Rubble subcolony might be deterring night-herons from nesting there. However, apparently vegetation density in 29 neither deterred night-herons from nesting in the Rubble nor did it prevent predators from destroying most of the nests. The large numbers of nesting gulls, although fewer in 29 than in recent years, has likely contributed to the high night-heron predation rates. Although we assumed much of the predation on night-heron nests was by gulls, common ravens were observed taking eggs from night-heron nests, and likely destroyed many nests. The use of remote cameras could help to confirm the role of various species as predators on Alcatraz. Habitat changes are a major concern on Alcatraz. Mirrorbush has declined precipitously over the course of the study, primarily a result of senescence, and ivy has proliferated in some areas, including the Dock, Rubble, and Foghorn subcolonies, previously dominated by mirrorbush. An increase in the percentage of nests found in ivy reflects this apparent trend. 15

20 The Greenhouse subcolony, a relatively small area located adjacent to a walkway open to year-around visitor use, was once home to a significant night-heron population, reaching 51 nests in 23. However, as of 28, night-herons had been replaced by a snowy egret nesting colony second in size only to West Marin Island in the North San Francisco Bay area. Agonistic interactions are higher in snowy egrets in the pre-egg phase (Burger, 1978), a factor that may have contributed to the total displacement of night-herons by the snowy egrets in the Greenhouse in 28 and 29. The visual and physical barrier presented by the dense vegetation at this subcolony (fig, rose, Albizia) is apparently sufficient to prevent human-caused disturbance of the nesting birds and should be maintained. Barricades and signs alerting visitors to the presence of nesting wading birds have also likely helped reduce visitor disturbance. Large numbers of night-herons were observed to use the dense century plants both upslope and downslope of the Agave Trail. Despite its difficult access, we were able to locate 41 nests, 28.7% of the island s total in 29. The increased use (26-29) of this habitat, is further justification for retaining the closure of this area to human visitors during the breeding season. Seasonal exclusion of visitors from the South Coast nesting area has contributed to the preservation of these subcolonies, which, in 29, had 91% of the nesting night-herons on the island. Alcatraz remained a significant breeding site for both night-herons and snowy egrets in San Francisco Bay. Acknowledgements We thank Lara Rachowicz, Andrew Greene, Patrick Furtado, Chris Perry, Rachel Townsend, Aaron Sunshine, and Matt Griffis of the National Park Service for their invaluable assistance with logistics and field work. Christian Hellwig and Stephanie Bishop assisted with 16

21 nest searches. Sara Acosta of PRBO Conservation Science provided data on Western and California gulls. Sara Acosta of PRBO Conservation Science provided data on Western and California gulls. John Kelly of the Cypress Grove Research Center of Audubon Canyon Ranch provided information on North San Francisco Bay breeding night-herons and snowy egrets, and Caitlin Robinson-Nilsen of the San Francisco Bay Bird Observatory provided information on breeding by night-herons and snowy egrets in the South Bay. Funding was provided by the U.S. Geological Survey and by the Golden Gate National Parks Conservancy. References Cited Blus, L J., Rattner, B.A., Melancon, M.J., and Henny, C.J., 1997, Reproduction of Blackcrowned Night-Herons related to predation and contaminants in Oregon and Washington, USA, Colonial Waterbirds, v. 2, no. 2, p Burger, J Competition between cattle egrets and native North American herons, egrets, and ibises. The Condor, v. 8, no. 1, p Henny, C.J., Blus, L.J., Krynitsky, A.J., and Bunck, C.M., 1984, Current impact of DDE on Black-crowned night-herons in the intermountain west, Journal of Wildlife Management, v. 48, no. 1, p Hensler, G.L., and Nichols, J.D., 1981, The Mayfield method of estimating nesting success: a model, estimators and simulation results, Wilson Bulletin, v. 93, no. 1, p Hothem, R.L., and Hatch, D., 24, Reproductive success of the black-crowned night-heron at Alcatraz Island, San Francisco Bay, California, , Waterbirds, v. 27, no. 1, p

22 Howell, J.A. and Pollak, T., 1991, Wildlife habitat analysis for Alcatraz Island, Golden Gate National Recreation Area, California, p in Wildlife Conservation in Metropolitan Environments (Adams, L.W. and Leedy, D.L., Eds.) NIUW Symposium Series. 2, National Institute for Urban Wildlife, Columbia, Maryland. Kelly, J.P., Etienne, K., Strong, C., McCaustland, M., and Parkes, M.L., 26, Annotated atlas and implications for the conservation of heron and egret nesting colonies in the San Francisco Bay area, ACR Technical Report , Audubon Canyon Ranch, Marshall, CA, 236 p. Mayfield, H, 1961, Nest success calculated from exposure, Wilson Bulletin, v. 73, no. 3, p Mayfield, H. F., 1975, Suggestions for calculating nest success,. Wilson Bulletin v. 87, no 4, p Robinson-Nilsen, C., Schacter, C., and Demers, J. 29. Colonial waterbird nesting summary for San Francisco Bay, 29. Unpublished Report, San Francisco Bay Bird Observatory, Milpitas, CA, 12pp. Tremblay, J., and Ellison, L.N., 198, Breeding success of the Black-crowned night-heron in the St. Lawrence Estuary, Canadian Journal of Zoology, v. 58, no. 7, p Wolford, J.W. and Boag, D.A., 1971, Distribution and biology of Black-crowned night-herons in Alberta, Canadian Field-Naturalist, v. 85, p

23 Table 1. Black-crowned night-heron reproductive chronology, Alcatraz Island, Year Median Initiation Date Range of Initiation Dates Colony Visits Observation dates Apr 24 March 11 July April 9 August May 26 March 4 July April 26 Jul Apr 2 March 28 May 3 3 April 28 May Apr 16 March 24 July 15 2 April 2 August May 25 March 18 July 8 22 April 1 August Apr 11 March 28 June 7 26 April 28 June Apr 15 March 4 June 4 29 April 1 June Apr 13 March 28 May 6 22 April 12 June Apr 15 March 21 June 14 9 April 1 July May 27 March 12 June April 17 July 2 13-May 4 April 16 July April 8 August May 1 April 18 June April 24 July May 16 March 23 June 1 22 April 1 July May 27 March 2 June April 14 July 24 3-May 12 March 22 June 12 3 April 19 July 25 2-May 23 March 7 June April 15 July May 3 April 14 June 12 3 May 2 July May 22 March 29 June April 1 August May 12 April 16 June April July May 7 April - 2 July April 2 August Mean 6-May 25 March 27 June April 17 July 1 When present, snowy egret nests were also monitored during these periods. 2 Except in 199 and 1993, observations were discontinued at most sites before nesting was completed, primarily to prevent adverse interspecific competition with Western Gulls. 19

24 Table 2. Number and percentage of black-crowned night-heron nests per subcolony, Alcatraz Island, SUBCOLONY Mean Totals S. COAST % of total 39% 46% 54% 48% 58% 65% 71% 83% 8% 83% 87% 75% 82% 44% 5% 47% 72% 75% 83% 88% 66.5% Tunnel Bench Rubble Rubble W Dock Aux. Dock CENTRAL % of total 43% 39% 34% 39% 31% 29% 21% 1% 8% 5% 6% 7% 1% 46% 38% 39% 21% 2% 3% 2% 22.5% Cell house Greenhouse Rec. Yard Wall Shower Warden N. COAST % of total 18% 15% 13% 13% 11% 7% 8% 7% 12% 12% 7% 18% 9% 11% 12% 14% 7% 4% 14% 9% 11.% Foghorn Power Plant Total Numbers of nests estimated in 1991 and 1992 based on numbers monitored in 199 and in

25 Table 3. Estimates of clutch size and percentage nesting success (and 95% CI) of the black-crowned nightheron at Alcatraz Island, Nest success Clutch Year size Nests found Mayfield nests 2 Exposure days Hatching 3 Fledging 4 Overall (6-8) (84-95) (52-76) (72-92) (61-1) (77-99) (66-89) (96-1) (7-93) (88-1) (66-88) (86-98) (56-78) (71-87) (53-74) (64-8) (37-54) (7-86) (47-66) (85-98) (39-64) (77-98) (29-62) (85-1) (43-64) (86-1) (39-6) (82-98) (32-55) (29-55) (69-94) (44-68) (72-96) (22-38) (75-93) (28-54) (79-1) (17-34) (67-95) Mean Based on days of nest exposure (see Hensler and Nichols 1981). 2 Nests included in the analysis (Mayfield 1961, 1975). Nests that had either hatched or failed before they were found were rejected from the analysis; such nests would have no exposure days. 3 Predicted percent of monitored nests in which one or more eggs hatched. 4 Predicted percent of nests that hatched in which one or more chicks reached at least 15 days of age. 5 Predicted percent of monitored nests in which one or more chicks reached at least 15 days of age, calculated by multiplying Hatch Success by Fledging Success. 21

26 Table 4. Egg and chick survival of the black-crowned night-heron at Alcatraz Island, Year Total Monitored nests ,148 Successful nests ,782 Eggs observed to hatching (A) ,381 Failed eggs (B) % Hatchability ((A-B)/A) Hatched eggs/ monitored nest Hatched eggs/ successful nest Nests with fledged chicks ,54 Fledged chicks ,89 Fledged chicks/ monitored nest Fledged chicks/ hatched nest % hatched eggs that fledged Successful nests were those with at least one egg that was confirmed to have hatched. 2 Fledged chicks: the number of chicks observed to have survived to 15 days of age or more. 22

27 Table 5. Fates of black-crowned night-herons at Alcatraz Island, Nesting outcome Total Mean Monitored nests % predation % destroyed (other) % abandoned % unknown % hatched Number of hatched nests % predation % destroyed (other) % abandoned % unknown % fledged Number of fledged nests Fate of monitored nests (%). 2 Nests observed with at least one egg that hatched. 3 Fate of hatched nests (%). 4 Nests observed with at least one chick that survived to age 15 days. 23

28 Table 6. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, Nests per subcolony (% of island total) AUX BEN CH FH PP RUB RW TUN WAR TOTAL 2 (1.4) 47 (32.9) 1 (.7) 4 (2.8) 9 (6.3) 26 (18.2) 33 (23.1) 19 (13.3) 2 (1.4) 143 (1.) Nest fate per monitored nest % predation % destroyed % abandoned % unknown No. hatched nests (% of subcolony total) 1 (5.) 19 (4.4) (.) 1 (25.) 4 (44.4) 1 (3.8) 1 (3.3) 5 (26.3) (.) 41 (28.7) Fate per hatched nest % predation % destroyed % abandoned % unknown No. fledged nests (% of hatched nests) (.) 13 (68.4) (.) (.) 3 (75.) (.) 4 (4.) 1 (2.) (.) 21 (51.2) % fledged nests per subcolony total DOK, GH, REC, SH, and WAL not included; no nesting by black-crowned night-herons in

29 Table 7. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, AUX BEN CH DOK FH GH PP REC RUB RW SH TUN WAL WAR TOTAL Nests per subcolony (% of island total) 198 (6.3) 229 (7.3) 18 (.6) 188 (6.) 179 (5.7) 366 (11.6) 153 (4.9) 79 (2.5) 669 (21.3) 317 (1.1) 19 (.6) 582 (18.5) 19 (3.5) 42 (1.3) 3148 (1.) Nest fate per monitored nest % predation % destroyed % abandoned % unknown No. hatched nests (% of subcolony total) Fate per hatched nest 139 (7.2) 117 (51.1) 15 (83.3) 126 (67.) 18 (6.3) 21 (57.4) % predation % destroyed % abandoned % unknown (68.) 48 (6.8) 336 (5.2) 139 (43.8) 16 (84.2) 318 (54.6) 75 (68.8) 34 (81.) 1785 (56.7) No. fledged nests (% of hatched nests) 91 (65.5) 65 (55.6) 9 (6.) 94 (74.6) 62 (57.4) 116 (55.2) % fledged nests per subcolony total (76.) 37 (77.1) 162 (48.2) 68 (48.9) 13 (81.3) 176 (55.3) 58 (77.3) 31 (91.2) 161 (59.4) 25

30 Table 8. Estimates of clutch size and percentage nesting success (and 95% CI) of the snowy egret at Alcatraz Island, Nest Success Year Clutch Size Nests Found Mayfield Nests 2 Exposure days Hatching 3 Fledging 4 Overall (34-1) (68-1) (76-1) (1-1) (82-1) (1-1) (8-99) (95-1) (77-96) (1-1) (59-95) (1-1) Mean Based on days of nest exposure (see Hensler and Nichols 1981). 2 Nests included in the analysis (Mayfield 1961, 1975). Nests that had either hatched or failed before they were found were rejected from the analysis; such nests would have no exposure days. 3 Predicted percent of monitored nests in which one or more eggs hatched. 4 Predicted percent of nests that hatched in which one or more chicks reached at least 1 days of age. 5 Predicted percent of monitored nests in which one or more chicks reached at least 1 days of age, calculated by multiplying Hatching Success by Fledging Success. 26

31 Table 9. Fates of snowy egret nests at Alcatraz Island, Year Nesting Outcome Totals Mean Monitored Nests % predation % destroyed (other) % abandoned % unknown % hatched Number of hatched nests % predation % destroyed (other) % abandoned % unknown % fledged Number of fledged nests Fate based on monitored nests (%) 2 Nests observed with at least one egg that hatched. 3 Fate based on hatched nests (%) 4 Nests observed with at least one chick that survived to age 15 days. 27

32 Table 1. Egg and chick survival of the snowy egret at Alcatraz Island, Year Total Mean Monitored nests Successful nests¹ Eggs observed to hatching (A) Failed eggs (B) % Hatchability ((A-B)/A) 93.5% 9.3% 96.5% 89.6% 98.% 97.3% 94.2% Hatched eggs/monitored nest Hatched eggs/successful nest Nests with fledged chicks Fledged chicks² Fledged chicks/ monitored nest Fledged chicks/ successful nest % hatched eggs that fledged 31.% 64.3% 76.8% 72.8% 46.9% 48.3% 58.4% ¹ Successful nests were those with at least one egg that was confirmed to have hatched. ² Fledged chicks were those chicks observed to have survived to 1 days of age or more. 28

33 Alcatraz Island California Foghorn Recreation Yard Alcatraz Island Power Plant Wall Shower (No nests since 2) (No nests since 1998) Warden's House Auxiliary Dock Greenhouse 2 Meter s Tunnel Rubble West Dock Bench Bench Agave Rubble Figure 1. Black-crowned night-heron nesting subcolonies on Alcatraz Island, San Francisco Bay, California, during ; Snowy egrets nested only in the Greenhouse subcolony in 29 29

34 4 35 BCNH SNEG 4 35 BCNH SNEG 3 3 Nests Initiated Nests Initiated /11 3/18 3/25 4/1 4/8 4/15 4/22 4/29 5/6 5/13 5/2 5/27 6/3 6/1 6/17 6/24 7/1 Week and Start Date, 26 3/11 3/18 3/25 4/1 4/8 4/15 4/22 4/29 5/6 5/13 5/2 5/27 6/3 6/1 6/17 6/24 7/1 Week and Start Date, BCNH SNEG 4 35 BCNH SNEG 3 3 Nests Initiated Nests Initiated /11 3/18 3/25 4/1 4/8 4/15 4/22 4/29 5/6 5/13 5/2 5/27 6/3 6/1 6/17 6/24 7/1 Week and Start Date, 28 3/11 3/18 3/25 4/1 4/8 4/15 4/22 4/29 5/6 5/13 5/2 5/27 6/3 6/1 6/17 6/24 7/1 Week and Start Date, 29 Figure 2. Comparison of black-crowned night-heron (BCNH) and snowy egret (SNEG) nest initiations at Alcatraz Island during