Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 2010

Transcription

1 Reproductive Success of Black-crowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, Annual Data Summary Prepared for: Golden Gate National Recreation Area, National Park Service U.S. Department of the Interior U.S. Geological Survey Western Ecological Research Center

2 Reproductive Success of Black-crowned Night- Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 21 By Roger L. Hothem, Brianne E. Brussee, and Peter S. Coates U.S. GEOLOGICAL SURVEY WESTERN ECOLOGICAL RESEARCH CENTER 21 Annual Data Summary Prepared for: Golden Gate National Recreation Area National Park Service San Francisco, California Dixon Field Station USGS Western Ecological Research Center 6924 Tremont Road, Dixon, CA 9562 Sacramento, California 211 i

3 U.S. DEPARTMENT OF THE INTERIOR KEN SALAZAR, SECRETARY U.S. GEOLOGICAL SURVEY Marcia McNutt, Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information, contact: Center Director Western Ecological Research Center U.S. Geological Survey 32 State University Drive East Modoc Hall, Room 36 Sacramento, CA Cover photograph: Black-crowned Night-Herons (P. S. Coates) Recommended citation: Hothem, R.L., B.E. Brussee, and P.S. Coates Reproductive Success of Blackcrowned Night-Herons and Snowy Egrets at Alcatraz Island, San Francisco Bay, California, 21. Annual Data Summary. Prepared for Golden Gate National Recreation Area, National Park Service, San Francisco, California ii

4 Contents Abstract... 1 Introduction... 2 Methods... 2 Study Area... 2 Nest Monitoring... 2 Camera Monitoring of Night-Heron Nests... 3 Estimates of Reproductive Success... 4 Results and Discussion... 5 Black-crowned Night-Herons... 5 Nest Monitoring... 5 Area Trends... 6 Nesting Habitat... 6 Reproductive Success... 7 Camera Monitoring... 8 Snowy Egrets... 9 Nest Monitoring... 9 Nesting Habitat... 1 Reproductive Success... 1 San Francisco Bay Trends Conclusions Acknowledgements References Cited iii

5 Tables Table 1. Black-crowned night-heron reproductive chronology, Alcatraz Island, Table 2. Number and percentages of black-crowned night-heron nests per subcolony, Alcatraz Island, Table 3. Estimates of clutch size and percentage nesting success (and 95% CI) of black-crowned night-herons, 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. Fates of black-crowned night-heron nests monitored with micro-cameras on Alcatraz Island, Table 9. Estimates of clutch size and percentage nesting success (95% CI) of the snowy egret at Alcatraz Island, Table 1. Fates of snowy egret nests at Alcatraz Island, Table 11. Egg and chick survival of the snowy egret at Alcatraz Island, Figures Figure 1. Black-crowned night-heron nesting subcolonies on Alcatraz Island, San Francisco Bay, California, during Stars indicate locations of DVRs used to record night-heron activities iv

6 Figure 2. Comparison of black-crowned night-heron (BCNH) and snowy egret (SNEG) nest initiations at Alcatraz Island during Figure 3. Comparison of black-crowned night-heron (BCNH) and snowy egret (SNEG) nest initiations at Alcatraz Island during Figure 4. Weekly nest initiation dates by year for black-crowned night-heron nests found at Alcatraz Island, during weeks beginning 11 March 22 July, Figure 5. Range of initiation dates, median initiation dates, and numbers of nests monitored for blackcrowned night-herons at Alcatraz Island, (*Observations in 1992, 1996 and 1997 were terminated 4 6 weeks early because of excessive interspecific conflict between night-herons and Western gulls) Figure 6. 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 7. 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); and South Coast Area (Auxiliary dock, Dock, Bench, Rubble West, and Tunnel) (See Fig. 1) Figure 8. Nesting habitat used by black-crowned night-herons at Alcatraz Island, Figure 9. Mean clutch size (95% CI) for Black-crowned Night-Herons at Alcatraz Island, (Overall mean is shown by dashed line). See Table 3 for significant differences Figure 1. Mean incubation-period success (95% CI) for Black-crowned Night-Herons at Alcatraz Island, (Overall mean shown by dashed line)... 4 Figure 11. Mean nestling-period success and (95% CI) for Black-crowned Night-Herons at Alcatraz Island, (Overall mean shown by dashed line) v

7 Figure 12. Trends of gull (Acosta et al. 26 and S. Acosta, pers. comm.) and black-crowned nightheron nests on Alcatraz Island, Gulls were primarily Western gull, but totals include 5 (24), 14 (25), 21 (26), 23 (27), 21 (28), 14 (29), and 66 (21) California gull nests vi

8 Abstract As part of a monitoring program initiated in 199, we documented nesting chronology, habitat use, subcolony use, hatchability, incubation-period success, and nestling-period success for black-crowned night-herons (Nycticorax nycticorax) and snowy egrets (Egretta thula) at Alcatraz Island, San Francisco Bay, California. In 21, we monitored 196 night-heron nests, a 37% increase from the 143 nests found in 29 and 94 snowy egret nests, an increase from the 85 nests found in 29. As in the previous 6 years, all snowy egrets nested exclusively in the Greenhouse subcolony; night-herons nested in 11 other subcolonies. Black-crowned night-heron incubation-period success in 21 was 38%, nestling-period success was 76%, and overall reproductive success was 29%, all increases from 29, but all lower than the 21-year averages. Incubation-period success, nestling success, and overall success were all 1% for snowy egrets, but rainfall and earlier than expected nesting restricted visits to the snowy egret subcolony, and, as a result, the fates of more nests were unknown. Predation on snowy egret nests was not observed in 21 but could have occurred after monitoring ceased. The percentage of nightheron nests destroyed by predators in 21 (32%) was similar to the average observed over the past 13 years (3%). To better document nest fate, 21 night-heron nests were monitored using micro-cameras and digital video recorders (DVR). The cameras revealed that ravens preyed on more nests (both eggs and chicks) than previously assumed. The night-heron colony on Alcatraz was likely the largest in the North San Francisco Bay, and the total number was about 2.5 times more than were observed in all of South San Francisco Bay in 21. The 94 snowy egret nests at Alcatraz in 21 ranked second to West Marin Island s 175 nests in North San Francisco Bay, but the total was 3 more than were counted at all the sites surveyed in South San Francisco Bay. 1

9 Introduction We monitored reproduction by black-crowned night-herons and snowy egrets on Alcatraz Island for the 21 st consecutive year in 21. As in previous years, the primary objectives of this study were to: 1) conduct baseline monitoring to describe and estimate the distribution and abundance of black-crowned 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 nest distribution, abundance, productivity, and effects of disturbance on reproduction; and 4) compare annual and long-term variation and trends in the nesting populations. Largely due to the secretive nature of breeding night-herons, certain aspects of their reproductive behavior are not well known (Hothem et al. 21). In 21, we deployed micro-cameras on selected night-heron nests to assist in the evaluation of night-heron reproductive parameters, especially predation. Methods Study Area The study area is Alcatraz Island (37 o 49 N, 122 o 25 W), a 9.1-ha island in San Francisco Bay that the National Park Service has managed as a National Historic Landmark since 1973 (Howell and Pollak 1991) (Fig. 1). Nest Monitoring Nest searching and monitoring techniques generally followed the methods used in previous years (Hothem and Hatch 24). The first monitoring visit on 26 April was based on timing of recent breeding seasons and the presence of breeding night-herons and snowy egrets on 2

10 the island, as reported by park personnel. Our goal was to prevent colony abandonment by delaying visits to the subcolonies until nesting had become well established but before hatching. We searched previously occupied nesting subcolonies (Fig. 1) and other potential sites for active night-heron and snowy egret nests from 26 April to 26 July (Table 1). Although the Wall subcolony, thought to be abandoned by night-herons since 21 (Table 2), was not checked during the monitoring visits in 21, at least one night-heron chick was seen in nearby vegetation by a Park Service employee late in the season, indicating a possible nest (L. Young, pers. commun.). This subcolony should be checked for nests during future studies. As in all previous years, except 199 and 1993, monitoring was terminated early in certain subcolonies to prevent observer-induced adverse interactions between gulls and the wading birds (Table 1). The final visits were as follows: Dock and Recreation Yard (7 June), Power Plant and Foghorn (22 June), Auxiliary Dock (6 July), Cellhouse, Rubble, and Tunnel (13 July), and Warden s House (21 July). Monitoring in the Rubble West and Bench subcolonies continued up to 26 July, when the last cameras were removed. Monitoring of the snowy egrets at the Greenhouse subcolony was terminated on 1 May, earlier than at the other subcolonies because of the risk that monitoring posed to the large number of egret chicks present in the relatively small subcolony (~95 m 2 ). Early termination of nest monitoring, especially for the snowy egrets, prevented us from compiling a total census of the nests on the island and resulted in a higher percentage of unknown fate for nests that were initiated later in the season. Camera Monitoring of Night-Heron Nests In previous studies, videography has been used successfully to identify predators on greater sage-grouse (Centrocercus urophasianus) (Coates et al. 28). We used a similar procedure to identify predators on night-heron eggs or chicks and to evaluate night-heron nesting 3

11 behavior in 21. Between 2 May and 26 July, we deployed 12 digital video recording (DVR) systems with micro-cameras to monitor randomly chosen active nests in the Rubble, Rubble West, and Bench subcolonies. Micro-cameras (3 11 mm dimensions; EZ Spy Cam; Los Angeles, CA) were mounted.5 1. m from nests on nearby vegetation or attached to a camouflaged iron stake using rebar tie wire. Camera resolution was 48 TV-Lines and consisted of a 3.6-mm board lens. To record night images without disturbing the birds, each camera included 12 infrared light-emitting diodes; the wavelength of the light (95 nm) was invisible to vertebrates. Cameras and cables were disguised using camouflage duct tape and vegetation. Cables extended up to 1 m to a 4-channel H.264 DVR system (AV Tech Corp., Hong Kong, China) housed in a camouflaged casing and hidden under vegetation or beneath burlap. We attached 4 micro-cameras to each of three DVR systems. The DVRs were set to record nests at 1.3 frames per second continuously during all times of day and night to prevent missing predation activities. Extension cords connected the DVRs to a power source at the South Foghorn. The status of each nest was checked by scientists using a hand-held LCD video monitor (screen width, 18 cm) in the field during weekly island visits. We continued video-monitoring until after nest termination when the cameras were moved to nearby active nests, or, if an active nest were not available, the camera was removed. Estimates of Reproductive Success In 21, reproductive success was calculated similar to previous years (Hothem and Hatch 24). We used the Mayfield method (Mayfield 1961, 1975) to calculate incubationperiod and nestling-period success. We also calculated apparent rates of predation and other causes of nest failure. 4

12 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 the 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. Results and Discussion Black-crowned Night-Herons Nest Monitoring In 21, night-herons initiated nests (first egg laid) over a 111-day period, from 17 March to 6 July, 24 days longer than in 29 (Table 1). The first night-heron nest initiation was 3 weeks earlier than in the previous year, and 51% of the night-heron nests already had eggs by the time of the first monitoring visit. It is likely that some nests were initiated and terminated before the first visit. The median initiation date in 21 (24 April) was 11 days earlier than the 21-year average, and it was the earliest median initiation date since 1998 (Table 1). Nest initiation patterns are shown in Fig The earliest documented nest initiation occurred 8 days earlier in 21 than the overall average, and the latest initiation was 9 days later, resulting in a breeding period 17 days longer than the long-term (199 21) average (Table 1, Fig. 5). 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. 6). The total number of nests (196) found in 21 was 37% more than in 29 and 23% more than the 21-year average (159). 5

13 Area Trends We arbitrarily assigned each nesting subcolony to one of three geographic areas: the South Coast, the Central Island, or the North Coast (Fig. 7; Table 2). With 54 more nests in the South Coast subcolonies in 21 than in 29, the percentage of the total found in the South Coast area of the island increased from 88% in 29 to 92% in 21 (Fig. 7; Table 2), the highest observed during the 21-year study. Only four night-heron nests were found in the subcolonies of the Central Island area (2% of the island total), similar to the totals observed in 28 and 29 (Fig. 7; Table 2). Most of this decline was related to the abandonment of the Greenhouse subcolony in 28. Night-heron nest numbers in the North Coast area have been relatively stable for the past 11 years; 11 nests were found in 21 (Fig. 7; Table 2). This was similar to the average (11.4 nests) observed during 2 21, but it was about one-half the average for the previous 1 years (2. nests). Nesting Habitat In 21, 25.5% of the night-heron nests were in English ivy (Hedera helix), 24.% were in century plant (Agave americana), and 16.8% were in rubble, which consisted of the remains of demolished buildings (i.e., concrete, reinforcing rods, metal, scrap wood, and miscellaneous debris) (Fig. 8). In the 199s, night-heron nests were most often found in mirrorbush (Coprosma baueri), but in 21 only 1.7% were found in that habitat. Although we have not measured habitat availability during this study, it has been clear that the abundance of mirrorbush, especially in the Foghorn and Dock subcolonies, has declined significantly, while English ivy has appeared to increase, often colonizing former mirrorbush areas. 6

14 Reproductive Success We estimated that the mean clutch size for black-crowned night-herons in 21 was 2.62 eggs per nesting attempt, the lowest observed in the course of the study (Table 3, Fig. 9) and significantly lower (P <.5) than 6 of the previous years (199, 1992, 1993, 1997, 22, and 26). This low clutch size may have been related to the number of nests that were destroyed by predators, because replacement clutches tend to be smaller (Wolford and Boag 1971). The estimated incubation-period success (percentage of nests that hatched one egg) and 95% confidence interval (CI) for night-herons during 21 was 38% (29 49%), which was lower than the study average (55%), but an increase from the low (24%) observed in 29 (Table 3, Fig. 1). The estimated nestling-period success (percentage of nests in which one nestling survived to 15 days) and 95% CI for night-herons during 21 was 76% (68 85%), lower than the average for the 21-year study (86%) (Fig. 11). The overall success (combined incubation and nestling period success) of nesting night-herons on Alcatraz in 21 was 29% (Table 3). In 21, we observed that in successful nests that were monitored, 266 of the 27 eggs (98.5%) hatched (Table 4), compared with the overall study average of 94.6%. Of the 266 hatched eggs in 122 nests, we observed that 138 chicks (51.9%) in 76 nests survived to at least 15 days. Night-heron production averaged.7 fledglings per monitored nest (i.e., nesting attempt) or 1.13 chicks fledged per hatched nest (Table 4). Both values were similar to the 21- year means. 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 21 (See Camera Monitoring below and Table 5). Based on apparent nest success, we estimated that eggs in 32% of the monitored nests were destroyed by predators before 7

15 hatching (Table 5), higher than the average for the study (2.9%), but similar to the average predation rate during (29.7%). We also estimated that 15.6% of the successful nests (and 9.7% of all nests monitored) were destroyed by predators during the nestling phase, the highest value observed in the study and more than twice as high as the 21-year average (6.1%). Overall, we estimated that 41.3% of all the nests monitored in 21 were destroyed by predators. Predation was especially heavy in the Rubble subcolony in 21. Of the 51 monitored nests, 23 (45.1%) were destroyed by predators during the incubation period, and 7 (13.7%) were destroyed during the nestling period, for a total of 58.8% destroyed by predators (Table 6). We observed that 24 of the marked nests were re-used after predation or fledging in 21. However, we were unable to confirm whether these were actually replacement clutches or if they were first-time clutches. The incubation-period abandonment rate in 21 was 3%, which is similar to the 21-year average of 3.5%. No nest was considered to be abandoned during the nestling period in 21. The percentage of nests with unknown fates, which included those that were either lost or which were not rechecked for any reason, was 3% during incubation and 19% during the nestling period, compared with the 21-year average of 19.3% and 3.3%, respectively (Table 7). Camera Monitoring We monitored 22 black-crowned night-heron nests during 2 May through 26 July in the Rubble West (6), the Bench (15), and the Rubble (1) subcolonies. Cameras were installed during the incubation period at 19 nests. Eggs in 12 of those hatched, while eggs in six were destroyed by predators before hatching, and one nest was abandoned when found. Resident common ravens were documented to have preyed on eggs at five nests, while a night-heron preyed on eggs at one nest. Chicks in 8 of the 12 nests were observed to survive to fledging (15 days), but chicks in two 8

16 of those nests were subsequently killed by ravens. Chicks in two of the nests were killed by predators (one by a night-heron and one by a raven) before they reached 15 days, the chick in one nest died at 5 days of unknown causes, and chicks in one nest survived to 12 days of age when the camera was removed. Cameras were placed on three nests after the chicks had hatched. Chicks in two of the three nests survived to at least 15 days, while chicks in the third nest were destroyed by a night-heron at about 1 days of age (Table 8). Video evidence revealed that common ravens both consumed eggs on the nest and removed eggs from the nest, presumably for later consumption or for feeding to their young (Kelly et al. 25). After nests were depredated, shells left in the nest were often removed by the night-herons. Thus, the presence or absence of predated eggshells in or near the nest may be insufficient evidence for predator identification. Additional video-monitoring studies could improve our understanding of night-heron nesting ecology, thus assisting park managers in determining appropriate strategies to improve night-heron reproductive success. Snowy Egrets Nest Monitoring The first observed occurrence of snowy egret nesting on Alcatraz Island during this study was three nests in the Tunnel subcolony in Since that time, we have monitored a total of 491 snowy egret nests on Alcatraz Island, with annual totals ranging from none in 22 to 94 in 21 (Fig. 6). Since 24, all snowy egret nests on Alcatraz have been located in the Greenhouse subcolony. During 24 27, both snowy egrets and night-herons nested in the Greenhouse, but, starting in 28, only egrets have been found in that colony. Based on linear regression, during the period 24 21, the numbers of snowy egret nests increased significantly in the 9

17 Greenhouse subcolony (r 2 =.863, F = 31.43; P =.2), while the numbers of night-heron nests in the same subcolony decreased significantly (r 2 =.724, F = 13.1; P =.15). In 21, snowy egret nests were initiated over a 42-day period between 29 March and 1 May (Fig. 2), but because nesting was initiated about 2 weeks earlier than in 29, 72% of the snowy egret nests monitored in 21 had eggs in their nests by the first monitoring visit. The median initiation date for egrets in 21 was 12 April, 12 days earlier than that observed for the night-herons. The earlier nesting by the egrets, especially in 28 and 29 (Fig. 3) may have played a role in preventing night-herons from nesting in the Greenhouse subcolony. Nesting Habitat Of the 94 snowy egret nests monitored in 21, 43% were in plume albizia, 26% were in blackberry, and 23% were in fig trees (Ficus carica). The remaining nests were found in Viburnum spp. (6%), bear s breeches (Acanthus mollis) (2%), and mirrorbush (1%). Reproductive Success The mean clutch size for snowy egrets in 21 was 3.16 eggs per nest (Table 9), significantly lower than the means in 25, 28, and 29 (One-way repeated measures ANOVA and Holm-Sidak method for pairwise multiple comparisons). However, the average clutch size for all snowy egret nests during (3.34) was significantly higher than that for night-herons for the same period (2.79) (Mann-Whitney Rank Sum Test; P <.1). Note that the mean clutch sizes reported for snowy egrets in previous progress reports were recalculated for this report to match criteria used for night-herons. As with the night-herons, we used the Mayfield method to calculate incubation-period, nestling-period, and overall success. Incubation-period, nestling-period, and overall success were 1

18 all 1% in 21 (Table 9). In 21, however, the presence of 94 simultaneous snowy egret nests with as many as 145 chicks, 66 of which were 1 days old or more, in a relatively small area (about 81 m 2 ) made unobtrusive monitoring difficult. In addition, the fourth scheduled visit to the subcolony (17 May) was cancelled because rainfall made monitoring unsafe for the survival of eggs and chicks. Therefore, to prevent observer-caused abandonment and egg and chick mortality, our last monitoring visit to the egret subcolony was 1 May. This earlier-thannormal termination of nest monitoring resulted in our inability to determine the fates of a high percentage of the nests, and the incubation-period fates of 43% of the nests and the nestlingperiod fates of 59% of the nests were unknown (Table 1). Incubation-period and overall success were higher than those calculated for the previous six breeding seasons (Table 9), and 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 San Francisco Bay area, lower than that observed at Alcatraz in 25 21, but similar to that observed in 24 (Table 9). In previous years, predation on snowy egret eggs and chicks was low, ranging from % in 25 to 7% in 28 and 29. However, in 21, neither chicks nor eggs in any of the 94 monitored nests were observed to be destroyed by predators. As described for the night-herons previously, at least one pair of common ravens, a species known to prey on snowy egret eggs at nearby (16 km north) West Marin Island (Kelly et al. 25), has been present on Alcatraz since at least It is unclear why predation rates on snowy egrets on Alcatraz have been so consistently low. 11

19 No snowy egret nest was considered abandoned in either the incubation period or nestling period in 21. Since 24, the average abandonment rate during the incubation period has been 1.1%, while the nestling period rate has been 1.3% Eggs hatched in 54 of the 94 egret nests (57%) in 21 (Table 11). Six of the 151 eggs observed to the hatching date failed. Hatchability (96.%) of snowy egret eggs was slightly lower than that observed in 29 (97.3%), but higher than the 7-year average (94.5%) (Table 1). Egret egg hatchability in 21 was lower than night-heron hatchability (98.%), but higher than the average night-heron hatchability for the 2-year study (94.3%) (Table 4). Of the 145 egret eggs that hatched, we observed that 66 chicks (45.5%) survived to at least 1 days (1.22 chicks fledged per successful nest;.7 chicks fledged per monitored nest), and two chicks were found dead (Table 11). San Francisco Bay Trends The total numbers of night-heron nests (absolute numbers) at Alcatraz were 37% higher in 21 than 29. 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 decreased by about 17%. The peak number of night-heron nests on Alcatraz was 23.7% of the North Bay total in 21 compared with 9.7% of the North Bay total in 29. The percentage of night-heron nests found at Alcatraz compared with the Central Bay area colonies (West Marin, Yerba Buena, Red Rock, Brooks, and Alcatraz islands) was 4.9% in 21 compared with 17.4% in 29. This increase in the percentage at Alcatraz can be attributed primarily to the 51% decline in estimated numbers of night-heron nests on West Marin Island and the 13% increase at Alcatraz in

20 Tokatlian et al. (21) reported that 8 night-heron nests were counted in South San Francisco Bay by the San Francisco Bay Bird Observatory (SFBBO), a total similar to those in 28 and 29, but fewer than the 133 counted in 27. Lake Cunningham in San Jose had 24 nests, the most in the South Bay in 21. Counts by SFBBO were conservative because they reported the peak number of nests and only estimated night-heron nest numbers at colonies that also had other species of interest (i.e., great blue herons (Ardea herodias), great egrets (Ardea alba), or snowy egrets). 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 have increased each year since 27, increasing from 79 to 94. Based on data from Audubon Canyon Ranch (E. Condeso, pers. commun.), the total peak numbers of active nests (including 87 at Alcatraz) at all North San Francisco Bay colonies were similar in 29 and 21. Nests on Alcatraz comprised 21.% of the North Bay population in 29 and 23.6% in 21. 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) increased from 3.6% in 29 to 46.% in 21, largely because of a decline in snowy egret nests at West Marin Island from 175 in 29 to 12 in 21. Snowy egret nests have declined in the South Bay from 272 nests in 25 to 64 in 21, fewer than were monitored at Alcatraz in 21 (Tokatlian et al. 21). Conclusions During 21, we documented reproduction by black-crowned night-herons and snowy egrets at Alcatraz Island for the 21 st consecutive year. In 21, 196 night-heron nests were monitored, an increase from the 143 nests monitored in 29. Snowy egret nesting in 21 was similar to 28 and 29, but reached a study high of 94 nests. As in 29, incubation-period, 13

21 nestling-period, and overall success for night-herons were less than the overall averages for the study. All categories of reproductive success were higher for snowy egrets than the night-herons in 21, and snowy egret success rates were higher than those previously observed in the study. We assumed, based on limited evidence, that predation was the most significant limiting factor for night-herons nesting on Alcatraz, especially during the 12 years prior to 21. In previous years, we have attributed much of that predation to Western gulls (Hothem and Hatch 24). We based this conclusion on the observed significant increase in the number of gull nests (linear regression: r 2 =.767; F=63.44, P<.1) on the island from 199 to 21 (Fig. 12) (S. Acosta, pers. commun.), combined with the significant increase in predation on night-heron nests during the same period (linear regression: r 2 =.68; F=31.98, P<.1). In addition, the correlation between total gull nests and percentage predation on night-heron nests was also significant (linear regression: r 2 =.562; F=25.42, P<.1). Previously, we have observed gulls aggression toward night-herons, occasionally attacking and sometimes killing night-heron chicks, but we had never actually observed gull predation on eggs. In 21, however, we used video cameras to try to confirm our assumptions about predation. Based on the results of monitoring 11.2% of the nests with micro-cameras, it is clear that common ravens as well as night-herons, were predators on both night-heron eggs and chicks, at least in the Bench subcolony in 21. No video evidence of gull predation on eggs or chicks in the nests was recorded in 21. Park records (L. Young, pers. commun.) and those of PRBO Conservation Science (S. Acosta, pers. commun. 211) indicate that common ravens first were observed on Alcatraz in 1997 with nesting first observed in The increase from 3% predation on night-heron nests in 1997 to 22% in 1998 and as high as 46% in succeeding years may have been related to the 14

22 presence of nesting ravens on the island. Further study using videography in other nesting subcolonies should help to clarify the roles that corvids and gulls may be playing in predation on night-heron nests. Although predation has been a limiting factor for snowy egrets at other nesting sites within San Francisco Bay (Kelly et al. 25), predation on snowy egrets, which averaged 4.1% during 24 21, has not been found to be significant at Alcatraz. The use of video cameras would be helpful for the detection of otherwise undocumented predation and also to document possible anti-predatory behavior by the egrets. Habitat changes are a major concern on Alcatraz. Mirrorbush has declined 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. The number of night-heron nests located in the Rubble subcolony increased from 26 in 29 to 51 in 21, a 96% increase. We previously hypothesized that the density of the vegetation in the Rubble subcolony might be deterring night-herons from nesting there. However, apparently vegetation density has neither deterred night-herons from nesting in the Rubble subcolony, nor has it prevented predators from destroying 88% of the nests in 29 and 43% of the nests in 21. The Greenhouse subcolony, a relatively small area located adjacent to a walkway open to year-around visitor use, was once home to a significant population of breeding night-herons, totaling 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 San Francisco Bay area. The visual and physical barrier presented by the dense vegetation at this subcolony (fig, rose, Albizia) 15

23 appears to present a barrier sufficient to prevent human-caused disturbance of the nesting egrets 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 documented nesting in the dense century plants in the Agave portion of the Bench subcolony (Fig. 1), and we monitored 54 nests, 27.6% of the island s total, in this area in 21. The retention of the black-crowned night-heron as a breeding species on Alcatraz Island will require preservation of the Bench and the other South Coast subcolonies where 92% of the island s night-herons bred in 21. The prevention of undue disturbances that might reduce reproductive success or even cause the birds to abandon the site should be a high priority. Alcatraz is a significant breeding site for both night-herons and snowy egrets in San Francisco Bay, with nest numbers exceeding 1 in all but one of the past 21 years for the nightherons and approaching 1 for the snowy egrets. Because the breeding populations of both night-herons and snowy egrets at Alcatraz Island have grown to comprise > 2% of the Bay area s totals, wise long-term management actions at Alcatraz for both species are likely essential to the vitality and stability of these populations within the San Francisco Bay area. Acknowledgements We thank Lara Rachowicz, Rachel Townsend, Suzanne Byron, and Laura Young of the National Park Service for their invaluable assistance with logistics and field work. Assistance with video monitoring of night-heron nests was provided by Zachary Lockyer and Michael Casazza. Sara Acosta of PRBO Conservation Science provided data on gulls nesting on Alcatraz. John Kelly and Emiko Condeso of the Cypress Grove Research Center of Audubon Canyon Ranch provided information on North San Francisco Bay breeding night-herons and snowy 16

24 egrets, and Caitlin Nilsin of the San Francisco Bay Bird Observatory provided information on nesting by night-herons and snowy egrets in the South Bay. We thank the Golden Gate National Parks Conservancy for their generous support of this study. References Cited Blus, L J., Rattner, B.A., Melancon, M.J., and Henny, C.J Reproduction of Blackcrowned Night-Herons related to predation and contaminants in Oregon and Washington, USA, Colonial Waterbirds, 2: Coates, P. S., J. W. Connelly, and D. J. Delehanty. 28. Predators of Greater Sage-Grouse nests identified by video monitoring. Journal of Field Ornithology 79: Henny, C.J., Blus, L.J., Krynitsky, A.J., and Bunck, C.M Current impact of DDE on Black-crowned night-herons in the intermountain west. Journal of Wildlife Management 48:1 13. Hensler, G.L., and Nichols, J.D The Mayfield method of estimating nesting success: a model, estimators and simulation results. Wilson Bulletin 93: Hothem, Roger L., Brianne E. Brussee and William E. Davis, Jr. 21. Black-crowned Night- Heron (Nycticorax nycticorax), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: Hothem, R.L., and Hatch, D. 24. Reproductive success of the black-crowned night-heron at Alcatraz Island, San Francisco Bay, California, Waterbirds 27: Howell, J.A. and Pollak, T Wildlife habitat analysis for Alcatraz Island, Golden Gate 17

25 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, MD. Kelly, J.P., K.L. Etienne, and J.E. Roth. 25. Factors influencing the nest predatory behaviors of common ravens in heronries. The Condor 17: 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 Nest success calculated from exposure, Wilson Bulletin 73: Mayfield, H. F Suggestions for calculating nest success. Wilson Bulletin 87: Tokatlian, K., C. Robinson-Nilsen, and J. Bluso Demers. 21. Colonial waterbird nesting summary for San Francisco Bay, 21. Report prepared for Don Edwards San Francisco Bay National Wildlife Refuge and California Department of Fish and Game, 12 pp. Tremblay, J., and Ellison, L.N Breeding success of the Black-crowned night-heron in the St. Lawrence Estuary. Canadian Journal of Zoology 58: Wolford, J.W. and Boag, D.A Distribution and biology of Black-crowned night-herons in Alberta. Canadian Field-Naturalist 85:

26 Table 1. Black-crowned night-heron reproductive chronology, Alcatraz Island, Median Range of Colony Year Initiation Date Initiation Dates 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 Apr 17 March 6 July April 26 July Mean 5-May 25 March 27 June April 19 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

27 Table 2. Number and percentages of black-crowned night-heron nests per subcolony, Alcatraz Island, SUBCOLONY S. COAST % of total 39% 46% 54% 48% 58% 65% 71% 83% 8% 83% 87% 75% Tunnel Bench Rubble Rubble W Dock Aux. Dock CENTRAL % of total 43% 39% 34% 39% 31% 29% 21% 1% 8% 5% 6% 7% Cell house Greenhouse Rec. Yard Wall Shower Warden 2 4 N. COAST % of total 18% 15% 13% 13% 11% 7% 8% 7% 12% 12% 7% 18% Foghorn Power Plant Total

28 Table 2 (continued). Number and percentages of black-crowned night-heron nests per subcolony, Alcatraz Island, SUBCOLONY Mean Totals S. COAST % of total 82% 44% 5% 47% 72% 75% 83% 88% 92% 67.7% Tunnel Bench Rubble Rubble W Dock Aux. Dock CENTRAL % of total 1% 46% 38% 39% 21% 2% 3% 2% 2% 21.5% Cell house Greenhouse Rec. Yard Wall Shower.9 19 Warden N. COAST % of total 9% 11% 12% 14% 7% 4% 14% 9% 6% 1.7% Foghorn Power Plant Total Numbers of nests estimated in 1991 and 1992 based on numbers monitored in 199 and in

29 Table 3. Estimates of clutch size and percentage nesting success (and 95% CI) of blackcrowned night-herons, Alcatraz Island, Nest success Year Clutch size found nests 3 days period 4 period 5 Overall 6 2 Nests Mayfield Exposure Incubation Nestling D (6-8) (84-95) ABCD (52-76) (72-92) CD (61-1) (77-99) BCD (66-89) (96-1) ABCD (7-93) (88-1) ABCD (66-88) (86-98) ABCD (56-78) (71-87) BCD (53-74) (64-8) ABCD (37-54) (7-86) ABCD (47-66) (85-98) ABCD (39-64) (77-98) 21 3 ABCD (29-62) (85-1) BCD (43-64) (86-1) ABCD (39-6) (82-98) ABCD (32-55) ABCD (29-55) (69-94) BCD (44-68) (72-96) ABC (22-38) (75-93) AB (28-54) (79-1) ABCD (17-34) (67-95) A (29-49) (68-85) Mean Based on days of nest exposure (see Hensler and Nichols 1981). 2 Mean clutch sizes sharing capital letters are not significantly different based on One-way analysis of variance and Holm- Sidak multiple comparison procedures (P >.5). 3 Nests included in the analysis (Mayfield 1961, 1975). 4 Predicted percent of monitored nests in which one or more eggs hatched. 5 Predicted percent of nests that hatched in which one or more chicks reached at least 15 days of age. 6 Predicted percent of monitored nests in which one or more chicks reached at least 15 days of age, calculated by multiplying Incubation-period success by Nestling-period success. 22

30 Table 4. Egg and chick survival of the black-crowned night-heron at Alcatraz Island, Year Total Mean Monitored nests , Successful nests , Eggs observed to hatching (A) , Failed eggs (B) % Hatchability ((A-B)/A) Hatched eggs/ monitored nest Hatched eggs/ successful nest Nests with fledged chicks , Fledged chicks , 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. 23

31 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. 24

32 Table 6. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, 21. Nests per subcolony 1 (% of island total) AUX BEN CH DOK FH PP REC RUB RW TUN WAR TOTAL 4 (2.) 54 (27.6) 1 (.5) 5 (2.6) 2 (1.) 9 (4.6) 1 (.5) 51 (26.) 3 (15.3) 37 (18.9) 2 (1.) 196 (1.) Nest fate per monitored nest % predation % destroyed % abandoned % unknown No. hatched nests (% of subcolony total) 2 (5.) 36 (66.7) 1 (1.) 3 (6.) 2 (1.) 9 (1.) (.) 27 (52.9) 21 (7.) 2 (54.1) 1 (5.) 122 (62.2) Fate per hatched nest % predation % destroyed % abandoned % unknown No. fledged nests (% of hatched nests) 2 (1.) 22 (66.1) 1 (1.) 2 (66.7) 1 (5.) 8 (88.9) (.) 14 (51.9) 13 (61.9) 12 (6.) 1 (1.) 76 (62.3) % fledged nests per subcolony total GH, SH, and WAL not included; no nesting by black-crowned night-herons observed in

33 Table 7. Fates of monitored nests per subcolony for black-crowned night-herons, Alcatraz Island, Nests per subcolony (% of island total) AUX BEN CH DOK FH GH PP REC RUB RW SH TUN WAL WAR TOTAL 22 (6.) 283 (8.5) 19 (.6) 193 (5.8) 181 (5.4) 366 (1.9) Nest fate per monitored nest % predation % destroyed % abandoned % unknown (4.8) 8 (2.4) 72 (21.5) 347 (1.4) 19 (.6) 619 (18.5) 19 (3.3) 44 (1.3) 3344 (1.) No. hatched nests (% of subcolony total) Fate per hatched nest 141 (69.8) 153 (54.3) 16 (84.2) 129 (66.8) 11 (6.8) 21 (57.4) 113 (69.8) % predation % destroyed % abandoned % unknown (6.) 363 (5.4) 16 (46.1) 16 (84.2) 338 (54.6) 75 (68.8) 35 (79.5) 197 (57.) No. fledged nests (% of hatched nests) 93 (66.) 87 (56.9) 1 (62.5) 96 (74.4) 63 (57.3) 116 (55.2) 87 (77.) % fledged nests per subcolony total (77.1) 176 (48.5) 81 (5.6) 13 (81.3) 188 (55.6) 58 (77.3) 32 (91.4) 1137 (59.6) 26

34 Table 8. Fates of black-crowned night-heron nests monitored with micro-cameras on Alcatraz Island, 21. Nest number Subcolony Camera installed Hatch date Predation on eggs (date and predator) Predation on chicks (date and predator) Days chick(s) observed to survive Date chick(s) last seen alive Notes 164 RW 5/2/21 6/1 6/15 dead chick unknown cause 26 RW 5/2/21 5/ / RW 5/2/21 5/ /14 353A RW 5/2/21 6/9 47 7/26 41 BEN 5/21/21 5/25 6/2 by night-heron night-heron killed chicks; raven ate failed egg 456 BEN 5/21/21 5/28 6/18 by raven 458 BEN 5/21/21 6/1 26 7/6 46 BEN 5/21/21 5/22 by raven 467 BEN 5/21/21 6/3 by night-heron 471 BEN 5/21/21 6/2 6/18 by raven 218A BEN 5/21/21 5/28 by raven 223A BEN 5/21/21 6/8 35 7/ BEN 5/28/21 6/5 43 7/18 39A BEN 5/28/21 6/1 35 7/6 232A RUB 6/7/21 6/5 6/15 by night-heron 184 RW 6/14/21 6/ /26 224A RW 6/14/21 Nest abandoned when found 41A BEN 6/14/21 6/22 by raven 179 BEN 6/22/21 7/2 by raven 19 BEN 6/22/21 7/2 by raven 211A BEN 6/22/21 7/ /26 Chick alive when camera removed 191 BEN 6/28/21 7/13 7/24 by raven 27

35 Table 9. Estimates of clutch size and percentage nesting success (95% CI) of the snowy egret at Alcatraz Island, Nest Success Year Clutch Size Nests Found Mayfield Nests 1 Exposure days 2 Hatching 3 Fledging 4 Overall (34 1.1) (68 1.) (76 1.1) (1 1) (82 1.) (1 1) (8 99) (95 1) (77 96) (1 1) (59 95) (1 1) (1 1) (1 1) Mean 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. 2 See Hensler and Nichols (1981). 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. 28

36 Table 1. 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. 29

37 Table 11. 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% 96.% 94.5% 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% 45.5% 55.1% 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. 3

38 Alcatraz Island California Figure 1. Black-crowned night-heron nesting subcolonies on Alcatraz Island, San Francisco Bay, California, during Stars indicate locations of DVRs used to record night-heron activities. 31