Colony Stability of Cave-Nesting Australian Swiftlets in Queensland: What Are the Impacts of Severe Weather Events?

Australian Field Ornithology 2013, 30, 131–151

Colony stability of cave-nesting Australian Swiftlets in Queensland: What are the impacts of severe weather events?

Michael K. Tarburton and Shirley R. Tarburton

School of Science and Technology, Pacific Adventist University, Private Mail Bag Boroko, National Capital District, Papua New Guinea (Present address: 3 Freda Court, Blackburn South VIC 3130, Australia) Email: [email protected]

Summary. A study of 31 Australian Swiftlet Aerodramus terraereginae colonies at Chillagoe, Queensland, over 28 years and a study of two colonies at Finch Hatton Gorge near Mackay, Qld, over 25 years have shown that some colonies are more vulnerable than others to extinction from severe weather events. Flooding and strong cyclones can also change breeding practices, forcing Swiftlets to use different chambers or caves, or even change the time of breeding. Extreme weather can cause the extinction of exposed colonies and severe reduction of protected colonies.

Introduction Nesting in caves has its advantages for swifts, just as nesting in tree-hollows has for birds in general. These include lower predation (e.g. Tarburton 2009) and lower losses because of high winds. But do these advantages always convert into reality? It is possible that clustered nesting may make at least some populations of swifts vulnerable to local weather events that could lead to local extinction. The Australian Swiftlet Aerodramus terraereginae, like all swiftlets studied to date, has a slow reproductive rate (e.g. clutch of one twice a year compared with a clutch of four twice a year for the Welcome Swallow Hirundo neoxena), long incubation period (27–28 days compared with 16 days for the Welcome Swallow), and long nestling period for its size (46–51 days compared with 19 days for the Welcome Swallow) (Tarburton 1988a, 1993). This means that replacement of losses caused by catastrophic events is slower than for many other bird species including most passerines. The two eggs that are laid each year will be increased to three only if one of the first two eggs or young nestlings is lost (Tarburton 1988a). In late 2010 and into early 2011, several cavers from the Chillagoe Caving Club (CCC) reported that all the caves entered in the Chillagoe–Mungana district of Queensland had been flooded or had deep water in them, the result of La Niña summer rains. This district is where most known breeding colonies of Australian Swiftlets are located (Table 1, Figure 1), so a census of as many colonies there as possible was essential to determine the effect of such weather events on the total Swiftlet population. In addition, it was hoped that a study of the effects of changed rainfall on this Queensland species would help to better understand the effects of increased rainfall on other species of swiftlets. 132 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

Figure 1. Locations of known breeding colonies of Australian Swiftlets in Queensland.

Table 1. Number of known, recently active Australian Swiftlet colonies in Queensland.

District Number of recent colonies Chillagoe–Mungana 42 Mitchell–Palmer 15 Cairns–Cooktown 7 Family Islands 5 Tully 4 Broken River 4 Innisfail 4 Mackay 2

Methods Visits to eight Australian Swiftlet colonies in the Chillagoe–Mungana district in April 2011 confirmed flooding in the caves, and prompted the need for a population-wide survey (Tarburton 2011a). During November and December 2011, as many colonies as possible Vulnerability of Australian Swiftlet colonies 133 were visited to determine how many of them had been affected and how permanent the destruction might be in the colonies that had been eliminated or reduced. The 28 colonies visited were all those listed for 2011 in Tables 2–4. CCC members accompanying us to one or more of these caves were K. Offer, D. Mitchell, A. Slee, V. Christensen, D. Irvin, C. Head and B. Dane. Either F. Pittorino or L. Parnell (Queensland Parks and Wildlife Service staff) accompanied visits to all caves in national parks. By June 2011 it had become clear that, in the period between December 2007 and March 2011, Queensland’s Australian Swiftlet populations had been heavily impacted, and it was decided to try censusing colonies in the non-breeding season by a less intrusive method that did not involve entering caves or counting nests. The new technique was to be at the entrances of the caves where Swiftlets breed and sleep—either before first light, to count birds leaving the roost for the day-long feeding trip, or from 1600 h to count them entering the cave for their evening sleep. Although this method is likely to reduce negative impacts on breeding birds, there were several difficulties to be overcome. A maximum of only two cave colonies per day could be censused, whereas up to five colonies per day could be censused by counting nests. Some colonies could be censused only in the evening, because Swiftlets exited from up to five cave openings (which could not be simultaneously watched by just two observers), but entered through only one or two entrances. In addition, as exiting birds flew low along the contours in many directions, they were more difficult to detect than when entering the cave. Another limitation is that censusing at cave entrances instead of counting nests is accurate only during the non-breeding season as an unknown number of birds is already inside the cave brooding or feeding young at any given time in the breeding season. Several caves were entered to compare nest counts with entrance counts. All previous censuses had been undertaken in the breeding season and were based on the number of nests in current use, thus indicating the number of breeding pairs. From our experience, one visit in November–January will register both early and late nesters because incubation (27–28 days) and nestling (46–51 days) periods are much longer than for most bird species of the Swiftlet’s size (Tarburton 1988a, 1993). We assume that one census visit made at this time is sufficient to determine if a particular colony is extant and, if so, its size. A follow-up census was organised for the non-breeding season during June and July 2012. It covered the same caves, as well as the few that were missed previously, and was based on the number of birds leaving a cave at daybreak or entering at sunset. The only known colonies in the Chillagoe–Mungana district that were not censused in 2011–2012 were Satu Lagi (which could not be relocated) and Project 31 (because the station owner did not allow visits at that time). The results of these censuses were compared with our censuses since 1985 and with those of other observers. Colony size during the breeding season was determined by counting completed nests in use, as indicated by the sparkle of fresh saliva on them. This method was demonstrated to be reasonably accurate (± 14%) by comparison with two mark- and-recapture methods and counts of sleeping White-rumped Swiftlets Aerodramus spodiopygius in Fiji (Tarburton 1987). During censuses, a search was undertaken from each cave entrance to the nest-site of the colony for signs of predation. If elongated scats (indicating pythons or tree-snakes) or Children’s Pythons Antaresia childreni or Green Tree Snakes Dendrelaphis punctulata were located, then a designation of predation by a reptile was assigned to that colony for that visit. If pile(s) of Australian Swiftlet flight-feathers (remiges) were located, the feathers were counted and the total divided by ten to estimate the approximate number of Swiftlets killed by a mammal (presumed, and in some cases known, to be a cat Felis catus). 134 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

This study was extended from the Chillagoe–Mungana district to monitor historical data from the Queensland coastal and offshore colonies of Australian Swiftlets as well, and to look more closely at any correlations between previous declines in these colonies and heavy rainfall in the locality. Australian Swiftlets on Dunk Island are known to feed on the mainland (Banfield 1925; Higgins 1999), so on 25 December 1988 we stood on the coastal point at South Mission Beach and, with the aid of binoculars, counted the Swiftlets returning between 1622 h and 1900 h to roost on Dunk Island. On 8–9 September 1989, Schulz (1991) visited four active colonies and two former colony sites on Dunk Island, and counted the nests in use. After Cyclone Larry passed over the region on 20 March 2006, Adrian Walker repeated our census procedure at South Mission Beach, four times in February 2007, to collect comparative data. Bedarra Island has also been censused on occasion (Busst 1956; Orrell 1967; Smyth et al. 1980), so available data are compared with those of Walker from a visit after Cyclone Larry. For brevity, breeding seasons (historically October–March: Higgins 1999) are referred to by the year that they start. Declines are measured as percentages from the data in Tables 2–3 and 6. Chillagoe’s monthly rainfall for 1902–1991 was obtained from Vince Kinnear at the Chillagoe Post Office, and for 1992–2012 from the Chillagoe office of the Queensland Parks and Wildlife Service. Finch Hatton rainfall was obtained from the Bureau of Meteorology but, when that station closed in 1991, rainfall data from the Bureau for Mirani Weir (21 km to the south-east) were used. All these data are now available from the Bureau of Meteorology webpage (www.bom.gov.au).

Results and discussion Chillagoe–Mungana colonies The years and population counts for all visits to Australian Swiftlet colonies in the Chillagoe–Mungana district are in Tables 2–4. Locations are shown in Figure 2, and nests, colonies and caves on the front cover and Figures 3–4 and 7–8. Rainfall data indicate that in 1991 Chillagoe received its heaviest rainfall total for any January (1902–2012) (Table 5), and February received twice the average rainfall for that month. In 1989 (our last census before the January–February 1991 flood), the Keef’s Cavern (CH 24) colony had 185 pairs of nesting Swiftlets, but the number had declined 51% to only 91 pairs in 1991 (Table 3). This colony is in a small chamber at the lower end of a rock chute, down which flows rainwater which, in a flood, would be likely to trap any birds roosting in the colony. Keef’s Cavern colony is safe from predators but not from flooding rains. The abandonment of the Tower of London (CH 5) middle breeding site before 1985 (Table 2) was probably because of flooding of the lower part of the chamber where the Swiftlets nested, as the only entrance was low and well below the high- water line on the cave walls. The 20 marks of old nests visible on the wall were above the waterline, but eggs would have addled and nestlings starved by the floodwater preventing parental access. There is almost no chance that flooding of caves in the Chillagoe–Mungana district would occur in the Swiftlets’ non- breeding season (in the dry season, traditionally April–September: Table 5). In the 1985 breeding season, Alan Cummins (CCC) took us to Capricorn Cave (CH 176). He was unaware that the Swiftlet colony there had disappeared until we arrived at its former location, and found nothing but a little, very old guano. Other Vulnerability of Australian Swiftlet colonies 135

Figure 2. Locations of breeding colonies of Australian Swiftlets in the Chillagoe– Mungana region, Queensland. Numbers refer to cave numbers designated by the Chillagoe Caving Club (see Tables 2 and 3).

CCC members confirmed that this colony had disappeared in 1981 (M. Godwin & G. Greaves pers. comm.), and rainfall data show that in January 1981 Chillagoe received more than double the average rainfall for that month. It is likely that this heavy flooding ended the Capricorn Cave colony. We checked Capricorn Cave twice in June–July 2012 for Swiftlets leaving at sunrise or arriving at sunset, but there was still no replacement colony in that limestone outcrop (karst tower). For each of the years 2007 to 2012, Chillagoe received in one or more of the summer months more than twice the average rainfall for that month, and in 136 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

0 0 0 0 0 0 11

0 10

0 0 0 0 06 1 (d)

0 0 0 0 01

0 0 0 99

P 0 0 0 0 P 91 (a) (a)

2 2 0 0 M 89 160 Year 1

4 6 0 88 1 1

0 0 0 0 0 M 23 86 0 N 253

0 0 0 85 0 N

M 82 (a) Few 0 (b) 41 (b) 41

6 81 (b)

51 79 (b)

(b) Few 1971 : (a) Chillagoe Caving Club (b) Pecotic (c) Chillagoe (1982), (1983), Caving Club Ham (pers. (1997), (d) R.

CCC # = Official Australian Speleological Federation Cave Number as designated through the Chillagoe Caving Club M = evidence for predation by mammals; N = marks of old nests; P = birds present, no bird or nest count provided. Other Cave name Tower of London: Front Middle Rear Royal Arch Cave: Swiss Cottage Snakey Cavern But Good Cave Chinese Cavern Capricorn Cave Wallabadori Pope John Paul 1 Barramundi Eyes Project 31 Swiftlet Scallops 1 Another Golgotha Western Pit . Census data for Australian Swiftlets at Chillagoe–Mungana caves, Qld, where earlier reports were not verified or where colonies where or verified not were reports earlier where Qld, caves, Chillagoe–Mungana at Swiftlets Australian for data Census . of data in parentheses CCC # CH 5 CH 9 CH 46 CH 133 CH 138 CH 176 CH 192 CH 227 CH 309 CH 312 CH 322 CH 451 CH 511 Table 2 Table went extinct. procedures; sources comm.). Blank cells = no census undertaken in that year. Vulnerability of Australian Swiftlet colonies 137

Table 4. Census data (counts of nests and of birds entering caves) from the Chillagoe– Mungana area (including Metal Hills—see Figure 2), Qld, for caves, discovered in 2011– 2012, containing Australian Swiftlet colonies. * = census by A. Cummins & B. Dane.

Cave # Cave name Tower Nest Bird Date count count CH 532 Kerrin’s Dilemma #1 Royal Arch 123 27.6.12 Entrance CH 533 Krystelle’s Delight Royal Arch 97 27.6.12 CH 535 Dirk’s Backyard Spring 11 7.12.11 Dirk’s Backyard Spring 22 26.6.12 CH 536 Lisette’s Art Shoppe Spring 12 12.12.11 Lisette’s Art Shoppe Spring 53 25.6.12 CH 537 Vertical Mums Ryan Imperial 25 5.7.12

CH 541 Kerrin’s Dilemma #2 Royal Arch 70 28.6.12 Entrance CH 542 Ella Bella Cave Kurrajong 21 19.7.12 CH 543 Beefeater Pot Queenslander 97 16.7.12 Beefeater Pot Queenslander 42* 24.8.12 CH 544 Gruntagrike #2 Ryan Imperial 226 10.7.12 Entrance CH 547 Mikael’s Drop In Kurrajong 15 20.7.12 CH 550 Cam’s Shaft Kurrajong 52 23.7.12 CH 552 Telicia's Twin Kurrajong 5 22.7.12 CH 553 First Prussik Cave #2 Royal Arch 23 1.7.12 CH 554 Joseph’s Pit Markham 38 28.7.12 CH, MH 1 Granite Rock Cavity Upper Metal Hill #3 14 26.7.12

CH, MH 2 Granite Rock Cavity Middle Metal Hill #3 111 26.7.12

CH, MH 3 Granite Rock Cavity Lower Metal Hill #3 227 26.7.12

Total 1219

almost all Swiftlet colonies the nests were washed from the sloping roofs of the caves during the breeding season. Whereas in non-flooding seasons Swiftlets usually have to repair only the rim of a nest, it appears that in 2007–2012 most nests had to be completely rebuilt. Populations of Swiftlets there were reduced by the destruction of eggs and nestlings, and also some adults. The floodwaters were still sitting in Swiftlet Swallet (CH 398) on 22 April 2011, when K. Offer and MKT conducted a census and had to swim from the bottom of the 20-m caving 138 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

850 1139 1796 1956 1436 Year (1980), 1989 45 M et al. 80 M [200] 156 419 140 283 Dec. 1997 73 56 385 257 168 160 280 2011 1988 Nov. 54 M 1989 15 14 60 88 Oct. 1998 1981 101 58 (f) 264 2006 7 5 18 102 Sep. 1975 6 (e) 1976 2004 73 (g) 4 8 39 82 Aug. 1970 1975 2 (e) 7 4 61 27 Jul. 1938 2002 60 (g) 11 25 95 94 Jun. 1950 1964 123 2001 58 M 250 (c, d) 4 13 14 136 May 1930 43 17 265 63 30 1993 279 1963 1911 Apr. 250 (c) 413 158 138 426 1976 Mar. 1992 29 (g) 1959 198 (g) 25 (b) 517 216 295 568 Feb. 1956 112 1991 14 M 1953 20 (a) 219 377 753 690 Jan. 1991 M = Evidence for predation by mammals (number in square brackets indicates approximate number of fatalities). Other

1990 30 M 1951 40 (a) Census data for Australian Swiftlets in the two Dooloomai Falls caves in the Qld, Finch Hatton Gorge, near Mackay, Mean and maximum monthly rainfall (mm) for Chillagoe 1902–2006 and 2007–2012, showing increases in the mean between showing increases in and 2007–2012, for Chillagoe (mm) 1902–2006 monthly rainfall Mean and maximum Table 6. Table in the years shown. (e) Smyth (d) Chisholm (1966), (c) Seton (1965), sources of data in parentheses: (b) Robertson (1962), (a) Wheeler (1959), (f) Crouther (1983), (g) T. Brickhill pers. comm. Table 5. Table November and March in the last 6 years. Cave Upper Lower Cave Upper Lower Month Mean 1902–2006 Max. 1902–2006 Year of maximum Mean 2007–2012 Max. 2007–2012 Vulnerability of Australian Swiftlet colonies 139

Figure 3. Fledgling Australian Swiftlet on remains of water-damaged nest in Crocodile Pot, 25 April 2011. Photo: Michael K. Tarburton

Figure 4. Australian Swiftlet nests, Bottle Cave (CH 501) colony, Chillagoe–Mungana area, Qld, 15 December 2011. Photo: Michael K. Tarburton 140 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

Table 3. Census data (breeding pairs from counts of current nests until 2011, and bird count in June–July 2012) for Australian Swiftlets for all caves at Chillagoe–Mungana, Qld, where Swiftlet populations survived beyond 1999. Evidence for predation by: mammals (M) or reptiles (R) (numbers in square brackets indicate approximate numbers of fatalities); P = large number of birds present; CCC # = Official Australian Speleological Federation cave number as designated through the Chillagoe Caving Club procedures. Other sources of data in parentheses: (a) Smyth et al. (1980), (b) Pecotic (1983), (c) G. Greaves & T. Wilson pers. comm., (d) B. Dane pers. comm., (e) A. Cummins pers. comm., (f) D. Irvin pers. comm., (g) D. Flett pers. comm. Blank cells = no census undertaken in that year.

Nests Birds CCC # Cave name 1972 1979 1980 1981 1982 1985 1986 1988 1989 1990 1991 1993 1999 2000 2001 2003 2005 2006 2010 2011 2012

CH 9 Royal Arch Cave*: SC 122 (b) 37 (b) M 18 (b) 12 (b) 15 26 62 77 96 85 112 119 M [1] 57 R 19 37

CH 24 Keef’s Cavern 11 (b) 109 185 Flood 91 126 M [40] 90+ 85

CH 26 Clam Cavern 23 (b) 32 (b) 33 (b) 11 M 11 M 17 32 37 72 217 R 65 M [50] 20 M [8] 37 63

CH 30 Stop Press Cave 4 M 15 27 54 R M 14 M [20] 35 29

CH 52 Swiftlet Cave 129 189 R 145 235 217 177 160 103 250

CH 81 New Southlander Cave*: O 30 (b) 22 14 R 15 0 (e)

CH 81 New Southlander Cave*: L 1 18 (e)

CH 124 Flow Cavern 13 (a) 6 (b) 8 M 19 30 31 33 M 39 M [3] 35 M [2+] 22 M R 17 CH 144 Christmas Pot 29 97 127 78 M [50] 99 36 144

CH 146 Guano Pot 81 R 88 98 101 132 98 71 109 58 51

CH 147 Crocodile Pot 35 (c) 30 (d) 50 (d) 120 253

CH 149 Christmas Pot*: L 29 40 97 78 42 36 40

CH 167 Crack Pot 486 (b) R 194 188 176 180 R 230 91 M [100] 481

CH 169 Squeeze Pot 135 100 141 99 236

CH 187 Gordale Scar Pot 400 (b) 169 162 R 119 141 R 129 265 327 182 M [6] 161 287

CH 221 September Cave 118 R 180 188 100+ R 172 R, M [5] 131 M [33] 108 108

CH 252 Good Black Friday 64 R 20 (d) 109+ R 248

CH 306 Mudlark Cave 17 (b) 31 28 R 18 12 14 25 46 R 29 R 51 0 M 0

CH 362 Hercules Cave*: R 8 22 32 52 64 81 38 12 0 33 41

Hercules Cave*: F 23 25 R 39 41 56 9 23 7 R 0 0 0

CH 374 Swiftlet Scallops 2 49 45 50 78 6 M [50] 59 0 1

CH 379 Tarby’s Pot 180 179 141 101 100 117 170 146 R 130 R 168 230

CH 380 Golgotha Cave*: S 48 R 49 R 54 R 66 R 46 46 R 62 R 35+ R 95 R 211

Golgotha Cave*: B 69 R 61 112 91 R 62 55 R 10 R 0 1

CH 381 Swiftrimlet Cave 25 29 43 55 89 96 48 M 78 M [2] 54 40 33 66

CH 397 Shirl’s Triple Twirl 21 R 37 50 69 79 71 R 49 14 36

CH 398 Swiftlet Swallet 264 257 258 483 355 202 193 380

CH 412 Vertical Terraces 25 (f) R 35 111 70

CH 459 Swiftlet Surprise P (g) 42 112 447

CH 496 Satu Lagi 25 0

CH 499 Cactus Cleft 67 43

CH 501 Bottle Cave 30 M 45 14 19 R [3], M [30] 44 141

Total 3994

*Royal Arch Cave: SC = Swiftlet Cavern; New Southlander Cave: O = original colony, L = 80 m below original colony; Christmas Pot: L = lower entrance; Hercules Cave: R = rear colony, F = front colony; and Golgotha Cave: S = side colony, B = bottom colony Vulnerability of Australian Swiftlet colonies 141

Nests Birds CCC # Cave name 1972 1979 1980 1981 1982 1985 1986 1988 1989 1990 1991 1993 1999 2000 2001 2003 2005 2006 2010 2011 2012

CH 9 Royal Arch Cave*: SC 122 (b) 37 (b) M 18 (b) 12 (b) 15 26 62 77 96 85 112 119 M [1] 57 R 19 37

CH 24 Keef’s Cavern 11 (b) 109 185 Flood 91 126 M [40] 90+ 85

CH 26 Clam Cavern 23 (b) 32 (b) 33 (b) 11 M 11 M 17 32 37 72 217 R 65 M [50] 20 M [8] 37 63

CH 30 Stop Press Cave 4 M 15 27 54 R M 14 M [20] 35 29

CH 52 Swiftlet Cave 129 189 R 145 235 217 177 160 103 250

CH 81 New Southlander Cave*: O 30 (b) 22 14 R 15 0 (e)

CH 81 New Southlander Cave*: L 1 18 (e)

CH 124 Flow Cavern 13 (a) 6 (b) 8 M 19 30 31 33 M 39 M [3] 35 M [2+] 22 M R 17 CH 144 Christmas Pot 29 97 127 78 M [50] 99 36 144

CH 146 Guano Pot 81 R 88 98 101 132 98 71 109 58 51

CH 147 Crocodile Pot 35 (c) 30 (d) 50 (d) 120 253

CH 149 Christmas Pot*: L 29 40 97 78 42 36 40

CH 167 Crack Pot 486 (b) R 194 188 176 180 R 230 91 M [100] 481

CH 169 Squeeze Pot 135 100 141 99 236

CH 187 Gordale Scar Pot 400 (b) 169 162 R 119 141 R 129 265 327 182 M [6] 161 287

CH 221 September Cave 118 R 180 188 100+ R 172 R, M [5] 131 M [33] 108 108

CH 252 Good Black Friday 64 R 20 (d) 109+ R 248

CH 306 Mudlark Cave 17 (b) 31 28 R 18 12 14 25 46 R 29 R 51 0 M 0

CH 362 Hercules Cave*: R 8 22 32 52 64 81 38 12 0 33 41

Hercules Cave*: F 23 25 R 39 41 56 9 23 7 R 0 0 0

CH 374 Swiftlet Scallops 2 49 45 50 78 6 M [50] 59 0 1

CH 379 Tarby’s Pot 180 179 141 101 100 117 170 146 R 130 R 168 230

CH 380 Golgotha Cave*: S 48 R 49 R 54 R 66 R 46 46 R 62 R 35+ R 95 R 211

Golgotha Cave*: B 69 R 61 112 91 R 62 55 R 10 R 0 1

CH 381 Swiftrimlet Cave 25 29 43 55 89 96 48 M 78 M [2] 54 40 33 66

CH 397 Shirl’s Triple Twirl 21 R 37 50 69 79 71 R 49 14 36

CH 398 Swiftlet Swallet 264 257 258 483 355 202 193 380

CH 412 Vertical Terraces 25 (f) R 35 111 70

CH 459 Swiftlet Surprise P (g) 42 112 447

CH 496 Satu Lagi 25 0

CH 499 Cactus Cleft 67 43

CH 501 Bottle Cave 30 M 45 14 19 R [3], M [30] 44 141

Total 3994

Figure 5. Australian Swiftlet nests in upper colony at Dooloomai Falls, Finch Hatton Gorge, near Mackay, Qld, 24 November 2011. Photo: Michael K. Tarburton

Figure 6. Site of lower colony of Australian Swiftlets at Dooloomai Falls, Finch Hatton Gorge, near Mackay, Qld, 12 January 2002. Photo: Michael K. Tarburton Vulnerability of Australian Swiftlet colonies 143 ladder in 2.5-m-deep water, containing dead Swiftlets, microbats and guano, to reach the census point. The high-water line was 5 m up the wall, above 12 of the nests. This colony is on a huge vertical wall with approximately 80% of the wall of the cave above the high-water line, but after the recent series of wet seasons (2007–2012) this colony was the smallest it has ever been since we discovered it in 1986 (Table 3). As the Swiftlets come in from entrances near the top of the tower (and not through any low entrance or passage that might be flooded, as is the case for the middle colony at the Tower of London Cave), it must have been the sheer volume of water running over the nests that caused significant losses to the Swiftlets’ breeding efforts. In some Swiftlet colonies, such as Crocodile Pot (CH 147), some late replacement broods were laid (indicating that at least some young were successfully raised), enabling those colonies to increase. The egg of the fledgling that we found on 25 April 2011 would have been laid on ~8 February. All previous known laying dates were between 5 October and 16 January (n = 113). Figure 3 shows the last fledgling from the 2010 season in the Crocodile Pot’s main colony and shows that water flowing over the wall there has dissolved most of the nests. The photograph on the front cover shows what a normal nest at this late stage of development looks like. The two oldest nestlings photographed by G. Anderson in Clam Cavern (CH 26) on 12 October 2011 came from eggs laid on approximately 12 and 25 August, which is 2–3 months earlier than usual. Not all colonies of Swiftlets bred earlier in response to the heavier summer rains, as photographs taken in Crocodile Pot 4 days earlier showed that all Swiftlets there were either incubating or still building nests. Some colonies therefore responded to the extra water by breeding early, but others did not. Other colonies, like those in Shirl’s Triple Twirl (CH 397) and Swiftlet Cavern (CH 9), suffered a decline (Table 3) and moved to a different wall in the same cave chamber. Australian Swiftlets sometimes abandon a nesting chamber for another in the same cave, as happened in New Southlander Cave (CH 81) and probably Golgotha Cave (CH 380), during this 2007–2011 period of flooding. In addition, a colony can be extinguished from a complete tower, as was the Capricorn Cave (CH 176) colony in 1981; there was still no sign of this colony in 2012 (Table 2). Although the extinction of the colony in Mudlark Cave (CH 306) by 2010 (Table 3) is certain, the fate of the birds there is not certain. It will be interesting to see how long the location of water-soaked cave walls and chambers persists in the surviving birds’ memory after this 2007–2011 wet period. In total, these five years (2007–2011) of flooding destroyed the Swiftlet breeding colonies in four caves: Mudlark (CH 306), Hercules (CH 362) front colony, Rescue (CH 69), and Satu Lagi (CH 496). These caves had no breeding Swiftlets in the 2011 season (Table 3). Two other colonies [Swiftlet Scallops 2 (CH 374) and Golgotha (CH 380) (bottom colony)], where all nests had been destroyed and contained no sleeping birds, were rechecked in December 2011 (when they each contained just one pair of breeding birds: Table 3) and June 2012. Nests of the rear colony in Hercules Cave had also been completely cleared off the wall, and 144 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

Figure 7. Opening to Guano Pot (CH 146) cave, Chillagoe–Mungana area, Qld, 16 December 2011. It is 25 m feet to the bottom of the cave. Photo: Michael K. Tarburton

Figure 8. Entrance no. 1 to Kerrin’s Dilemma (CH 541) cave, Chillagoe–Mungana area, Qld, 1 December 2011. Photo: Michael K. Tarburton Vulnerability of Australian Swiftlet colonies 145 there were no roosting Swiftlets in October 2011 (D. Irvin pers. comm.). However, in December 2011, we found 33 active nests there (Table 3), demonstrating that not all colonies where nests and young have been destroyed by flooding are terminated, particularly if at least some adults survive. From the increase in the Golgotha side colony, it is likely that the birds deserting the bottom colony were accommodated in the side colony. Swiftlets from both colonies in Golgotha Cave use the same entrance, although they use five different exits.

Coastal mainland colonies Heavy rains in the 2006 breeding season at Finch Hatton and Mackay suggested to members of local bird and tramping clubs that the two Australian Swiftlet colonies at Dooloomai Falls in Finch Hatton Gorge would have been washed away. However, the colony above the falls showed a 28% increase (101 nesting pairs in 2006 compared with 73 in 2004) and the colony below the falls was 115% larger than at our last census (264 nesting pairs in 2006 compared with 123 in 2001) (Table 6). The 2007 season, however, had higher rainfall lower down the Gorge than at the Swiftlet colonies (550 mm of rain in 4 hours early in the morning of 17 January 2008: Peter & Wendy Bennett, W. Swadling pers. comm.). This caused major flooding throughout the Dooloomai National Park and in the settled lower part of the Gorge, preventing access for days. We visited the Swiftlet colonies on 7 February 2008 and found at the upper Dooloomai Falls colony that floodwaters had not reached the nests, nor washed them off the walls. However, at the colony 100 m downstream of the falls only 22 nests had not been submerged (and these contained nestlings or eggs) but most of the nests that had been submerged (>100) had lost their contents (only two rotten eggs and a dead nestling were found in these nests) and were coated with silt. This was a 91% decline in the number of nests from the previous season. More heavy rain fell on 14 February 2008. Tess Brickhill and others from Mackay (which received 600 mm rain in 24 h), who were familiar with the Australian Swiftlet colonies, thought that the nests in both Dooloomai Falls colonies would have been flooded (T. Brickhill pers. comm.). However, less rain fell in the Gorge and it was believed that Swiftlet colonies there would not have been flooded (W. Swadling pers. comm.). Therefore, rainfall near a nesting location does not always indicate what actually happens to the nesting colony. In the next severe weather event, Severe Tropical Cyclone Yasi crossed the coast near Mission Beach on 3 February 2011. Although this is well north of Mackay, much rain fell over the next 4 weeks in the Finch Hatton Gorge area (e.g. 787 mm at Mirani Weir in March). This very high rainfall reduced the lower Swiftlet colony in size, but did not destroy it. The lower colony had decreased by 39% in the 4 years since the previous census in January 2007, and the upper colony had increased by 66% (see Figure 5 for upper colony, in November 2011, and Figure 6 for site of lower colony at Dooloomai Falls). Nests surviving in the lower colony were the highest ones, with well over 100 of the lowest nests still appearing sodden, misshapen, and unusable. In summary, flooding of nest-sites of Australian Swiftlet colonies can have 146 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton variable effects, depending on whether or not the adult birds are killed in the flood. For example, the lower Dooloomai Falls (in Finch Hatton Gorge) and Keef’s Cavern (CH 24, near Chillagoe) colonies were both covered, by 6–7 m of water during December 1990 and January 1991. The former appears to have suffered no significant reduction as a result, but the latter was halved. Thus, nesting in caves (as at Chillagoe) can exacerbate the risk of colony reduction or even extinction from heavy rainfall compared with colonies breeding under granite boulders (as in Finch Hatton Gorge), which do not trap water around the nests as easily as do caves.

Declines in offshore island colonies Queensland’s offshore colonies were the first breeding colonies of Australian Swiftlets known to Europeans. Those on Dunk Island (Banfield 1912) and Timana (Thorpe) Island (Busst 1956) were both located before that above the Dooloomai Falls in Finch Hatton Gorge. That on Bedarra Island was the fourth discovered (Busst 1956). None of the offshore colonies has been regularly censused, so there are few data on the effect on them of cyclonic winds and rain. However, combining the work of four observers before and after Cyclone Larry gives some indication of the destruction that it wrought on the Dunk and Bedarra Island colonies. Severe Tropical Cyclone Larry crossed the tropical north Queensland coast near Innisfail during the morning of 20 March 2006. Both Dunk and Bedarra Islands, 45 km south of Innisfail, were just inside the area of greatest impact. At its peak, the cyclone registered winds of up to 290 km/h and was graded category five, the highest category for a cyclone (Arvier 2006; Bureau of Meteorology 2006). Six colonies of Australian Swiftlets were found on Dunk Island in September 1989 (Schulz 1991); four contained fresh nests, some with eggs (total = 173 nests). The previous season we had counted 417 Swiftlets returning to Dunk Island from Mission Beach, indicating that Schulz’s (1991) September count was slightly conservative, probably because it was very early in the breeding season. In the first breeding season after Cyclone Larry, Adrian Walker repeated our census procedure from South Mission Beach, obtaining the following data: 186 Swiftlets (12 February 2007), 174 (14 Feb.), 275 (27 Feb.) and 283 (28 Feb.) (A. Walker pers. comm.). The increase through the month is most likely explained by fledglings joining their parents to forage on the mainland each day. The average of the 2007 counts (230) is 45% lower than our count of 417 in 1988, and is probably best explained by the destruction of nestlings in the colonies by Cyclone Larry (March 2006, i.e. too late for adults to lay replacement clutches for that breeding season). The four active breeding sites in 1989 were all on the exposed northern and eastern coasts of Dunk Island, under boulders at the top of the beaches; only one appeared to be out of the reach of cyclone-driven waves. Waves, just like the rising waters of Dooloomai Creek in Finch Hatton Gorge and the cave water at Chillagoe, could wash away eggs and nestlings, flood eggs or drown nestlings. It is also likely that a food shortage for the Swiftlets occurred in the weeks following the cyclone, and this might have reduced the survival of inexperienced or weak birds. Schulz (1991) also suggested that the size of these colonies was influenced more by weather events than by human disturbance (contra Orrell 1967). Vulnerability of Australian Swiftlet colonies 147

On Bedarra Island (10 km south of Dunk Island) in 2007 in the last week of January (usually the peak of the breeding season), there were no active Swiftlet nests. Only five old nests remained on the roof of the former breeding colony. Cyclone Larry could also have been responsible for the demise of this colony. When there is no sign of nest-building or occupied nests in December or January, our experience suggests that a colony has been eliminated or that the remaining Swiftlets from it have moved to a nearby site. The Bedarra Island colony is known to have survived a previous cyclone without damage: Busst (1956) had been recording the number of active nests there for 18 weeks when Cyclone Agnes (category 3) struck on 6 March 1956, but when he revisited the colony on 23 March he found the colony undamaged. The Swiftlet colonies on both Bedarra and Dunk Islands are not in limestone caves but (like the mainland coastal colonies) are under granite boulders, suspended on other boulders. These effects of Cyclone Larry on offshore colonies of Australian Swiftlets had been anticipated by a study finding similar effects on the closely related White- rumped Swiftlet in Samoa (Tarburton 2011b). Censusing of cave-nesting colonies of White-rumped Swiftlets in the 4 years following two category 4 cyclones, in 1990 (Cyclone Ofa) and 1991 (Cyclone Val), that sat over Samoa for up to 6 days, found that four colonies had become extinct, and 38 had been reduced from hundreds or thousands of pairs to between two and 103 nesting pairs (MKT unpubl. data).

Newly discovered colonies Fifteen Australian Swiftlet colonies have been discovered on our last two trips to Chillagoe. The caves containing these colonies were all discovered by looking for Swiftlets entering caves near sunset. Each cave’s official Australian Speleological Federation number and name as well as the population of Swiftlets for each colony are given in Table 4. Not all colonies could be accessed but, in those that could be, there were deep guano piles beneath the nests, indicating that these colonies had not been established recently (i.e. they were not colonies relocated from elsewhere, although some birds from abandoned colony sites may have moved to them). Some or all of the unentered new colonies could be relocated colonies, but this could occur only where the adult birds had not been drowned by flooding. One Australian Swiftlet colony has relocated within the same cave in this wet period. When we censused New Southlander (CH 81) colony in the 2006 breeding season, there was a single nest 80 m from the traditional colony (which contained 15 nests). At the end of the 2011 breeding season, however, there were no nests in the traditional location but 18 nests at the newly established site where we had noticed the pioneering pair in 2006 (A. Cummins pers. comm.). The total known population of Australian Swiftlets in the Chillagoe–Mungana district as at July 2012 is estimated at 5213 birds (determined by adding the population counts for Tables 3–4). We believe that this is significantly below what it would normally be. By comparing the total number of nesting pairs in colonies that were censused in both 2006 and 2011, and allowing two birds per nest, we determine that that portion of the Chillagoe–Mungana population declined from 2692 to 2128 birds, a loss of 564 birds (21%) in the six wet years (2006–2011). 148 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton

The census data also document a decline in the last 6 months of the present study. From the 2011 nest count (November–December, and assuming one pair of birds per nest) compared with the bird census of 2012 (June–July), there was a decrease of 259 Swiftlets. In this 6-month period, only Crocodile Pot (CH 147), Swiftlet Cave (CH 52) and Bottle Cave (CH 501) colonies had significant population increases, totalling 110 birds; the September Cave (CH 221) colony declined from 108 nesting pairs to 108 birds, so this colony had lost half of its breeding stock.

Other causes for decline in Australian Swiftlet populations Other causes for population declines in Australian Swiftlets have been suggested. For example, during excessively wet seasons the upper Dooloomai Falls and three Family Island colonies suffer from nest-material failure (nest collapse) (Smyth 1976; Pecotic 1983) when located under granite shelters that are not adequately waterproof. Crouther (1983) disputed that rain is the cause for nest failure at Dooloomai, suggesting that the rain early in 1975 could have allowed earlier breeding, and that in other years when breeding numbers were known, high rainfall did not always correlate with low Swiftlet numbers. Rainfall data from the Finch Hatton Post Office substantiate Crouther’s claim for the 1975 breeding season. However, there are too few population data, and as they are not taken from comparable stages of the breeding season, a generalised relationship between rainfall and the falling of nests cannot be validated. Crouther (1983) found that moth larvae had weakened Australian Swiftlet nests during the 1981 season. She reported the loss of some eggs and nestlings from failure of the nest structure, but believed that the majority of nestlings had fledged by the time that water had weakened the nests. MKT’s daily visits to Guano Pot (CH 146) found that some eggs and nestlings were lost because water washed a few nests off the wall, so moths and excessive water flow can both cause Swiftlet breeding failure. However, we also found, from daily nest inspections in December and January over 2 years, that in a very dry season (part of El Niño) Swiftlet nests at Chillagoe had less saliva in them than usual and, as a consequence, some fell off the wall, taking their contents (eggs and/or nestlings) with them. Data collected since 1983 have shown that the suggested inverse correlation between rain and nest numbers does not always hold. December 1988 was a very wet month, but the number of nests (385) in the rediscovered lower Dooloomai Falls colony was high in early January. The following December (1989) was dry but the number of nests was low (80) (Table 6). It has been shown that increased rainfall increases the Swiftlets’ food supply and thereby the reproductive increase for that season (Tarburton 1988a). The present study shows, however, that if rainfall reaches flood levels it can decrease the population of Swiftlets. This decline may be caused by reduced foraging time during prolonged heavy rain or the physical loss of nests and their contents from cave walls. Other factors can also complicate the relationship between rainfall and reproductive output. The population of the upper Dooloomai Falls colony declined each year between 1988 and 1992, including both wet and dry Decembers. Fresh flight-feathers on the ground in four of the five years indicate that mammalian predator(s) contributed to the population decline of the Swiftlets. Vulnerability of Australian Swiftlet colonies 149

Orrell (1967, p. 30) suggested that frequent human visitation was probably responsible for the extinction of the Australian Swiftlet colonies on Dunk Island, whereas the ‘rigorously protected and seldom visited’ colony on Bedarra Island persisted. Orrell’s judgement about the Dunk Island colonies was based on second-hand information, and the fact that Schulz (1991) later found Swiftlets on Dunk Island means either that the birds had changed location or recovered from small numbers, or simply that the quoted friends of Orrell had not gone to the correct locations in December or January. Other evidence for human impacts on Australian Swiftlet colonies in Queensland is discussed elsewhere (Tarburton 1988b) but suggests that cavers or researchers using roosting caves in daylight during the Swiftlets’ non-breeding season do not adversely affect the birds. Visits to Swiftlets’ roosting/nesting caves during the evenings of the non-breeding season or at any time during the breeding season need not impact the breeding birds negatively if (1) the birds’ passage to their nests is not impeded, (2) bright lights are not brought suddenly onto the birds or nests, (3) noise levels are kept low, and (4) visits are infrequent. Following these guidelines will eliminate or drastically reduce losses to a Swiftlet colony caused by humans. Another cause for the extinction of a colony is the collapse of the rock under which the Swiftlets nest and roost. This is believed to be the cause of the extinction of the small colony that once inhabited Timana Island in the Family Group (Busst 1956).

Other observations Variation in timing of breeding and relocation of colonies (within a cave or to a different cave) by Australian Swiftlets in response to varying rainfall have already been discussed. It is not known if other apodids behave in a similar way except possibly the vulnerable Seychelles Swiftlet Aerodramus elaphrus. Chantler & Driessens (1995) suggested that one possible reason for the disappearance of one colony of that species and the increase of another might be caused by a change in water distribution patterns on the island [although Chantler (1999) and BirdLife International (2013) have subsequently proposed other reasons for the decline of that species].

Conclusion This paper is the first report on the effects of heavy rainfall on nesting Australian Swiftlets. Heavy rainfall and strong winds such as those in North Queensland during recent La Niña events have been shown to affect the stability of Swiftlet colonies in several ways. They can cause the demise of a breeding colony by either washing down the rockface on which the nests are anchored or partially dissolving the nests, or by the cave or creek bed filling up until floodwater drowns the nestlings or causes eggs to fail. In addition, adult Swiftlets may be prevented from reaching their nests if water has filled the entrance passage to a cave. Offshore and coastal colonies, under granite boulders, may also be reduced or destroyed by high waves or heavy rainfall, though few details are available. Some eggs and young are lost from nests weakened by extremely dry periods during an El Niño 150 Australian Field Ornithology M.K. Tarburton & S.R. Tarburton period. We suggest that studying cave geology to predict which Swiftlet colonies are susceptible to heavy rainfall would be an onerous undertaking, and it is easier and more predictive to visit as many caves as possible after wet seasons to learn which colonies have survived and which have not.

Acknowledgements We thank officers and members of the Chillagoe Caving Club (K. Offer, D. Mitchell, A. Slee, V. Christensen, D. Irvin, C. Head, A. Cummins, M. Cummins and B. Dane) and the Queensland Parks and Wildlife Service (J. Barton, L. Little and M. Cockburn) for help in locating and reaching some of the caves; our son Kerrin and daughter Krystelle for their help in the field, and Tess Brickhill for censusing Dooloomai in two of the years that we did not; Denis Jeffery for local knowledge on the tracks to Dooloomai Falls; and Adrian Walker for offshore island censuses. One of the trips to Queensland was funded by a grant from the Frank M. Chapman Memorial Fund of the American Museum of Natural History, and another two were partly funded by Pacific Adventist University and BirdLife Australia. We also thank referees Charles Collins and Mark Antos and editors James Fitzsimons and Julia Hurley for comments on this paper.

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Received 11 September 2012