Raptor Predation on Wintering Shorebirds
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RAPTOR PREDATION ON WINTERING SHOREBIRDS G. PAGE AND D. F. WHITACRE Point Reyes Bird Observatory Box 3,221 Bolinas, California 94924 Although high mortality rates have been dem- and mud flats form the major feeding habitat for onstrated for shorebirds by the analysis of large numbers of shorebirds that winter on the estuary. band recoveries and studies of marked birds (Martin-LGf 1961; Boyd 1962; Holmes 1966; METHODS Soikelli 1970), there is little information on Using a spotting scope and binoculars, we watched the causes of shorebird mortality. Goss-Cus- many diurnal raptors while they hunted on Bolinas Lagoon. When we saw a raptor eat prey, we col- tard (1970) theorized that flocking by some lected the remains and identified the prey to species shorebirds on the wintering grounds is a pro- bv comuarine the remains with study skins. We also I _ tective response against avian predators. How- searched for-piles of feathers and bther remains of ever, there exists no information demonstrat- shorebirds eaten by raptors and identified these in the same manner. We counted the number of pri- ing the extent to which raptors affect wintering maries in each pile of feathers, and if 11 or more pri- shorebird populations. Consequently, it was maries were present, we considered the remains to our purpose in this study to examine in detail represent one bird. If fewer than 11 primaries were the relationship between raptors and shore- present, we considered the remains to represent part birds and determine the impact of the raptors of a bird. To estimate the number of birds repre- sented by the partial remains, the total number of on the size of winter shorebird populations. left wing primaries contained in all the partial remains Although the raptors we studied were mainly was divided by the average number of primaries per the Merlin (F&o columharius) and Short- wing in the remains representing an entire bird. eared Owl (Asia flammeus) and the shore- We studied the composition of the owls ’ diets by analyzing pellets collected regularly at their roosts. birds were mainly the Dunlin (C&&is a@na) We removed all the bones from each pellet and de- and Least Sandpiper ( CaZi&is minutilla), we termined the species to which they belonged by com- have described all the raptor-shorebird inter- parison with known skeletons. In both winters we found many bones from either Robins (Tu&s migra- actions that we saw. The study was conducted torius) or Varied Thrushes (Ixoreus naevius) in the during the winters of 1971-72 and 1972-73 on pellets of Short-eared Owls. Although we could not Bolinas Lagoon, Marin County, California. separate the bones of these two species, there was strong evidence that most were from Varied Thrushes. STUDY AREA The lengths of the humeri, ulnae, and tarsometatarsi from the Dellets were sinnificantlv different (P < Bolinas Lagoon is a 570-ha estuary at the southern 0.05), fro& the lengths of ihe correiponding bon& of edge of Point Reyes Peninsula on the central Cali- Robins, but were nit significantly different (P > 0.05) fornia coast (fig. 1). The entire east side of the es- ( Steel and Torrie 1960:74) in length from the corre- tuary is bordered by Bolinas Ridge, which rises 460 sponding bones of Varied Thrushes. During the two m above sea level. The west side of Bolinas Lagoon winters of study, we found the feathers of 11 Varied is bordered by wooded hills, pastures, and the steep, Thrushes and 2 Robins at the Short-eared Owls ’ roost. 40-m high, wooded face of the Franciscan Mesa. To In this paper, therefore, we have referred to all these the south, a low sand spit separates the estuary from bones as Varied Thrush remains. The presence in a the ocean, except for a narrow opening at the es- pellet of one or more identifiable bones of an animal tuarys’ southwestern corner. On the north, the es- we considered to represent one prey item and bones tuar; is bordered by the lower slopes of the Bolinas from two individuals of the same species in a pellet to Ridge and the wooded floor of the Olema Valley. represent two prey items, even if major skeletal ele- Kent Island lies close to the southwestern edge bf ments were missing. We calculated the proportion of Bolinas Lagoon, separated by a narrow channel from each prey species in the owls ’ diet by converting the the Franciscan Mesa and Pine Gulch Creek delta number of individuals of each species in the pellets to (fig. 1). The majority of the islands’ 40 ha is salt a percentage of the total prey represented in the pel- marsh (Salicornia uirginica, Spartina foliosa, and Dis- lets. tichlis spicata), but a small area on the southwestern During February and March 1973, we kept a Short- corner is a few feet higher than the rest and is cov- eared Owl in captivity to obtain an es&ate of the ered by grass and several Monterey pines (Pinus number of Dellets Droduced dailv bv an owl under mdiata) and Monterey cypresses ( Cupressus maccro- natural co&tions. -T?;heShort-caged Owl‘ was a wild car)?a). A narrow row of sand dunes extends east bird that had been found with a broken wing. The from this high area 460 m along the southern edge of wing had recently mended when we received it. The the island aid provides cover for roosting Short-eared bird was kept in an outdoor cage, where it was rela- Owls. A few hundred hectares of tidal sand and mud tively inactive except when it ran over the cage floor flat border Kent Island, the Pine Gulch Creek delta, or flew between two perches. We fed it dead Cali- the north and south ends of the estuary, and the sand fornia voles ( Microtus califomicus) and deer mice spit on the estuarys’ southern boundary. These sand (Peromyscus maniculatus)‘ and recorded the total [731 The Condor 77:73-83,, 1975 74 G. PAGE AND D. F. WHITACRE adult female Merlin wintered there from early September 1972 to April 1973. After examin- ing many study skins of Merlins, we were con- fident that the bird was an adult female and that it was of the subspecies (F. c. bendirei). Between 21 September 1972 and 28 February 1973, we watched the Merlin on 119 days and saw her eat 131 vertebrate prey items, of which 109 were shorebirds, and collected 12 additional shorebird remains from her eating perches. During her stay, she hunted shore- birds on Bolinas Lagoon almost every day. The falcon usually arrived early in the morning and spent the entire day on the estuary, al- though she sometimes left after an early after- PACIFIC OCEAN noon meal. Her nocturnal roost was not lo- cated but it was at least 0.8 km from the FIGURE 1. Map of Bolinas Lagoon. estuary. On six occasions we saw her chase other Merlins away from the estuary. weight of rodents eaten in each meal. We varied the The Merlin selected hunting perches in re- number of rodents in each feeding, the number of sponse to the distribution of her major prey, feedings per night, and the time intervals between small sandpipers. At low tide, Least Sand- feedings. Weight of the owl was monitored throughout pipers, Western Sandpipers, and Dunlins fed the experiment. To estimate the weight of prey consumed daily on the mud flats and the Merlin hunted them by the raptors on the estuary, we calculated the aver- from perches nearby. When the tide was high age weight of each prey species. These averages were and the small sandpipers came into the salt based on the weights of 12-30 individuals from Point marsh to feed or to roost, she hunted them Reyes Bird Observatory banding records and from study skin data. To estimate the number of shorebirds from perches in the salt marsh. During very available to raptors, we took six censuses per month high tides, Least Sandpipers and Dunlins con- of the shorebirds on Bolinas Lagoon in the winter of gregated in pastures adjacent to the estuary 1971-72, and three censuses per month in 1972-73. and she hunted them from fence posts border- The estuary was divided into three areas and the In mid-November heavy birds in each area were counted or estimated simul- ing the pastures. taneously. On some censuses small sandpipers [Least rains and high tides flooded the estuary and Sandpipers, Dunlins, and Western Sandpipers (Cali- much of the adjoining pastures. Many Dun- dris mu&)] could not be separately identified. These lins and some Least Sandpipers left Bolinas birds always made up less than 20% of those that Lagoon and moved to pastures on Bolinas were identified and were incorporated into the census totals as Least Sandpipers, Dunlins, and Western Sand- Mesa, 1.2 km west of the estuary. The Merlin pipers according to the relative abundance of these followed and hunted the sandpipers there. species on the census. The average number of shore- When an opportunity arose, she often attacked birds from each months’ censuses from October other prey between hunts at shorebirds, and through February was calculated and the highest of these averages was used as an estimate of the winter occasionally, instead of hunting shorebirds, population of each shorebird. To measure the abun- she remained on the margin of the estuary and dance of California voles during the winter, we hunted passerines. She sometimes caught fly- trapped voles in three areas on Kent Island in I972- ing grasshoppers and lepidopterans when they 73, using live traps made after the pattern of Brown were available in the fall and spring.