Quantitative ornithology with a commercial marine radar: standard‐target calibration, target detection and tracking, and measurement of echoes from individuals and flocks
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Abstract:
Summary Marine surveillance radars are commonly used for radar ornithology, but they are rarely calibrated. This prevents them from measuring the radar cross‐sections (RCS) of the birds under study. Furthermore, if the birds are aggregated too closely for the radar to resolve them individually, the bulk volume reflectivity cannot be translated into a numerical density. We calibrated a commercial off‐the‐shelf marine radar, using a standard spherical target of known RCS. Once calibrated, the radar was used to measure the RCS of common and roseate terns ( Sterna hirundo L. and Sterna dougallii Montagu) tracked from a land‐based installation at their breeding colony on Great Gull Island, NY, USA. We also integrated echoes from flocks of terns, comparing these total flock cross‐sections with visual counts from photos taken at the same time as the radar measurements. The radar's calibration parameters were determined with 1% error. RCS measurements made after calibration were expected to be accurate within ±2 dB. Mean tern RCS was estimated at −28 dB relative to one square meter (dBsm), agreeing in magnitude with a simple theoretical model. RCS was 3–4 dB higher when birds’ aspect angles were broadside to the radar beam compared with head‐ or tail‐on. Integrated flock cross‐section was linearly related to the number of birds. The slope of this line, an independent estimate of RCS, was −32 dBsm, within an order of magnitude of the estimate from individual birds, and near the middle of the frequency distribution of RCS values. These results indicate that a calibrated marine radar can count the birds in an aggregation via echo integration. Field calibration of marine radars is practical, enables useful measurements, and should be done more often.Keywords:
Sterna
Ornithology
Tern
Flock
Terns are a highly migratory group of seabirds that are found worldwide. In Ireland, there are five species of commonly breeding tern: Little Tern Sternula albifrons, Roseate Tern Sterna dougallii, Arctic Tern S. paradisaea, Common Tern S. hirundo and Sandwich Tern S. sandvicensis. Prior work has demonstrated that whilst many Irish tern species, including Common and Roseate Terns, are increasing in abundance, the productivity of these species can be low. Multiple factors may influence the ability of adult terns to successfully raise chicks, including food availability, provisioning rates, colony density, dependence effects, and/or disease. Here, we investigated factors contributing to the mortality of young terns from Rockabill Island in the Republic of Ireland, which supports the largest breeding population of Roseate Terns in Europe. To better understand the factors contributing to the deaths of young birds, we analysed the macroscopic and microscopic characteristics of necropsies of 60 young Common, Arctic and Roseate Terns. Of the carcasses that we examined, 41 showed congested blood circulation in the lungs and head simultaneously, and of the remaining 19 birds, only five presented a clear cause of death. Here, we outline descriptions of these carcasses in addition to recommendations of further investigations that might help to confirm the causal factors leading to young tern mortality.
Tern
Sterna
Hirundo
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Sterna
Tern
Hirundo
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While nest usurpation and subsequent incubation of eggs and even brooding of chicks from other species has been reported for Common Terns (Sterna hirundo), such behavior is considered rare. We report an observation of a Common Tern pair usurping the Least Tern (Sternula antillarum) nest and brooding the Least Tern chicks. While the Least Tern pair attempted to provide care for the chicks, the Common Terns displayed aggressive behavior and defended the nest. Though both species attempted to feed the chicks, no feeding events were observed due to harassment from the other species. Neither pair was observed nesting prior to or following this event, and all chicks are believed to have been lost to predation. We discuss the possible scenarios leading to the observed usurpation event, the possibility that usurpations are more common than previously believed, and the need for different monitoring methods to elucidate the causes of usurpations.
Sterna
Tern
Hirundo
Paternal care
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Tern
Sterna
Hirundo
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Hirundo
Sterna
Tern
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1993年と1994年に，利根川中流域でコアジサシの繁殖状況を調査した結果，繁殖期に継続的に観察された個体は1993年，1994年ともに約100羽であった。コロニーの規模は成鳥10数羽のものから最大でも100羽とすべて小規模なものであった。営巣が確認された場所は1例を除いてすべて中州であった。環境要因，人為的要因による影響を受け易く，コロニーが小規模であったため繁殖効率が低いものと推定された。標識調査では1993年，1994年にそれぞれ32羽，18羽の雛に足輪とカラーリングを装着した。また1993年には佐波郡玉村町五料利根川のコアジサシのコロニー内において，国内初のアジサシの造巣と抱卵行動が記録された。
Tern
Sterna
Hirundo
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Isle aux Dames in the Bay of Morlaix, northern Brittany (48°41’N 3°53’W), holds one of the largest mixed tern colonies in France, with Sandwich Terns Sterna sandvicensis, Roseate Terns S. dougallii and Common Terns S. hirundo, and this island has been the main French Roseate Tern colony since the 1980s (Cadiou et al. 2004; Le Nevé 2005). Annual surveys are conducted to obtain data on breeding numbers and productivity. In 2007 the census of the colony was carried out on 12 June by a team of eight experienced and inexperienced people. Different teams covered different areas of the colony, counting nests, using different colour of small paper marks put inside to avoid double-counting, and reporting nest-contents.
Tern
Sterna
Hirundo
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