Nests f Swainsn's Hawks in Slan and Yl Cunties Laurence J. Resseguie, 01 Hrtaleza Place, Davis, CA 95618 I began mnitring a small number f nests f Swainsn's Hawks (Bute swainsni) in Yl Cunty in 1998. I gradually expanded my area f bservatin ver several years while fcusing n the cunties f Slan and Yl. In 004 I attempted t search the flat areas f Yl Cunty as well as Slan Cunty east f Vacaville. All f these nests have been reprted t the Califrnia Natural Diversity Database maintained by the Department ffish and Game. This article reprts what I fund in 004 and 005. Mst f my bservatins have been made frm public rads, but private rads have been used where permissin has been granted. Mst bservatin has been frm a substantial distance in rder t minimize disturbance f the birds and because many nests are inaccessible. Fr that reasn, eggs were nt seen, and chicks were nt detected while small. In this reprt, I cnsidered a nest active if the breeding cde as used fr the Eurpean Bird Census Cuncil's (EBCC) Eurpean breeding bird atlas prject (Hagemaijer and Blair, 1997) was seven r greater. The list f thse cdes (mitting situatins which were inapplicable because f the species r because f the methds f bservatin) is as fllws: Pssible breeding: 1 Species bserved in breeding seasn in pssible nesting habitat Prbable breeding: Pair bserved in suitable nesting habitat in breeding seasn 4 Permanent territry presumed thrugh territrial behavir n at least tw different days a week r mre apart at the same place 5 Curtship and display 6 Visiting prbable nest site 7 Agitated behavir r anxiety calls frm adults 9 Nest building Cnfirmed breeding: 11 Used nest r eggshells fund 1 Recently fledged yung 1 Adults entering r leaving nest site in circumstances indicating ccupied nest (including high nests, the cntents f which cannt be seen) r adult seen incubating 14 Adult carrying fd fr yung 16 Nest with yung seen 90 CVBC Bulletin/Fall 007
Only the suthern part fy 010Cunty was searched in 005 s the findings are divided int Slan Cunty, the prtin f Yl Cunty searched nly in 004 and the prtin f Yl Cunty searched in bth years. Large areas were nt searched in bth areas because access was nt btained r by chice. Substantial areas knwn r presumed t cntain nests f Swains n 's Hawks which were nt searched include the cities, University f Califrnia campus, riparian strips alng the principal waterways (Sacrament River, Cache Creek, Putah Creek, mst f Willw Slugh) and the Yl Bypass. Sme nests which were accidentally fund in sme f thse lcatins have been included. RESULTS AND DISCUSSION The chice f an EBCC cde f at least 7 t define active nest sites was arbitrary. It yielded 144 active sites detected in Slan Cunty in 005 (Table 1). Chsing an EBCC cde f at least 10, which is the lwer limit fr cnfirmed nesting, reduces the number t 10. Chsing an EBCC cde f at least, which is the lwer limit fr prbable nesting, raises the number t 166. Using all applicable EBCC cdes raises the number t 196. In Slan Cunty, 106 fledglings were detected at 7 nest sites in 004, and 161 fledglings were detected at 109 nest sites in 005. In Yl Cunty, 147 fledglings were detected at 104 nest sites in 004, and 40 fledglings were detected at 0 nest sites in 005. Twnships T7N RIE, T7N RE and T6N RIE are cntiguus and lie entirely within Slan Cunty with a cmbined area f apprximately 79.7 square kilmeters (108 square miles). Using an EBCC cde f at least seven t define active sites, 6, and sites were detected in these twnships in 005 fr a density f.5nests/1o square kilmeters (84.nests/1 00 square miles). Using an EBCC cde f at least 1, thse numbers becme, and 5 fr a density f.61 nests/1o square kilmeters (9.5nests/1 00 square miles). N valid inference can be drawn frm these data abut pssible change in ppulatin between 004 and 005. The amunt f time and effrt which were available t search and mnitr the excessively large area in 004 were entirely inadequate. Substantially mre time and effrt were applied t a much smaller area in 005 but were still inadequate t apprach cmplete detectin f either nests r fledglings. Substantially mre nests and fledglings must have been missed in 004. Blm (1980) attempted an inventry f Swainsn's Hawk nesting sites fr the entire state in 1979. He fund 110 sites and estimated the ttal number f pairs in Califrnia t be 75. Fur fthe 110 were reprted t be in Slan Cunty, all described as "near Davis" which is in Yl Cunty n. the brder with Slan Cunty. Tw were in Valley Oaks (Quercus [bata) and tw in unidentified trees. His definitin fr a nesting territry was "...the Vlume 10, Number 4 91
lcatin (%) ()11 (6)5 (7)4 (7)9 (4)0 (6)6 (10) (5) (8) (1) 5 n 11 where 511 nn nly 005 (%) 8464 1 4I 5 11 18 I 144 10645 50 8(11) 9(4) (10) I in (4) () (1) () (6) at in004 bth least part104 years 4 searched 16 ne 7n 60 I (15) () adult Swainsn's hawk was bserved, even if the Eucalyptusl 005 in nest Oak Willw Other expanded f Cttnwd 004 Slan 196 was data and part which nt t Cunty wuld 005 searched include fund. resulted (nests yield were Later, recently with frm ant number during EBCC searched use fledged f the cde fall sites pst-fledgling nestlings." applicable 7 r 005. fr greater). Slan This EBCC perid, Cunty definitin cdes. this even Vet definitin applied larger tparts than was my Table Sycamre extic I. Tree types used by Swainsn's Hawks fr nests inyl Slan Cunty and Yl cunties Black lcust I = Mst f the eucalyptus were Blue Gum (Eucalyptus glbulus), but a few were Red Gum (E. camaldulensis). = Other extic includes Almnd (Prunus dulcis), Arizna Ash (Fraxinus velutina), Dedar Cedar (Cedrus dedara), elm (Ulmus spp.), hackberry (Celtis spp.), Osage Orange (Madura pmifera), pine (Pinus spp.), palm. Sme f the willws and ne f the walnuts were als extics. 9 CVBC Bulletin/Fall 007
Blm (1980) reprted the identified tree types used fr nesting in the Central Valley in 1979 t be cttnwd (6), ak (1), willw (1), and eucalyptus (1). One third f the nests fund in Slan Cunty in 004 and 005 were in eucalyptus, and anther third in walnuts in Slan Cunty as well as Table. Swainsn's Hawk nest sites envirnments bserved in Slan and Yl cunties in 004 and 005 (nests with EBCC cde 7 r greater). Occupied farmstead includes rural residences with structure similar t a farmstead regardless f the ccupatins f the residents. Vacant farmsteads mayr may nt still have buildings. A nest in a radside tree in a farmstead is classed as farmstead. Mst f the radside nest sites were n public rads, but sme were n private rads. Grve is a small area with the structure f a frest, with cntinuus canpy; the trees are ccasinally native but usually eucalyptus. Nest site envirnment Slan Cunty 004 005 n (%) n (%) part searched nly in 004 n (%) Yl Cunty part searched bth years 004 005 n(%) n (%) Occupied Farmstead 9(8) 4(41) 19 (18) 8 (18) 9 (18) Vacant Farmstead 1 (1 14 (10) 6 (6) 6 (1) (6) Radside (1). 46 (44) 46 (4) 1 (9) 18 (6) Pasture r Crpland 19 (18) (16) 8 (6) 6 (1) 7 (14) Urban 4 5 5 Grve 4 Native Wdland Orchard Riparian 6 4 7 Cemetery Ttal 104 144 106 45 50 Vlume 10, Number 4 9
the larger part f Yl Cunty which was searched nly in 004 (Table 1). Eucalyptus were less frequent in Yl Cunty, but in the suthern part accunted fr mre than a fifth fthe sites. Tw nests in Slan Cunty in 004 were in palms. Each fledged ne. One f these sites was in a Black Walnut (Juglans califrnica) in 005. The ther repeated nesting in the palm in 005 and fledged tw. The latter had nested in a small pine adjacent t the palm in 001 fledging ne and in 00 fledging three. It is clear frm Tables 1 and that the distributin f tree types in which Swainsn's hawk nests are fund depends n the area searched. Inhad searched by bat instead f autmbile, I wuld have encuntered a different distributin f tree types and site types. Many areas in Slan Cunty have nly eucalyptus. Half the nests in the part f Slan Cunty which was searched were in extic trees. The nly native tree available in numbers in that part f the cunty is Black Walnut, and it is absent frm many areas in the cunty. These distributins cannt be generalized t the Central Valley as a whle. As the type f tree used fr nesting is greatly influenced by the types f trees available, n inference can be drawn abut preferences f Swains n's Hawks fr tree types r site types frm surveys f this srt. The nests fund in rchard trees in 004 and 005 were in the same rchard and in trees very clse tgether. N chicks were detected in either. Perhaps this was cincidental, but perhaps rchard spraying practices cause such lcatins t be ppulatin sinks. LITERATURE CITED Blm, P. H. 1980. The Status f the Swainsn's Hawk in Califrnia, 1979. State f Califrnia, Department f Fish and Game and United States Department f the Interir, Bureau f Land Management, Sacrament Hagemeijer, W. J. M. and M. J. Blair, eds. 1997. The EBCC Atlas f Eurpean Breeding Birds: Their Distributin and Abundance. T. & A. D. Pyser, Lndn. 94 CVBC Bulletin/Fall 007