NESTING BIOLOGY OF THE BAIRD SPARROW IN SOUTHWESTERN MANITOBA

Very few studies have documented aspects of Baird's Sparrow (Ammodramu s bairdii) nesting biology, apparently because of difficulty in locati ng their nests. Subsequently much of the information regarding the bre eding biology of the Baird's Sparrow is based on small samples of nest s and anecdotal information. We studied the nesting biology of the Bai rd's Sparrow in southwestern Manitoba, during 1991-1992. Baird's Sparr ows arrived in the first two weeks of May and initiated clutches as ea rly as 25 May. Clutch initiation peaked between 29 May and 4 June with a second smaller peak occurring in mid- to late July. Seventy-six nes ts were located with a mean clutch size of 4.6 eggs. The incubation pe riod extended 11-12 days and young fledged between 8 and 11 days of ag e. Mayfield nest success was 37% with predation being the primary caus e of nest loss. Brown-headed Cowbirds (Molothrus ater) parasitized 36% of the nests with 67% of these nests containing more than one cowbird egg ((x) over bar = 2.0 +/- 0.2 S.E., range = 1-4). Hatching success of non-parasitized nests was significantly higher than that of parasit ized nests. In addition, significantly fewer young kedged from success ful parasitized nests than from successful non-parasitized nests resul ting in an average cost of 1.1 Baird's Sparrow fledglings per parasiti zed nest. Egg removal by cowbirds was likely the primary cause of lowe red productivity in parasitized nests. Baird's Sparrows appear to be a good quality host for cowbirds in southwestern Manitoba as 21% of cow bird eggs laid fledged young with 0.5 cowbirds hedging per parasitized nest.