Nesting Phenology of Landbirds in Baja California

Abstract

Breeding seasons of birds in high and mid-latitudes generally are short and are sharply constrained by suitable environmental conditions. In subtropical regions, there generally is a distinct season but the season is longer and there is more out of season breeding (Immelmann 1971). In deserts, climatic effects (especially precipitation) have a pronounced effect on the initiation and termination of (Serventy 1971). Southern Baja California is an extremely arid subtropical region; thus climatic factors may have a large impact on annual variation in the season of birds. The literature on the seasons of landbirds in Baja California is scarce and, for some species such as the Black-throated Sparrow (Amphispiza bilineata), contradictory. Based on observations and collections, van Rossem (1945) suggested that Black-throated Sparrows on Margarita, Magdalena, and Espiritu Santo Islands breed in February whereas those on the adjacent peninsula breed in October. Banks (1963a) found evidence of on Cerralvo Island in the fall of 1960 and 1961, and collected several birds in condition in April and May 1962. Collections on Magdalena Island in April 1963 indicated that had occurred in the spring (April or May) and the previous fall (Banks 1964). Banks found no evidence for February on either island and concluded that the season for Black-throated Sparrows extends from mid-May through September or October (Banks 1963a, 1964). Collections from Partida, Monserrate, Danzante, and Carmen Islands led Banks (1964) to conclude that started in early May on these islands and that the March date suggested by van Rossem (1945) for the gulf islands was too early. Data presented here on the phenology of Blackthroated Sparrows and other landbirds on Coronados Island and an adjacent location on the Baja California peninsula (hereafter referred to as the mainland) in 1984 and 1985 indicate that phenology changes between years, perhaps explaining the differing conclusions reached by van Rossem and Banks. I conducted field work from 1 January to 3 June 1984 and from 4 January to 25 June 1985 on Coronados Island and the adjacent Baja peninsula (hereafter referred to as the mainland) as part of a study of the landbird communities in the two locations. At each location I established two 10-ha study plots where I color-banded birds, mapped territories, and searched for nests. When a nest was found, it was periodically checked (usually at 1to 4-day intervals) until the young fledged or the nest failed. If a nest was found before laying was completed, the date at which incubation started (initiation date) was recorded as the midpoint between the nest check when incubation had started and the previous check (unless the hatching date or fledging date provided a more accurate estimate). If laying was completed, I estimated initiation date by extrapolation based on hatching date, fledging date, or the size of the young when the nest was found and the number of days for incubation and nestling period for each species. Only nests for which the initiation date could be estimated to +4 days were included in the analysis. I obtained sufficient data for the 2 years to compare the nesting phenology of five species: Black-throated Sparrows, Verdins (Auriparusflaviceps), Costa's Hummingbirds (Calypte costae), and Blue-gray (Polioptila caerulea) and Black-tailed (P. melanura) gnatcatchers. Nesting began in early January in 1984. In 1985, however, no nests were initiated until early February and it was not until late February or early March that nesting began in earnest (Fig. 1). The median nest initiation date for 1984 was 40 days earlier than in 1985 (Table 1, P < 0.0001). I did not see any juveniles nor did I find any nests that were well advanced in early January 1984 or 1985. Thus, I feel confident that had not occurred for several months before my arrival in either year. The earliest nest initiation date for each species was 23 to 52 days earlier in 1984 than 1985 (Table 2). Median initiation dates for Black-throated Sparrows, Verdins, and Costa's Hummingbirds were significantly earlier in 1984; there was no significant difference for either of the gnatcatcher species (Table 1). The lack of significant difference for the gnatcatchers probably was due to the small sample size for these species because the earliest initiation date was considerably earlier in 1984 than 1985 for both species (Table 2). The earlier median dates in 1984 were not due to the extended field season in 1985. When I consider only those nests that were discovered on or before day 151 (the last day in 1984 that a nest whose initiation date could be de-