Effects of fertilizer-induced reduction of invertebrates on reproductive success of Loggerhead Shrikes(Lanius ludovicianus)

Abstract

We examined the effect of spraying the common fertilizer, sodium ammonium nitrate, on cattle pastures in central Florida. Shrikes are considered good indicators of habitat quality. Hence, eight pairs of shrikes whose territories were sprayed were considered as the treatment group, and four pairs in unsprayed territories as controls. The fertilizer was sprayed on 18 June 1992, when the resident shrikes were into their second and third nests, and within 24 h the dominant bahia grass and patches of blackberry bushes became chlorotic. The eight treatment pairs expanded their territories significantly by 138.5% on average, and the smaller their initial territory, the greater the change. The total number of insects collected in the sprayed pasture in the first three weeks was extremely low compared to the number in the unsprayed pasture. In the control pairs no loss of eggs, young, or adults owing to abiotic causes was observed. However, in the treatment pairs seven eggs, two nestlings, and eight fledglings disappeared, or died from causes attributed to the spraying. In addition, seven females and one male disappeared, and one territory was abandoned. Our observations suggest the need for additional studies to examine the effects of supposedly „environmentally friendly” fertilizers and show the value of monitoring breeding bird populations in agricultural systems.