Spatial dynamics in breeding performance of a predator: the connection to prey availability

Using nationwide long-term data on goshawk and grouse populations in Finland we study the spatial dynamics of the numbers of breeding northern goshawk (Accipiter gentilis) pairs, goshawk brood size and offspring sex ratio and their connection to the abundance of grouse. Our first large-scale data comprise of observations on goshawk nests during 1986–2001 pooled to 21 different regions. The second set are annual (1989–1998) observations of brood size and offspring sex ratio (females over the sum of females and males) in goshawk nests all over the country, aggregated to 50 km grid level (n =28 grid units). The third set comprises counts (1989–2001) of four species of woodland grouse, split to adults and juveniles, also given in the same 50 km grid units. Using these data, we show that the annual numbers of northern goshawk nests in the different regions fluctuate in synchrony. Synchrony is also found in long-term fluctuations of northern goshawk brood size and offspring sex ratio. Moreover, synchrony is found in annual numbers of grouse juveniles and adults, the main prey for the northern goshawk. In the brood size and offspring sex ratio of the goshawk, as well as in the annual numbers of grouse juveniles and adults the degree of synchrony falls off with increasing distance. However, only in sex ratios and in grouse dynamics are the slopes of synchrony vs distance roughly matching. We also found that sex ratio either vs grouse juveniles or grouse adults has a more matching spatial dimension (50 km radius) that sex ratio vs brood size. These observation lend support to the hypothesis that goshawk offspring sex ratio and grouse abundance are interconnected. Despite the reason, consequences of spatial coupling in sex ratio could have repercussions on other life history events.