The influence of microtines on polygyny, productivity, age, and provisioning of breeding Northern Harriers: a 5-year study

Abstract

Breeding Northern Harriers, Circus cyaneus, and their principle prey, the vole Microtus pennsylvanicus, underwent synchronous fluctuations in New Brunswick between 1980 and 1984. Microtines were abundant in 1980 and 1983 and were significantly tracked by the number of nesting harriers (r = 0.90), the number of polygynous males (r = 0.89), the number of harem females (r = 0.90), and the mean clutch size (r = 0.94), but not the reproductive success of successful females (r = 0.72). Male nest defence likewise exhibited a strong relationship (r = 0.99, n = 3) with prey abundance, but nest predation did not. An unexpected association with prey abundance was the greater proportion of young females (≤ 2 years) breeding at vole lows; the reverse was true for yearling males. Young females that did breed at vole highs were significantly more productive than were old females breeding at highs. The difference arose principally through nest predation. Successful females also consistently reared significantly greater proportions of their hatchlings when voles were increasing than when they were decreasing. Our results suggest that New Brunswick harriers were affected by prey fluctuations in most aspects of their reproduction and population dynamics. Significant correlations between male food provisioning rates and clutch size and reproductive success over 3 years provide a proximate mechanism through which fecundity may vary annually. They may also provide a proximate pathway mediating for polygyny.

Diet composition influences annual breeding success of Montagu's HarriersCircus pygargusfeeding on diverse prey

“…Ultimately, results from this study suggest that Montagu's Harriers in Madrid may be considered as hare specialists, despite an overall diverse diet, apparently showing numerical responses to that prey (in terms of reproductive output, although not in terms of breeding numbers). Similar results have been found for other harrier species with overall diverse diet, but which responded numerically to vole abundance in the pre-laying period (Simmons et al 1986, Redpath et al 2002. Such responses have also been found in other generalist raptor species (Watson et al 1992, Viñuela & Veiga 1992, Wiklund 2001, where abundance of one particular prey type influences breeding density or performance.…”

Does timing of breeding matter less where the grass is greener? Seasonal declines in breeding performance differ between regions in an endangered endemic raptor

“…A wide spread in timing of breeding has been reported in other raptors from the Southern Hemisphere [e.g., 8 months for the Black Sparrowhawk (Accipiter melanoleucus), Martin et al 2014], including other harrier species, such as the African Marsh Harrier Circus ranivorus (9 months, Simmons 2000, Simmons et al 2005, the Cinereous Harrier (Circus cinereus) (7 months, del Hoyo et al 1994) or the Spotted Harrier (Circus assimilis) (5 months, del Hoyo et al 1994). This contrasts to what is usually observed in harrier species breeding in the Palearctic, for which the timing of breeding rarely exceeds 3 months [e.g., Pallid Harrier (Circus macrourus), Montagu's Harrier (Circus pygargus), Hen/Northern Harrier (Circus cyaneus/hudsonius), (Schipper 1979, Simmons et al 1986, Arroyo et al 1998, Simmons 2000, Amar et al 2005, Terraube et al 2009). These large scale differences in the extent of the timing of breeding are likely related to climate, the Northern Hemisphere being in general characterized by a more pronounced seasonality in rainfall and temperature regimes (Garcia and Arroyo 2001, Redpath et al 2002a) than the Southern Hemisphere, limiting in time the conditions that are suitable for successful breeding, and also suggest that conditions for breeding are suitable over a longer time for Black Harriers, as for other African raptors.…”

Long‐term increase in the fecundity of hen harriers in Wales is explained by reduced human interference and warmer weather

“…. Table 2 Number of breeding attempts 1990-1995 that were recorded as being on land that was managed by gamekeepers (= gamekeeper) or not managed by gamekeepers ( =no gamekeeper), and numbers of these breeding attempts that were recorded as having failed due to predation or having failed due to human interference Clutch size is an indicator of pre-laying food availability in harriers (Simmons et al, 1986;Salamolard et al, 2000;Redpath et al, 2001Redpath et al, , 2002aAmar et al, 2003) and so it seems likely that the low clutch sizes in Wales after the mid1990s were a sign of reduced pre-laying food availability. The decline in clutch size was concomitant with an increasing population, and while a density-dependent effect on clutch size is unlikely (Whitfield & Fielding, 2008) it is possible that with an increasing population a greater proportion of females bred at a younger age (Newton, 1979) and clutch size increases with age in many birds (Hamann & Cooke, 1986).…”

“…Explanatory variables in our initial model included annual rates (nest failures per breeding attempt) of human interference and predation (predominantly predation by red fox), as the two major attributable causes of breeding failure (see 'Results'), clutch size as an indicator of pre-laying food availability in harriers (Simmons et al, 1986;Salamolard et al, 2000;Redpath et al, 2001Redpath et al, , 2002aAmar et al, 2003) and weather variables, using correlations between weather variables and measures of breeding performance as an exploratory filter for their subsequent inclusion in the initial 'full' model. Successive GLMs were run after removal of variables with P40.10 as the criterion for removal: model performance was judged using the Akaike information criterion (AIC: Akaike, 1974).…”

“…Despite claims that driven grouse shoots were abandoned in Wales in the 1970s (Tapper, 2004), they continued at least until 2001 in a part of north Wales (Offord, 2002) such that in 1998 about 27% of Welsh harriers bred on managed grouse moors (Sim et al, 2001), declining to about 13% in 2004 (Sim et al, 2007), reflecting the decline of Welsh grouse shooting (Offord, 2002) and the recent expansion of the Welsh harrier population. Food availability can also influence harrier breeding performance (Simmons et al, 1986;Salamolard et al, 2000;Redpath, Thirgood & Leckie, 2001;Redpath et al, 2002a;Amar, Redpath & Thirgood, 2003). Without data on potential food abundance, however, we could not examine this potential factor directly, but as clutch size is considered to be an indication of food availability during egg laying (Simmons et al, 1986;Salamolard et al, 2000;Redpath et al, 2001Redpath et al, , 2002aAmar et al, 2003), we could indirectly assess whether there were long-term changes in food abundance by looking at long-term changes in clutch size.…”