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In a study of almost 16 000 nest records from seven swallow species across the entire Western Hemisphere, clutch sizes decline with relative laying date in each population, but the slope of this decline grows steeper with increasing distance from the equator. Late-laying birds at all latitudes lay clutches of similar sizes, suggesting that latitudinal differences may be driven primarily by earlier-laying birds. Focused comparisons of site-years in North America with qualitatively different food availability indicate that food supply significantly affects mean clutch size but not the clutch size-lay date regression. Other studies on the seasonality of swallow food also indicate that steeper clutch size-lay date declines in the North are not caused by steeper earlier food peaks there. The distribution of lay dates grows increasingly right-skewed with increasing latitude. This variation in lay-date distributions could be due to the predominance of higher quality, early-laying (and large-clutched) individuals among populations at higher latitudes, resulting from latitudinal variation in mortality rates and the intensity of sexual selection. Our results underscore the importance of studying clutch size and lay date in tandem and suggest new research into the causes of their joint geographic variation.

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Philornis Meinert, 1890 (Diptera: Muscidae) is a genus of Neotropical dipterans that parasitise birds. The currently used external morphological characters to distinguish between species within this genus present some limitations. We used the second internal transcribed spacer region (ITS2) of the rRNA gene as a molecular marker to differentiate adult specimens of Philornis identified morphologically as Philornis torquans and Philornis seguyi from different localities. Specimens identified as P. seguyi from Magdalena (Buenos Aires Province) showed an ITS2 sequence different from that for P. torquans, whereas all other specimens of P. seguyi had sequences identical to those for P. torquans. These findings do not necessarily confirm that specimens from Magdalena indeed belong to P. seguyi, nor that P. seguyi is a valid species. Instead, they alert us about the potential for species misidentification when using morphological characters alone. The use of molecular approaches to aid the identification of Philornis spp. will shed light on the systematics of this group. P. torquans is reported for the first time in Mendoza Province and Uruguay.

Parasitic nest flies Philornis spp. are a driving force threatening the extinction of bird species endemic to Neotropical islands such as the Gal apagos, where introduced Philornis downsi negatively impacts reproductive success of na€ ıve avian hosts. Elsewhere in the Neotropics, such as in the Caribbean region where Philornis nest flies are native, effects of Philornis on host productivity are poorly known. We manipulated parasitism by the native Hispaniolan nest fly Philornis pici on a critically endangered endemic host, Ridgway's hawk Buteo ridgwayi, to study the impact of nest fly myiasis on hawk breeding success with the goal of providing a management option for endangered species until broad-scale solutions can be found. Our treatment protocol was enough to reduce P. pici abundance by 89% and increase probability of fledging by 179% for treated nestlings. Our results indicate that parasitism by nest flies decreases survival and fledging success of nestling Ridgway's hawks and is a possible factor in the decline of the species. To the best of our knowledge, this work represents the first quantitative evidence of nest fly impact on survival or productivity in a non-passerine host.

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The examination of Philornis Meinert specimens recently collected in Argentina prompted this study, in which the presence of the previously recorded species is discussed and the literature concerning them is revised. From the seven species previously recorded from Argentina, we recognise three: P. torquans (Nielsen, 1913); P. blanchardi Garcia, 1952; and P. seguyi Garcia, 1952, herein considered as a senior synonym of P. nielseni Dodge, syn. nov. Philornis pici (Macquart, 1854) is considered a doubtful record and P. angustifrons (Loew, 1861) has been mistakenly recorded in Argentina. Finally, P. umanani Garcia, 1952 is considered an unrecognisable species (nomen dubium). Historical data and comments on each of these species are given.